PREDATION PRESSURE AS A DETERMINANT OF PARENTAL BEHAVIOR IN
NEOTROPICAL CICHLIDS
Luciana Ramirez
B. S., Universidad de Buenos Aires, 2003
THESIS
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
BIOLOGICAL SCIENCES
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
Fall
2008
© 2008
Luciana Ramirez
ALL RIGHTS RESERVED
ii
PREDATION PRESSURE AS A DETERMINANT OF PARENTAL BEHAVIOR IN
NEOTROPICAL CICHLIDS
A Thesis
by
Luciana Ramirez
Approved by:
__________________________________, Committee Chair
Dr. Ronald M. Coleman
__________________________________, Second Reader
Dr. Jamie M. Kneitel
__________________________________, Third Reader
Dr. Brett Holland
Date: ___________________________
iii
Student: Luciana Ramirez
I certify that this student has met the requirements for format contained in the University
format manual, and that this thesis is suitable for shelving in the Library and credit is to
be awarded for the thesis.
_______________________________________________ ___________________
Dr. James W. Baxter, Graduate Coordinator
Date
Department of Biological Sciences
iv
Abstract
of
PREDATION PRESSURE AS A DETERMINANT OF PARENTAL BEHAVIOR IN
NEOTROPICAL CICHLIDS
by
Luciana Ramirez
Understanding the evolution of various forms of parental care is a key question in the
field of evolutionary ecology. The study of parental behavior is inseparable from
Williams’ principle and parental decisions should be analyzed in terms of present
reproductive costs diminishing the expected future reproduction of an individual. Due to
the diversity in parental behavior exhibited within the family, cichlid fishes present an
excellent opportunity to study the evolution of parental care.
The experiments presented herein address two different aspects of the evolution of
mouthbrooding in cichlid fishes: first, if mouthbrooding may have had its origins as a
response to predation pressure; and second, if these fishes are able to adjust their
investment in offspring defense in relation to the perceived risk for their young. By
presenting potential predators of their offspring to parenting females of two different
species of Neotropical cichlids, I studied the changes in their parental response.
When they perceived high risk for their offspring, females of the substrate spawning
convict cichlid (Archocentrus nigrofasciatus) used their mouths significantly more in
offspring retrieving, a behavior that can be considered an incipient state of
v
mouthbrooding. This may constitute evidence that high predation threat on the young
acted as a selective pressure promoting mouthbrooding.
Delayed mouthbrooder Gymnogeophagus balzanii females were capable of adjusting
their investment in parental care by delaying by 33.9% the time of first releasing their
offspring in response to the perception of predation risk for her young. Delayed
mouthbrooding is considered to be an intermediate state between substrate spawning and
immediate or pure mouthbrooding.
Together, these results prove that female cichlids are capable of adjusting the amount
of parental investment in response to predation threat for their offspring and suggest that
in some Neotropical cichlids, maternal mouthbrooding may have evolved from biparental
substrate spawning as a result of male desertion in environments with high predation
upon fry.
, Committee Chair
Dr. Ronald Coleman
vi
ACKNOWLEDGMENTS
I am grateful to the Guy Jordan Endowment Fund of the American Cichlid Association
and the Mark Tomasello Fund of the Pacific Coast Cichlid Association for funding this
research. I thank the entire department of Biological Sciences of California State
University, Sacramento and to my Supervisory Committee (R. M. Coleman, J. M. Kneitel
and B. Holland) for their patience and efforts to help me every time I needed it. Special
thanks to Ron Coleman for having given me the opportunity of working in his laboratory,
for his guidance and his always wise advice and constant encouragement. Thanks to “The
Lab”, where I spent many hours during these years and where I met wonderful people,
who always gave me their support and friendship and made this adventure a wonderful
experience. Thanks to my husband Gabriel for his unconditional love and constant
support in every aspect of my life. Thanks to my family and friends for always being
there for me.
vii
TABLE OF CONTENTS
Page
Acknowledgements……………….………………………………………………….…..vii
List of Tables………………….….………………………………………………….…....x
List of Figures…………………….………………………………………………….…...xi
Chapter 1. GENERAL INTRODUCTION...………………….…………………….….....1
Williams’ Principle………………………………………………………………..1
Parental Care and Williams’ Principle…………………………………………….2
Factors Determining the Investment in Parental Care………………………..…...3
Parental Care Adjustments and Predation………………….……………………..5
Diversity of Egg Tending Behaviors in Cichlid Fishes…….……………………..6
Evolution of Mouthbrooding Behavior……………………..…………….……….8
Objectives………………………………………………………………………..12
Hypotheses…….…………………………………………….…………………...13
Chapter 2. ASSESSMENT OF PREDATION RISK AS A FACTOR PROMOTING THE
USE OF THE MOUTH IN PARENTAL CARE ACTIVITIES………………………...14
Introduction………………………………………………………………………14
Methods…………………………………………………………………………..15
Study Species……………………………………………………………..15
Experimental Design……………………………………………………..15
Results……………………………………………………………………………17
viii
Conclusions.……………………………………………………………………..20
Chapter 3. ASSESSMENT OF PREDATION THREAT AS CAUSE OF AN
INCREMENT IN THE TIME ALLOCATED TO MOUTHBROOD.………………….23
Introduction……………………………………………………………………...23
Methods………………………………………………………………………….23
Study Species….………………………………………………………….23
Experimental Design……………………………………………………..24
Results……………………………………………………………………………26
Conclusions………………………………………………………………………28
Chapter 4. FINAL DISCUSSION..……………………………………………………...33
LITERATURE CITED….……………………………………………………………….36
ix
LIST OF TABLES
Table
Page
Table 1. Differences between mouth brooding and substrate brooding .............................9
x
LIST OF FIGURES
Figure
Page
Figure 1: Mean Total Activity for each treatment (n=10). Female convict
cichlids used their mouths differently depending on the level of
predation. Females used their mouth significantly less when
subjected to the NoRod treatment (One way ANOVA, F2,27 =16.38,
p<<0.01) than when subjected to either the Model or the Rod (Tukey
HSD test, p> 0.05). Total Mouth Activity is the total number of days
in which the female used her mouth for parental purposes. Vertical
bars denote standard deviation. ................................................................................18
Figure 2: Total Mouth Activity as a function of Stage and Treatment. The
interaction among the factors Stage and Treatment was not
significant and there was no difference in mouth activity among
stages (Repeteated Mesures ANOVA F1, 27= 1.08, p=0.31). Notice
wider mean dispersion at the fry stage. Vertical bars denote 0.95
confidence intervals.. ................................................................................................19
Figure 3: Difference in the number of days until the first time of releasing
the fry in the delayed mouthbrooder G. balzanii. Parental females
under predation threat retained their offspring 33. 9% more time in
their mouths (Mann-Whitney U test: Z=2.97, p = 0.0023). Vertical
bars denote standard deviation... ..............................................................................27
Figure 4: Percent (%) time the fry spent outside the mother’s mouth over
the five days of observation. There were no statistical differences
among treatments (Mann-Whitney U test Z= 0.87, p= 0.39). Oral
shelter provided by female G. balzanii to their offspring decreases
over the five days of the experiment (Linear regression F1, 14= 13.08,
p= 0.003 for the Control treatment and Linear regression F1, 14=
17.24, p= 0.001 for the Predation treatment). Each point represents
the mean time that fry spent outside the mouth each day in the
presence and in the absence of a threat (n=4)...........................................................29
xi
1
CHAPTER 1
GENERAL INTRODUCTION
Williams’ Principle
Departing from the idea of fixed energetic resources which must be distributed
between somatic growth and reproduction, G. C. Williams (1966) recognized that every
reproductive event has a cost and that any effort put into the current brood is made at the
expense of the future broods. Therefore, organisms must allocate resources in a way that
maximizes their individual lifetime reproductive success and not just the number of
descendants immediately produced. The significance of considering the expected future
reproduction affects important life-history traits like age of sexual maturation and
fecundity. This statement is better known as Williams’ Principle and it was presented by
Gross and Sargent (1985) in a simplified model as:
LRS  P( RE )  F ( SE )
where LRS refers to the lifetime reproductive success. It can be understood as the number
of descendants or copies of a gene (Gross, 2005) that an individual is capable of leaving
throughout its lifespan. P is the present reproductive success. It depends on RE, or the
reproductive effort, which is the amount of energy or resources that a parent puts in
producing descendants, whether this is in gametes or in parental care. Finally, F refers to
the future reproductive success and it is function of the somatic effort, SE, because larger
2
individuals or individuals who are in better physiological condition, those who have
invested more resources in somatic growth, are assumed to be able to produce more
progeny in the future.
