Mating Systems & Parental Care
Chapter 18
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



Mating Systems
Parental Care

Who Invests in Offspring?
Parent-Offspring Recognition

Not covered in lecture
Parent-Offspring Conflict
Sibling rivalry
Chapter 18 2

Factors Affecting Type of Mating
System

Sexual Selection

Differential allocation of resources into gamete production and parental care

Sperm is cheap, eggs expensive
Chapter 18 3

Factors Affecting Type of Mating
System
Female Male
Limits to
Reproductive
Success
# of eggs produced
# of matings
1. Males should be competitive among themselves for opportunities to mate (Intrasexual competition)
2. Females should be choosy: any mating may involve a big investment on her part, so she should be selective about it (Intersexual competition)
Chapter 18 4

Sexual selection underlies the evolution of male competition and female choice.
Type of Mating System dependent on:
Amount of parental care required
Ecological factors
Chapter 18 5

Mating Systems



Most classifications of mating systems based on extent of bonding/association between male and female during mating.
Problem:

 judgment is needed to determined what constitutes an association
Subjective
Emlen & Oring (1977)
Chapter 18 6

Mating Systems


Emlen & Oring (1977):
Classification based on ability of one sex to monopolize or accumulate mates


Eliminates need for subjective judgment
Emphasizes the ecological and behavioural potential for monopolization
Chapter 18 7

Resource-based Mating Systems
Emlen and Oring (1977)
• the ecology of an organism may not permit males to have more than one partner.
•Females widely distributed and males cannot monopolize them.
•Females may mate with another male so monogamy may serve to guard the female.
•If males help rear young, fitness increases through increased young survival.
•The evolution mating systems is driven by the distribution of resources in the environment for both the male and the female.
Chapter 18 8

Mating Systems
Monogamy - 1 male, 1 female mate guarding mate assistance female enforced
Polygyny - 1 male, many females resource defense female defense scramble competition explosive breeding assemblage lek
Polyandry - 1 female, many males male defense resource defense
Promiscuity
Chapter 18 9

Mating Systems
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Males can produce lots of sperm almost continuously
Monogamy is not best for a male
Females often need help in raising offspring, so monogamy is good for her
Sexual selection theory and coupled with low parental investment of males suggest that polygynous mating systems should be most common.
Why are males?
Chapter 18 10

Mating Systems

Monogamy

(Association of) 1 male & 1 female



Neither sex is able to monopolize more than one member of the opposite sex
Mate for season only, or for life
Genetic constraints vs. social/ environmental factors (e.g., seabirds socially monogamous)
Chapter 18 11

Mating Systems

Polygamy

Matings with multiple partners; nonmonogamous systems

Polygyny : “many” females

Males able to monopolize >1 female

Polyandry : “many” males

Females able to monopolize >1 male
Chapter 18 12

Mating Systems

Promiscuity

Multiple matings by at least one sexes

Absence of prolonged associations
Chapter 18 13

Spatial distribution of resources influences type of mating system
Fig. 18.1
Dots = resources, circles = defended areas
Chapter 18 14

Polygamy - Types of Polygyny:


Resource-defence polygyny:
Males defend areas containing food or nesting sites that females need for reproduction


Territories differ quality (clumped distribution)
May reach a point where a female can do better by mating with an already-mated male on a good territory than an unmated male on a poor territory = polygyny threshold
Chapter 18 15

Polygyny
Threshold
Fig. 18-3
(a) (b)
The quality of territory at (b) is greater than at (a) . A female joining an already-mated male at (b) will have the same RS (c) as a female in a monogamous pair at (a) .
Chapter 18 16

Polygamy – Types of Polygyny

Resource-defence polygyny (cont’d):


Facultative (optional) polygyny :
Dependent on locale (clumped, defensible resources: polygyny ; vs. spread-out, nondefensible: monogamy )

Thus, same species may behave differently in different environments
Chapter 18 17

Polygamy – Types of Polygyny

Female-defence polygyny:


Females aggregate for reasons unrelated to mating
E.g., female elephant seals haul out onto land to give birth, sites limited
– Males compete to monopolize the grouped females, defend harem from other males
Chapter 18 18

Polygamy – Types of Polygyny

Male dominance polygyny


Neither resources nor females can be monopolized
Males gather and display, females choose based on quality of display