Parental Care and Williams’ Principle
Parental care was defined by R. L. Trivers (1972) as “any investment by the parents in
an individual offspring that increases the offspring’s chance of surviving (and hence
reproductive success) at the expense of parent’s ability to invest in other offspring”
Many studies on parental behavior have been done using fishes as subject models
because they present the whole range of states of parental care from no care at all to
biparental care, with male-alone care being predominant (Gross & Sargent, 1985). Forms
of parental investment in fishes include egg ventilation (fanning), nest building, egg and
fry defense from potential predators and food provisioning (Breder & Rosen, 1966;
Blumer, 1979; Wisenden et al., 1995; Barlow, 2000; Zworykin et al., 2000; Chong et al.,
2005, Karino & Arai, 2006). Some authors even include pre-spawning traits like yolk
provisioning to the eggs and provisioning of a safe place like a nest or any protective
covering as forms of parental care (Perrone & Zaret, 1979).
Williams’ Principle provides an adequate framework for the study of parental care
because it allows us to address basic questions in parental behavior such as which sex
should provide care and how much care should be provided at a single reproductive event
(Gross, 2005). The present work focuses on this last question of how much care a parent
should provide to its current offspring.
3
Factors Determining the Investment in Parental Care
According to Williams’ Principle, a parent should maximize its lifetime reproductive
success by regulating how much effort it puts into each reproductive event. The decision
on how much care a parent provides now to its offspring depends on four possible
determinants or a combination of them: size of the present brood, past investment,
genetic relatedness and future mating opportunity (Gross, 2005).
Brood size and past investment: The size of the clutch and the amount of parental
investment already provided (past investment) are important determinants of the intensity
of parental care. Coleman et al. (1985) determined that a parental bluegill sunfish,
Lepomis macrochirus, invests in its offspring according to both of these factors. When
researchers artificially reduced the clutch size at two different times, they found that nonmanipulated control clutches with more accumulated investment in defense and a larger
number of eggs were more protected from a model predator than late-reduced clutches,
with a high accumulated investment but fewer eggs. Finally, clutches reduced earlier,
with lower accumulated investment and few eggs were the least protected. More recently,
the same pattern was also observed in birds, where the initial size of the clutch
determined parental desertion rate in the common eider, Somateria mollissima (Bourgeon
et al., 2006). The authors observed that nest desertion was higher when the initial clutch
size was small and that females deserted their nests more during the first third of
incubation than later.
Genetic Relatedness: Experiments on certainty of paternity have demonstrated the
importance of genetic relatedness in parental investment. For example, male bluegill
4
sunfish are capable of making adjustments in their parental effort according to an
assessment based on the presence of sneaker males near their nests before egg hatching
and also based on olfactory clues after the eggs have hatched (Neff, 2003).
Future mating opportunities: Chances of future reproduction also determine how
much effort an individual invests in parental care of its current offspring. It is expected
that an individual with higher opportunities of mating in the future will invest less in
present offspring than another individual with lower mating expectations. Future mating
opportunities depend on factors that can be divided into intrinsic and extrinsic (Sargent &
Gross, 1993). Intrinsic factors are those related to an individual’s physiology or
condition, like health and size. As an example of an intrinsic factor, differences in body
size can explain unequal parental investment. In the convict cichlid fish, Archocentrus
nigrofasciatus, it was observed that large and small females differently appreciated samesize clutches. In fishes in general, larger females are capable of producing more eggs,
therefore a larger female values any given number of eggs less than a smaller female.
Thus, a smaller female invests more energy in defending the same number of eggs than a
larger female (Galvani & Coleman, 1998). Extrinsic factors can be understood as those
depending on the surrounding environment, like resource availability and predation risk.
One example of extrinsic factors affecting mating opportunity is the case of the European
starling, Sturnus vulgari (Smith, 1995). In this bird, male parental care decreased as more
females were available for courting and mating opportunity increased. It is not always
easy to discriminate among intrinsic and extrinsic factors, because the environment may
affect physiological conditions. As an example, it was demonstrated that female convict
5
cichlids were capable of adjusting the number of eggs they laid in relation to the quality
of the spawning sites presented. When presented with safer spawning places, females
produced more eggs than when more open and less secure shelters were provided
(Wisenden, 1993). Thus, females were capable of a physiological adjustment in their
gonadal investment in response to environmental conditions.
Parental Care Adjustments and Predation
Predation can be understood as an extrinsic factor, which can affect not only present
reproductive success but also future mating opportunity because a parent who is injured
while protecting its offspring may suffer a reduction in its fecundity. For example, an
injured individual may have reduced capabilities of attracting mates through combat
against conspecifics or it may have to allocate more resources to heal injuries and retain
fewer resources to produce gametes (Smith & Wootton, 1995). Besides partial reductions
in future fecundity, a parent might be killed by a predator while defending its offspring,
in which case it will not be able to produce descendants at all.
Thus, defense of the offspring against predators entails costs and an individual who is
capable of adjusting the amount of energy invested in defense according to the perceived
risk of predation is trying to maximize its lifetime reproductive value.
It is known that predation affects several important traits like feeding behavior (Foam
et al., 2005) and courtship (Magnhagen, 1991; Candolin, 1998). A classic example of
predation pressure affecting parental investment decisions is given by Trinidadian
guppies, Poecilia reticulata. In populations where predation upon adult guppies was
6
severe, there was a reduction in the age of sexual maturation, a shortening in the interbrood intervals and an increment in the number of eggs (Reznick & Endler, 1982). This
can be explained in terms of Williams’ principle by a lower expected future reproduction
in populations with greater predation on larger individuals, which causes an increment in
the present reproductive effort.
For a parent to be capable of changing its reproductive investment as function of the
perceived level of risk in the environment, it has to be capable of assessing the level of
threat in the surrounding media. The parent can use a series of indicators of the intensity
of the threat such as predator number, predator type and size, distance, threatening
behavior and frequency of attacking. Based on this information, the parent should be
capable of deciding how much effort to put into defending its offspring or whether it is
better to desert. Parental defense can be understood as a particular case of the threatsensitivity hypothesis (Helfman, 1989), which predicts that organisms will trade-off
predator avoidance against other activities in response to the magnitude of the predatory
threat. There is some evidence that parents can actually regulate the amount of care in
response to the perceived predation risk. For example, in natural populations of convict
cichlids, males desert most often from sites where predation pressure is lower, leaving the
female to care for the offspring alone (Wisenden, 1994).
Diversity of Egg Tending Behavior in Cichlid Fishes
One particular way to understand how individuals manage their parental investment is
by examining the egg tending behavior performed by fishes. Particularly, the freshwater
7
fish family Cichlidae presents a unique opportunity to study parental care due to the large
number of species within the family (over 2000) and its diversity in egg tending behavior
(Balshine-Earn & Earn, 1998; Barlow, 2000). For instance, egg care behavior ranges
from substrate spawning to delayed mouthbrooding to immediate mouthbrooding
(Keenleyside, 1991).
Substrate spawning is a common form of parental behavior in cichlid fishes, in
particular among Neotropical cichlids (Goodwin et al., 1998). Substrate spawners lay
their eggs on a clean surface like a rock, which they use as a nest or they may also seek
protection for their offspring in crevices.
Delayed mouthbrooding, or larvophilous mouthbrooding, is another form of parental
care in cichlids (Weidner, 2000), where parents lay their eggs on a substrate, but
immediately after the eggs have hatched, the non-free swimming embryos, known as
wrigglers, are collected into a parent’s mouth and they are orally incubated. After several
days of incubation, wrigglers become free swimming larvae, called fry, and they are
released for first time out of the mother’s mouth. The fry will leave the parent’s mouth
for foraging, but in the presence of any threat, the parent allows the young to come back
and seek refuge in its mouth. This process lasts several weeks, but as time passes, the fry
become more independent and spend more time outside until they are finally rejected by
the parent. Delayed mouthbrooding is a less common form of egg tending behavior in
cichlids and it only occurs in some members of the South American subfamily
Geophaginae (López-Fernández et al., 2005).
8
Immediate mouthbrooding, or the complete oral incubation of eggs, larvae and fry, is
an extended form of parental care in fishes (Oppenheimer, 1970) and it is commonly
observed particularly among African cichlids (Freyer & Iles, 1972; Goodwin et al, 1998).
The process of releasing the offspring in immediate mouthbrooding is the same as that in
delayed mouthbrooding. In both delayed and immediate mouthbrooders, oral incubating
females fast during the first period of incubation. Species differ in the amount of time
allocated to mouthbrooding and this period can last between one and six weeks (Freyer &
Iles, 1972; Schürch & Taborsky, 2005). Mouthbrooder and substrate spawning cichlids
present differences in several life-history traits and parental behavior (Table 1).
Evolution of Mouthbrooding Behavior
Evolutionary biologists strongly suspect that mouthbrooding evolved from substrate
spawning species when parents started to retain their offspring in the mouth for longer
periods of time while moving them from one spot to another when nesting conditions
were no longer favorable (Goodwin et. al, 1998). Possible adverse conditions include
lack of surfaces to attach eggs, rapid fluctuations in water levels and high predation
pressure (Oppenheimer, 1970; Timms & Keenleyside, 1975; Goodwin et al., 1998).
A curious behavior is observed in substrate spawning cichlids. Although they do not
present the oral adaptations required to mouthbrood, sometimes parents use their mouths
to move their offspring from one spot to another.
9
Table 1: Differences between mouth brooding and substrate brooding
Substrate brooders
Mouthbrooders
Elliptical shape and
Type of egg1
thicker eggshell with
Rounded, very rich in yolk
adhesive areas or
and thinner eggshell
stalks
Egg size & Number1
Many small eggs
Fewer bigger eggs
Fanning
Churning
Biparental
Female-alone; Biparental
Form of egg ventilation
provided by the
parents1, 3, 4
Most common gender
of the care giver2, 5
Independent offspring
in shorter period. Free
mouth for feeding and
Benefits for parents3
defense. Caves
provide extra
protection for the
young and the parent
High mobility to avoid
adverse environmental
conditions. No need to find
a nest and to defend it.
Provides a mobile refuge for
highly vulnerable fry
(...continued)
10
Table 1 (continued)...
Substrate brooders
Mouthbrooders
Exposition to adverse
environmental
conditions.
Competition for
Costs for parents4, 5
substrates. Can be
injured or killed while
defending the nest.
No feeding for long periods
of time; cannot use mouth in
defense
Offspring reaches the
most vulnerable fry
stage smaller
Representative
Neotropical
Cichlasoma
Geophagus
Genera5
1
: Weidner (2000), 2: Breder & Rosen (1966), 3: Barlow (2000), 4: Oppenheimer
(1970), 5: Goodwin et al. (1998).
11
This behavior is known as offspring retrieving and it is frequently observed during the
first days after hatching (Barlow, 2000). The parent digs small pits in the substrate using
its pelvic fins, the mouth and the body and the wrigglers are transferred from one of these
pits to another using the parent’s mouth. After embryos develop into free swimming
larvae, the parent also uses its mouth occasionally for gathering the young. This behavior
is presumably for keeping the offspring in one more easily defensible place (Breder &
Rosen, 1966).
Although it has long been suspected that retrieving behavior is a way to reduce
predation on the offspring, this has never been tested. If the behavior of moving and
gathering offspring using the mouth were actually a parental attempt to hide the offspring
from predators, it would be expected that high levels of risk would cause an increment in
the number of times that parents perform these behaviors. Indirect evidence that offspring
retrieving movements are parental attempts to protect the offspring was found by Reebs
(1994) while he studied circadian rhythms in convict cichlids. Offspring retrieving
behavior increased before night in response to light, probably, as an adaptive response to
nocturnal predators.
It is known that some animals, especially amphibians, exhibit some phenotypic
plasticity in the duration of offspring development as an antipredatory strategy. It is
believed that the duration of the most susceptible stage is the one that is shortened in
response to the risk of predation, whether it is the egg or the larvae stage (Evans et al.,
2007). In the specific case of delayed mouthbrooder fishes, if mouthbrooding were an
antipredatory response, an increment in the perception of risk of predation for the
12
offspring should trigger a delay in the time of the first release of the young and longer
periods of oral incubation. Both strategies would have the role of protecting the offspring
during the first days of being a fry, which are the most vulnerable (Lavery, 1995).
Because substrate spawning is considered the ancestral form of parental care and
mouthbrooding the derived state, by studying how substrate spawners use their mouths
for protecting their offspring and the reproductive behavior of delayed mouthbrooders, it
may be possible to better understand the transition between substrate spawning and
immediate mouthbrooding. The wriggler retrieving behavior may represent, from an
evolutionary perspective, an early stage in the development of the mouth as a tool of
parental care, while delayed mouthbrooding may be an intermediate state towards the
evolution of immediate mouthbrooding (Dupuis & Keenleyside, 1981). In turn, an
increment in the amount of time that a parent retains its offspring into its mouth as an
antipredatory response may have led to the behavior of permanent oral shelter. Thus,
predation is a factor that may have played a significant role in the evolution of
mouthbrooding.
Objectives
The objectives of this work were to study if predation pressure may have played a
role in the evolution of mouthbrooding and to determine if parents are capable of
adjusting the amount of parental investment they provide in response to an immediate
threat of predation for their offspring. First, I will assess if predation risk can be
considered a factor that promotes the use of the mouth as a tool of parental care in
13
females of the convict cichlid, Archocentrus nigrofasciatus, a substrate spawning cichlid.
Second, I will assess if predation threat triggers an increment in the amount of time
allocated to mouthbrooding in females of the delayed mouthbrooder cichlid
Gymnogeophagus balzanii.
Hypotheses
1. When substrate spawning females perceive increasing levels of predation
risk, they will perform more retrieving movements of their wrigglers and
they will use their mouths more often to gather the fry.
2. When delayed mouthbrooder females perceive high predation threat, they
will elongate the period of mouthbrooding, whether by delaying the time
of the first releasing of the offspring or by providing oral shelter for their
fry for longer periods.
14
CHAPTER 2
ASSESSMENT OF PREDATION RISK AS A FACTOR PROMOTING THE USE OF
THE MOUTH IN PARENTAL CARE ACTIVITIES
Introduction
Several causes have been proposed to explain the evolution of mouthbrooding.
Proposed factors include lack of surfaces to attach eggs, rapid fluctuations in water levels
and high predation pressure (Oppenheimer, 1970; Timms & Keenleyside, 1975; Goodwin
et al., 1998). In this experiment, I will assess if predation risk can be considered a factor
that promotes the use of the mouth as a tool of parental care in females of the convict
cichlid, Archocentrus nigrofasciatus, a substrate spawning fish.
Offspring retrieving (Barlow, 2000) is a typical parental behavior in Neotropical
substrate spawner cichlids in which a fish digs holes in the substrate and moves it
offspring from one hole to another using the mouth (Breder & Rosen, 1966). Because
substrate spawning is considered an ancestral form of parental care and mouthbrooding
the derived form (Goodwing et al., 1998), it may be supposed that the use of the mouth in
parental activities by substrate spawners constitutes an incipient state of mouthbrooding.
If predation pressure on the offspring where actually a factor promoting the use of the
mouth in parental activities, it will be expected that parents use their mouth more when
they perceive increasing levels of predation risk for their young.
15
Methods
Study Species
The convict cichlid, a Neotropical substrate spawning cichlid, was used as the
experimental subject. This cichlid is native to Central America from Guatemala to
Panama. They are sexually dimorphic, monogamous and present biparental brood care
with division of labor (Wisenden & Keenleyside, 1992; Coleman, 1993). The female lays
her eggs on a clean surface or on the walls of a crevice. Parental care includes fanning the
eggs to oxygenate them and the active defense of the offspring against intruders by
chasing predators away. The female also digs pits in the substrate using her pelvic fins
and performs offspring retrieving behavior, moving wrigglers from one pit to another. At
26º C the eggs hatch between 36 and 48 hours after spawning. After hatching, the small
larvae emerge. These larvae are unable to swim and they fed on their yolk sac. This
wriggler stage lasts almost five days. After this time, the offspring is capable of free
swimming and feeds by itself. Wild parental convicts keep providing defense for up to
six weeks before the fry disperse (Wisenden, 1995). In the wild, unattended offspring are
rapidly predated by small fishes especially conspecifics and minnows (Barlow, 2000).
Experimental design
One male and one female convict cichlid were randomly taken from stock tanks,
where both genders were kept separated and they were moved to smaller experimental
aquaria of 30 x 75 x 30 cm high. Light and temperature conditions were kept identical
and constant in every experimental aquarium (12 hours of light and 26ºC). Each
16
aquarium was equipped with one clay flowerpot to serve as a substrate where the female
attached her eggs and the bottom of the aquarium was covered with gravel. Three sides of
the aquarium were covered with an opaque white plastic to reduce outside disturbance.
After the female spawned, the fish were randomly assigned to one of three experimental
treatments. All treatments started 48 hours after spawning, the same day that eggs
hatched, which is the first day of the wriggler stage. Each replicate lasted ten days.
The first treatment consisted of presenting the parents with a threat model (Model
treatment). The model was constructed from a photographic print of the generalist cichlid
predator Lamprologus moorii (Axelrod et al., 1993, pp 820) of 44.6 mm standard length
(SL) attached to a plexiglass rod. The size of the model was chosen considering the size
of the parent such that the model was always smaller than the parental female. The model
was moved less than two cm from the offspring, without touching them, to generate the
threat of predation. The distance between the model and the offspring and the intensity of
the threat were both kept constant because the same person always moved the model. In
the second treatment, the rod alone was moved near the offspring without the model
attached to it (Rod treatment). These two treatments were considered to be two different
levels of threat. Threats lasted 90 seconds, which were divided into two intervals of 45
seconds each and were separated by a 45-second break. In the third treatment no threat
was presented (No Rod treatment), but an observer stood in front of the tank for 90
seconds. The recorded variable was number of days when the female used her mouth for
parental activities, whether it was for wriggler retrieving or for fry gathering.
17
A repeated measures ANOVA was performed to assess if there were significant
differences in female parental mouth activity among treatments and between stages.
Results
Overall, females in the No Rod treatment used their mouth in offspring retrieving and
gathering, also known as Total Mouth Activity (TMA), significantly less than females in
the other treatments (F2, 27= 16.38, p<< 0.01) (Figure 1).
The wriggler stage lasted on average 5.0 (± 0.7) days and no significant difference in
its duration was found among treatments (F2, 27= 0.83, p= 0.45). There was no difference
in the number of times that females used their mouth in parental activities between the
wriggler and the fry stages (F1, 27= 1.08, p=0.31) and no interaction between factors was
registered (F2, 27= 2.68, p= 0.09). Although the homogeneity of variance test for the fry
stage was marginally not significant (  2 = 5.4, df=2, p=0.06), the dispersion of the mean
values of mouth activity for each treatment was greater in the fry stage (F2, 27= 14.13, p<<
0.01) than in the wriggler stage (F2, 27= 2.54, p=0.1) (Figure 2). An independent analysis
of the fry stage showed significant differences among all treatments (Tukey HSD test,
p<0.05 for all comparisons) with females using their mouth in parental activities an
average of 0.5 times (± 0.7) in the No Rod treatment, 2.0 times (± 1.7) in the Rod
treatment and 3.5 times (± 1.3) when subjected to the Model treatment. The mean body
size of the females was 46. 6 (± 3.8) mm SL and this individual variation did not account
for the differences among treatments (F2, 27= 0.41, p= 0.67).
18
8
7
Total Mouth Activity
6
5
4
3
2
1
0
NoRod
Model
Rod
Treatment
Figure 1: Mean Total Activity for each treatment (n=10). Female convict cichlids used
their mouths differently depending on the level of predation. Females used their
mouth significantly less when subjected to the NoRod treatment (One way
ANOVA, F2,27 =16.38, p<<0.01) than when subjected to either the Model or the
Rod (Tukey HSD test, p> 0.05). Total Mouth Activity is the total number of days
in which the female used her mouth for parental purposes. Vertical bars denote
standard deviation.
19
5.0
4.5
4.0
Mouth Activity
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
Fry
Wrigglers
Stage
Figure 2: Total Mouth Activity as a function of Stage and Treatment. The interaction
among the factors Stage and Treatment was not significant and there was no
difference in mouth activity among stages (Repeated Measures ANOVA F1, 27=
1.08, p=0.31). Notice wider mean dispersion at the fry stage. Vertical bars denote
0.95 confidence intervals.
NoRod
Model
Rod
20
Parental females kept defending their offspring against the model, the rod and the
observer with the same intensity over time.
Conclusions
Female convict cichlids used their mouth in parental activities differentially in the
presence and in the absence of a threat to their offspring (Figure 1). This result reaffirms
the observation that wriggler retrieving and fry gathering using the mouth are both
parental behaviors performed as acts of protection of the offspring in response to the
presence of potential predators. This behavior may not be as effective as chasing
predators away, which is the first protective response, but it is used by females as a last
resort to protect their offspring. This behavior is less effective than chasing predators
away because, unlike delayed and immediate mouthbrooders, substrate spawners do not
possess the morphological structures that allow them to collect and retain large number of
offspring in their mouths at the same time. Thus, the offspring have to be moved in
several trips, a very few embryos or larvae at any one time. While doing this, the female
neglects most of her offspring and exposes herself to be injured by the predator, as she
cannot defend herself. This may be the reason why the response is not always immediate
and it may take some time after the threat has faded before the females decides that it is
safe enough to move her offspring to another place.
The results presented here are in concordance with the idea of predation as a selective
pressure on mouthbrooding.
21
Because predation threat causes an increment in the use of the mouth, in the past, it may
have been advantageous for a parent to retain its offspring longer or to be able to hold
more offspring at the same time. Other factors, like sudden changes in water levels,
cannot be discarded and they may have contributed, in combination with predation-risk
related factors, to the evolution of mouthbrooding.
It is interesting to note that differences in parental reaction were more accentuated
during the fry stage than in the wriggler stage. Even though females did not use their
mouth more during the fry than in the wriggler stage, they were more sensitive to
disturbance during the fry period and they used their mouth to gather the offspring
according to the intensity of threat presented (Model>Rod>No Rod) (Figure 2).
The role of past investment may be one explanation for the increasing sensitivity to
predation risk during the fry stage. This hypothesis proposes that parents will try to
minimize wastage of past investment (Coleman et al., 1985) by considering accumulated
investment to decide how much care to provide to the current offspring. The past
investment hypothesis was demonstrated on convict cichlids, which provided less defense
to clutches reduced in size in early stages than to clutches reduced in later stages (Lavery
& Keenleyside, 1990).
A different reason for more parental sensitivity to predators in the fry stage may be
that the offspring are more vulnerable when they first exit the nest and they become more
conspicuous to predators, which may attack from all sides (Smith & Wootton, 1995).
This is the nest crypsis hypothesis and it was experimentally confirmed to occur in
convict cichlids as well as in other Neotropical cichlids (Lavery, 1995). Also, because in
22
the wild the predominant types of cichlid offspring predator are smaller fishes than the
parents, often juvenile conspecifics and tetras, to defend a nest may be easier than to
defend the free swimming larvae. Thus, predation pressure upon eggs may be lower than
upon fry and females may be responding according to this pattern. According to this idea,
heavy predation upon fry may have caused parents to pick up their fry into their mouths
and to retain them for protection, transforming their mouths into mobile nests. Predation
on fry would be an even stronger selective pressure on uniparental care, independent of
the sex of the care giver.
An alternative hypothesis for the evolution of mouthbrooding is that heavy predation
on the eggs promotes mouthbrooding. This hypothesis is supported by the existence of
several fish families where males are egg bearers. It proposes that mouthbrooding would
have had its origins from the state of paternal substrate brooding, when males started to
retain the eggs in their mouths and they became the limiting resource as their
reproductive rates were lowered by the size of their oral cavities (Smith & Wootton,
1995). Therefore, females may have preferred and selected males with larger mouths
promoting the evolution of the oral structures for incubation. However, this does not
seem to be the case in cichlids where mouthbrooding evolved from the state of biparental
substrate spawning (Goodwin et al., 1998).
23
CHAPTER 3
ASSESSMENT OF PREDATION THREAT AS CAUSE OF AN INCREMENT IN THE
TIME ALLOCATED TO MOUTHBROOD
Introduction
Delayed mouthbrooder females fast during the period of oral incubation. Thus,
they face a decision about whether to release their young and start feeding themselves,
exposing the fry to predators when they are more vulnerable (investment in future
reproduction), or to prolong the oral phase of incubation and release larger and moreindependent young (investment in the current offspring). Because delayed
mouthbrooding cichlids may represent an intermediate state between substrate spawners
and immediate mouthbrooders (Dupuis & Keenleyside, 1981), studying delayed
mouthbrooders may put some light on the transition between these two forms of parental
care. The experiment presented here addresses an important aspect of the evolution of
mouthbrooding, namely, if predation pressure may be considered a factor that increases
the time that parental females retain the offspring in their mouth.
Methods
Species Study
The delayed mouthbrooder cichlid Gymnogeophagus balzanii was used for this second
experiment. G. balzanii is native to southeastern South America, from the Paraná and
Paraguay River basins. This species exhibits a polygynous mating system and is sexually
24
dimorphic with males developing a characteristic fatty nuchal hump (Casciotta et al.,
2005). During spawning, eggs are attached by the female to a clean surface, generally a
rock. The female always takes complete charge of incubating and rearing the offspring.
The process of guarding and ventilation of the eggs is also in complete charge of the
female, who fans them with her pectoral fins (Leibel, 1983). After 24-36 hours at 26º C
of substrate incubation, the female chews the larvae out of their egg-shells and starts the
oral incubation of the embryos. This stage lasts several days and the female does not feed
herself during the initial period. At this point, embryos are constantly churned in the
mouth of their mother to keep them oxygenated and they are never released. After this
first period, the free swimming larvae emerge out of the mother’s mouth, but in the
presence of any threat the fry are sheltered again in the female’s mouth until the danger
has passed. Parental care gradually decreases over time and after three or four weeks the
female completely stops providing oral shelter to her offspring.
Experimental design
G. balzanii were kept in breeding aquariums of 60 x 60 x 30 cm high in a ratio of
5♀:1♂. Fish in the breeding aquaria were fed once a day with commercial cichlid flakes
(Tetracichlid Flakes) and a mix of “bloodworms” and brine shrimp, Artemia salina.
Temperature was kept constant at 28º C in all aquariums.
Within 24 hours after a spawning, a female holding eggs in her mouth was moved to a
smaller experimental aquarium of 50.0 cm x 25.0 cm x 30.0 cm where she was randomly
assigned to one of two treatments. The first treatment was with potential offspring
25
predators present (Predation treatment). A glass container with juvenile convict cichlids,
Archocentrus nigrofasciatus, was placed into the experimental tank for 24 hours every
other day. The container consisted of a one-liter glass jar with gravel bottom and a plastic
top with holes in it, which allowed the exchange of gases, so females and fry were
capable of perceiving any odor cue released by the predators. The frequency of exposure
was chosen to resolve the trade-off between providing enough perceived risk of threat to
the young and the possibility that the parents will become acclimated to the presence of
the enclosed predators and therefore stop responding to them (Taborsky & Foerster,
2004) and the notion that females might not perceive enough risk of threat for the young.
The use of free predators would be more realistic, but inevitably, it would result in the
loss of some of the fry. Preliminary experiments showed that G. balzanii females
respond to the presence of potential offspring predators in the glass container by
attacking them repeatedly. I used juvenile convicts as predators because most egg and fry
predation in the wild is performed by other small cichlids or tetras and not by larger
predatory fishes (Barlow, 2000). Males were not included in the experiments because
single-female care is the common situation in nature (Weidner, 2000) and because males
could become fry predators. The second treatment was a control with no potential
predators present, only an empty container was introduced into the aquarium.
Preliminary experiments showed that direct observation of the parental activities was
not possible without disturbing the females because they were extremely sensitive to the
presence of observers. A female would collect her fry immediately if she detected any
movement in the room or she might even swallow her offspring if she suffered stress. To
26
avoid these problems, all activity in the experimental aquariums was observed using
webcams. To decrease the chances of disturbing the females, a remote PC access
software (Go to my PC®) was used. This software allows access to the webcams from
outside the experimental room. In addition, three sides of the experimental aquaria were
covered with opaque plastic to diminish any outside disturbance.
The recorded variables were the number of days until the first time the female released
her offspring and the time spent by the fry out of their mother’s mouth. In order to know
the number of days up to the first release, the experimental aquariums were checked six
times a day for ten minutes each time, up to the first observed releasing. To record the
time that fry spent out of their mother’s mouth after the first releasing, all activity in the
experimental aquariums was observed in six daily sessions for 15 minutes each, for five
consecutive days.
Differences among treatments were statistically assessed using a Mann-Whitney U test
for independent samples. The changes in times of oral incubation over time were assessed
with a linear regression for each treatment.
Results
Female G. balzanii that were orally incubating in aquaria with potential predators of
their offspring (n=8), retained their fry an average of 33.9% (Mann-Whitney U test
Z=2.97, p= 0.0023) more than control females (n=6) (Figure 3). Predation females
incubated for 8.3 (±0.8) days, while females in the Control treatment incubated for 6.2 (±
0.8) days.
27
9.5
9.0
8.5
Days
8.0
7.5
7.0
6.5
6.0
5.5
5.0
Predation
Control
Treatment
Figure 3: Difference in the number of days until the first time of releasing the fry in the
delayed mouthbrooder G. balzanii. Parental females under predation threat
retained their offspring 33. 9% more time in their mouths (Mann-Whitney U test:
Z=2.97, p = 0.0023). Vertical bars denote standard deviation.
28
However, no statistical difference among treatments was found in the time allocated to
mouthbrooding after the releasing of the fry (Mann-Whitney U test Z=0.87, p= 0.39). Fry
spent an average of 60.2% of total recorded time outside the mother’s mouth in the
Control treatment and 53.9% outside during the Predation treatment. When the first day
was not considered for both treatments, mean time outside was 72.9% for the Control
group and 61.4% for the Predation treatment (Figure 4). The reason underlying the
exclusion of data from the first day will be discussed later, but it was the only day where
the fry spent more time outside the mother’s mouth in the Predation group than in the
Control group.
As expected, over time, females provided less oral shelter to their fry in the Control
and in the Predation treatments (Linear regression: F1, 14= 13.08, p= 0.003 and F1, 11=
17.24, p= 0.001 respectively) and the fry spent most of the time outside the mother’s
mouth beyond the fifth day (Figure 4). The average SL of the females was 51.55 (± 4.8)
mm and this variation did not account for the observed differences in times among
treatments (F 1, 4= 0.6, p= 0.45). Females did not show signs of becoming accustomed to
the presence of predators in the glass containers, as they kept responding with the same
intensity over the time that the experiment lasted.
Conclusions
Females of the delayed mouthbrooder cichlid G. balzanii were capable of adjusting the
timing of parental care in response to the perceived risk of predation on their offspring by
delaying by almost 34% the time of first releasing her fry (Figure 3).
29
Figure 4: Percent (%) time the fry spent outside the mother’s mouth over the five days of
observation. There were no statistical differences among treatments (MannWhitney U test Z= 0.87, p= 0.39). Oral shelter provided by female G. balzanii to
their offspring decreases over the five days of the experiment (Linear regression
F1, 14= 13.08, p= 0.003 for the Control treatment and Linear regression F1, 14=
17.24, p= 0.001 for the Predation treatment). Each point represents the mean time
that fry spent outside the mouth each day in the presence and in the absence of a
threat (n=4)
30
This result is in concordance with other experimental data on parental care duration in
African cichlids, where females of the immediate mouthbrooder Ctenochromis horei
extended their oral incubation period 20% in the presence of a natural predator of their
young (Taborsky & Foerster, 2004). This flexibility in incubation times can be explained
in terms of benefits for the offspring. The most vulnerable moment for an offspring is
during the first days of being a fry when they are just starting to leave the mouth. The
vulnerability of the offspring decreases over time, as fry grows older, and improves their
swimming skills, which allow them to escape from predators more easily. Over time, G.
balzanii females decreased the time allocated to mouthbrooding independently of the
presence of the predators (Figure 4). This reduction in the amount of parental care over
time is also observed in other cichlids species (Lavery, 1995). However, the increment in
the chances of survival of the offspring also comes with an increment in the present costs
of reproduction for the female and consequently a reduction in her future reproductive
success. Because mouthbrooder females do not feed while incubating, the more time she
retains her brood in her mouth, the more her body condition decreases.
It is possible that by the moment of releasing her offspring, the female cannot retain them
longer because of her need to feed herself, even under the threat of predation for her
young. Even though average differences in time were not significant, G. balzanii females
under predation risk always released their fry for less time than Control females (Figure
4), except on the first day. During this first day, the fry of the females in the Predation
treatment spent more time outside than the fry of the Control group.
31
This may be because these females, who had incubated longer, had more energetic
requirements after longer fasts and they need more time foraging to fulfill these
requirements. The lack of significance among average retention times after the first
releasing may have been due to the low number of replicates or because the predator
presented was not enough threat for the female to cause a response. However, the
delaying in the first releasing of the fry suggests that this is not the reason for the result.
Another explanation could be that females are not capable of mouthbrooding any longer.
This is a possibility given the fact that after finishing a period of mouthbrooding, females
begin a period of resource storage where they have to save energy in the form of lipids to
use in the production of eggs (Mrowka & Schierwater, 1988).
One of the few studies of delayed mouthbrooders, Dupuis and Keenleyside
(1981), did not find evidence that predation threat causes a shortening in the substrate
phase of the incubation when they studied the effect of predation on parental care in the
biparental cichlid Bujurquina vittata (formerly Aequidens paraguayensis). They
concluded that the substrate phase cannot be shortened any more by parents because of
constraint in the time that the eggs need to hatch, which depends on temperature.
However, a study made on the fathead minnow, Pimephales promeleas, revealed that the
decision on whether to hatch sooner or not in the presence of predators depends on the
egg and it is based on chemical cues released by the predator (Kusch & Chivers, 2004).
Hatching times were not recorded here, but observation suggests a variation of 12 hours
or less among females (at 28º C). Also, as a difference from G. balzanii, which provide
female-alone care, the delayed mouthbrooder species studied by Dupuis and Keenleyside
32
has biparental care. It would be expected for a female alone to have more variation in the
picking up times because the predation threat for the offspring is higher for a single
female than for a couple with both members defending the nest. Besides these
assumptions, another plausible explanation is that this first substrate phase is actually not
very flexible because the predation risk on the eggs is low.
33
CHAPTER 4
FINAL DISCUSSION
Understanding the evolution of various forms of parental care and which fishes
perform which behavior is a key question in the field of evolutionary ecology in issues
like mating systems and sexual selection. The study of parental behavior is inseparable
from Williams’ principle and parental decisions should be analyzed in terms of present
reproductive costs diminishing the expected future reproduction of an individual.
The two experiments presented here have addressed the role of predation as a
selective pressure on the evolution of mouthbrooding and they have shown two things.
First, in both substrate spawner and delayed mouthbrooder cichlids, predation pressure
increments the use of the mouth in parental activities. Because the risk of predation for
the offspring increased the frequency with which substrate spawners used their mouths
and because delayed mouthbrooder females retained their young longer to avoid fry
predation, these responses may have constituted the first steps towards the evolution of
mouthbrooding. Second, consistent with Williams’ principle, G. balzanii females tried to
adjust the investment in parental care in response to the perception of predation risk for
their young by delaying the time of the first release of the offspring. The cost for females
is less time for storing energy to produce the next clutch. The benefits are that females
gain extra time for their offspring, delaying the exposure when fry are more susceptible,
so that they can leave the mouth larger and be more prepared to avoid predators.
34
Delayed mouthbrooding is especially interesting from an evolutionary perspective
because it may represent an intermediate state between substrate spawning and
immediate mouthbrooding (Dupuis & Keenleyside, 1981). Delayed mouthbrooders pay
some of the costs of being a substrate spawner and some of the cost of being a
mouthbrooder (see Table 1). However, under certain circumstances, females may not
have other evolutionary choices than mouthbrood. In Neotropical cichlids, there is a
correlation between mating systems and the mode of parental care (Goodwin et al.,
1998). Most substrate spawners are monogamous and exhibit biparental care, whereas
mouthbrooders are polygynous and present female-alone care. In biparental species, the
typical parental role division, with females in close contact with the offspring and males
patrolling the proximities of the nest (Itzkowitz et al., 2001), seems to put both genders
just one step away from the evolution of polygamy and female-alone care. For maternal
care to evolve, the female has to be capable of rearing her offspring successfully.
Mouthbrooding provides the female with a defensible, mobile and safe harbor for her
brood, which is enough protection against most offspring predators. The idea of females
using more their mouth in parental activities under predation pressure is also in
concordance with the hypothesis of maternal mouthbrooding evolving from the state of
biparental substrate brooding (Smith & Wootton, 1195) as a result of male desertion in
some Neotropical cichlids.
While the family Cichlidae has been a well studied group in general, delayed
mouthbrooders have received little attention. Furthermore, the behavioral ecology of
southern South American cichlids in general is poorly known. Future research on delayed
35
mouthbrooders should address the cost of mouthbrooding for females, offspring response
to predators and the study of wild populations to better understand the circumstances that
favor mouthbrooding.
36
LITERATURE CITED
Axelrod, H. R., Burgess, E. W., Pronek, N. and J. G. Walls (1993). Dr Axelrod’s atlas
of freshwater aquarium fishes. T.F.H. Publications, Neptune City, New Jersey.
Barlow G. W. (2000). The cichlid fishes: Nature’s grand experiment in evolution.
Perseus Publishing, Cambridge 335pp.
Balshine-Earn S. and Earn D. J. D. (1998). On the evolutionary pathway of parental
carein mouth-brooding cichlid fish. Proceedings of the Royal Society of London
Series B: Biological Sciences (London) 265: 2217-2222.
Blumer, L. S. (1979). Male parental care in the bony fishes. The Quarterly Review of
Biology 54: 149-161.
Bourgeon S., Criscuolo F., Bertile, F., Raclot, T., Gabrielsen G. W. and S. Massemin
(2006). Effects of clutch sizes and incubation stage on nest desertion in the
female Common Eider Somateria mollissima nesting in the high Arctic. Polar
Biology 29: 358–363
Breder, C. M. Jr. and D. E. Rosen (1966). Modes of reproduction in fishes. T.F.H.
Publications, Neptune City, New Jersey.
Candolin, U. (1998). Reproduction under predation risk and the trade-off between
current and future reproduction in threespine sticklebacks. Proceedings of the
Royal Society of London Series B: Biological Sciences (London) 265: 1171- 1175.
Casciotta, J. R., Almirón, A. E. and Bechara, J. (2005). Peces del Iberá. Hábitat y
diversidad. UNDP, Fundación Ecos, UNLP, UNNE, La Plata.
Chong, K., Ying, T. S., Foo, J., Jin, L. T. and A. Chong (2005). Characterization of
proteins in epidermal mucus of discus fish (Symphysodon spp.) during parental
phase. Aquaculture 249: 469– 476.
Coleman, R. M. (1993). The evolution of parental investment in fishes. Ph.D.
dissertation, University of Toronto, Canada.
Coleman, R. M., Gross, M. R. and R. C. Sargent (1985). Parental investment decision
rules: a test in bluegill sunfish. Behavioral Ecology and Sociobiology 18:59-66.
37
Dupuis, H. M. C. and M. H. A. Keenleyside. (1981). Egg care behaviour of Aequidens
paraguayensis (Pisces, Cichlidae) in relation to predation pressure and spawning
substrate. Canadian Journal of Zoology 60: 1794-1799.
Evans, J. P., Gasparini, C. and A. Pilastro (2007). Females guppies shorten brood
retention in response to predator cues. Behavioral Ecology and Sociobiology 61:
719- 727.
Fryer, G. and T. D. Iles (1972). The cichlid fishes of the great lakes of Africa: Their
biology and evolution. T.F.H. Publications, Neptune City, New Jersey.
Foam, P. E., Mirza, R. S., Chivers, D. P. and G. E. Brown (2005). Juvenile convict
cichlids (Archocentrus nigrofasciatus) allocate foraging time and antipredator
behavior in response to temporal variation in predation risk. Behavior 142: 129144.
Galvani, A. P. and R. M. Coleman (1998). Do parental convict cichlids of different
sizes value the same brood number equally? Animal Behaviour 56: 541– 546.
Goodwin, N. B., Balshine-Earn, S. and J. D. Reynolds. (1998). Evolutionary transitions
in parental care in cichlid fish. Proceedings of the Royal Society of London Series
B: Biological Sciences (London) 265: 2265-2272.
Gross, M. R. (2005). The evolution of parental care. The Quarterly Review of Biology
80:37-45.
Gross, M. R. and R. C. Sargent. (1985). The evolution of male and female parental care
in fishes. American Zoologist 25: 807- 822.
Helfman, G.S. (1989). Threat-sensitive predator avoidance in damselfish-trumpetfish
interactions. Behavioral Ecology and Sociobiology 24:47-58.
Itzkowitz, M., Santangelo, N. and M. Richter (2001).Parental division of labour and the
shift from minimal to maximal role specializations: an examination using a
biparental fish. Animal Behaviour 61: 1237- 1245.
Karino, K. and R. Arai (2006). Effect of clutch size on male egg-fanning behavior and
hatching success in the goby, Eviota prasina (Klunzinger). Journal of Experimental
Marine Biology and Ecology 334: 43- 50.
Keenleyside, M. H. A. (1991). Parental care. Pp 191-208 in: Cichlid fishes: behavior,
ecology and evolution (Ed., M. H. A. Keenleyside). Chapman & Hall, London.
38
Kusch, R. C. and D. P. Chivers (2004). The effect of crayfish predation on phenotypic
and life-history variation in fathead minnows. Canadian Journal of Zoology 82:
917- 921.
Lavery, R. J. (1995). Changes in offspring vulnerability account for the increase in
convict cichlid defensive behavior with brood age: evidence for the nest crypsis
hypothesis. Animal Behavior 49: 1177- 1184.
Lavery, R. J and Keenleyside, M. H. A. (1990). Parental investment of a biparental
cichlid fish, Cichlasoma nigrofasciatum, in relation to brood size and past
investment. Animal Behavior 40: 1128- 1137.
Leibel, W. S. (1983). Gymnogeophagus balzanii (Perugia 1891). Buntbarsche Bulletin
97: 11-20.
López-Fernández, H., Honeycutt, R. L. and K. O. Winemiller (2005). Molecular
phylogeny and evidence for an adaptive radiation of geophagine cichlids from
South America (Perciformes: Labroidei). Molecular Phylogenetics and Evolution
34: 227- 244.
Magnhagen, C. (1991). Predation risk as a cost of reproduction. Trends in Ecology and
Evolution 6: 183- 186.
Mrowka, W. and B. Schierwater (1988). Energy expenditure for mouthbrooding in a
cichlid fish. Behavioral Ecology and Sociobiology 22: 161- 164.
Neff, B. D. (2003). Decisions about parental care in response to perceived paternity.
Nature 422: 716- 719.
Oppenheimer, J. R. (1970). Mouthbreeding in fishes. Animal Behaviour 18: 493- 503.
Perrone, M. Jr. and T. M. Zaret (1979). Parental care patterns of fishes. The American
Naturalist 113: 351- 361.
Reznick, D. and J. A. Endler (1982). The impact of predation on life history evolution
in Trinidadian guppies (Poecilia reticulata) Evolution 36: 160- 177.
Sargent, R. C. and M. R. Gross (1993). Williams’ principle: an explanation of parental
care in teleost fishes. Pp 333-357 in The Behavior of Teleost Fishes, second edition
(Ed., T. Pitcher). Chapman & Hall, London.
Smith, C. and R. J. Wootton (1995). The costs of parental care in teleost fishes. Reviews
in Fish Biology and Fisheries 5: 7- 22.
39
Smith, H. G. (1995). Experimental Demonstration of a Trade-Off between Mate
Attraction and Paternal Care. Proceedings: Biological Sciences 260: 45- 51.
Schürch, R. and B. Taborsky (2005). The functional significance of buccal feeding in
the mouthbrooding cichlid Tropheus moorii. Behaviour 142: 265- 281.
Taborsky, B. and K. Foerster (2004). Female mouthbrooders adjust incubation duration
to perceived risk of predation. Animal Behaviour 68: 1275-1281.
Timms, A. M. and M. H. A. Keenleyside. (1975). The reproductive behaviour of
Aequidens paraguayensis (Pisces, Cichlidae). Zeitschrift fur Tierpsychologie 39:823.
Trivers, R. L. (1972). Parental investment and sexual selection. Pp 136-179 in: Sexual
selection and the descent of man, 1871-1971 (Ed., B. Campbell). Aldine, Chicago,
Illinois.
Weidner, T. (2000). South American eartheaters. Cichlid Press, El Paso, Texas.
Williams, G. C. (1966). Natural Selection and the costs of reproduction, and a
refinement of Lack’s Principle. The American Naturalist 100: 687- 690.
Wisenden, B. D. (1993). Female convict cichlids adjust gonadal investment in current
reproduction in response to relative risk of brood predation. Canadian Journal of
Zoology 71: 252- 256.
Wisenden, B. D. (1994). Factors affecting mate desertion by males in free-ranging
convict cichlids (Cichlasoma nigrofasciatum). Behavioral Ecology 5: 439- 447.
Wisenden, B. D. (1995). Reproductive behavior of free-ranging convict cichlids,
Cichlasoma nigrofasciatum. Environmental Biology of Fishes 43: 121- 134.
Wisenden, B. D. and M. H. A. Keenleyside (1992). Intraspecific brood adoption in
convict cichlids: a mutual benefit. Behavioral Ecology and Sociobiology 31: 263269.
Wisenden, B. D., Lanfranconi-Izawa, T. L. and M. H. A. Keenleyside (1995). Fin
digging and leaf lifting by the convict cichlid Cichlasoma nigrofasciatum: example
of parental food provisioning. Animal Behaviour 49: 623-631.
40
Zworykin, D. D., Budaeva, S. V. and A. D. Mochek (2000). Does parental fin digging
improve feeding opportunities for offspring in the convict cichlid? Environmental
Biology of Fishes 57: 443-449.