Usually on leks (mating areas where males congregate and defend small territories while displaying for females)
E.g., bowerbirds; sage grouse
Chapter 18 19

Polygamy – Types of Polygyny

Male dominance polygyny (cont’d)


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Mating success skewed in favour of a small number of males
Dominant males may have:



Preferred territory locations within lek
Most attractive display (e.g., bower)
Most elaborate ornaments, etc.
“Copying” behaviour seen in females
Chapter 18 20

Polygamy – Types of Polygyny


Scramble Polygyny
Males actively search for mates without overt competition

Large groups of females congregate, males concentrate only on inseminating females or fertilizing eggs, ignore other males (e.g., wood frogs, horseshoe crabs); OR

Widely disbursed females are actively pursued by males (e.g., thirteen-lined ground squirrels)
Chapter 18 21

Polygamy – Types of Polyandry

Resource-defence polyandry


Females control access to males indirectly by monopolizing critical resources e.g., spotted sandpiper – females compete for control of breeding territories
Chapter 18 22

Polygamy – Types of Polyandry

Female-access polyandry

Females do not defend resources essential to males, but they interact among themselves to limit access to males
 e.g., American jacanas – some females defend “superterritories” which encompass the territories of several males, prevent access to other females
Chapter 18 23

Polyandry


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Rare –
Females more brightly coloured, etc. (sexuallyselected traits)
Females show philopatry
(returning to natal territory after maturity)
:
True of males in polygynous species
Chapter 18 24

Ecology & Mating Systems


Mating systems related to resource distribution
Two sympatric (i.e., coexist in same area) species of Agelaius (Orians, 1961)
Red-winged blackbirds
Chapter 18
Tricoloured blackbirds
25

Ecology & Mating Systems

Red-winged Black Birds:

Usually polygynous



Male defend territory with 2 or 3 females
Male arrives 3-4 wk before females (often natal territory), defends territory until young are fledged (several months)
Rarely help raise young – females build nests and incubate
Chapter 18 26

Ecology & Mating Systems

Tricoloured Black Birds:

Monogamous



Nomadic colonies of 100-200,000 birds
Establish territories, find mates, build nests, lay eggs
(biparental care) – ALL WITHIN 1 WEEK!
Both sexes make large investment, but in a smaller timeframe
Chapter 18 27

Ecology & Mating Systems
Why so different?

Depends on Energy source (food availability)


RWBB: Stable diet of seeds and insects, available for several months
TCBB: Mass feeding flights, attack ephemeral, concentrated food sources, e.g., ripe rice/grain fields
Chapter 18 28

Mouse MHC Probe
Gibbs, et al. 1990. Realized reproductive success of polygynous Red-winged Blackbirds revealed by
DNA markers. Science 250: 1394-1397.
Chapter 18 29

Alternative Mating Tactics
Members of same sex can use different tactics to obtain matings

E.g., some males fight, others sneak copulations by being inconspicuous or mimicking females

“Sneaky”, satellite, or parasitic males

E.g., bullfrogs: Dominant male calls to attract females to his territory; satellite male hides at edge, waits for females to approach
Chapter 18 30

Explosive Breeding Assemblage
•Females become receptive for only a brief time
•They go to a breeding area when they are ready
•Males gather at the breeding location and compete for the female.
Chapter 18 31

Alternative Tactics

Morphology


 coho salmon: jacks vs. hooknoses
Hooknoses: built to fight
Jacks: smaller, mimic females, sneak fertilizations
(Fig. 18.8)
Chapter 18 32

Parental Care
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
Extent of parental care varies greatly across species
Ways to invest:


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DNA (release sperm/eggs, move on)
Prenuptial feeding (e.g., crickets) – spermatophore nourishes female, more and better quality offspring
Direct care of offspring (incubating, feeding, brooding) – birds/mammals/some fish…
Chapter 18 33


See Text pg. 151 (Chapter 10)
Chapter 18 34



Which Sex Invests?
Taxonomic differences also exist in patterns of parental investment:
 birds: monogamy / biparental care

 mammals: polygyny / maternal care fish: promiscuity/polygyny / no parental care or male care
Why these differences?
Chapter 18 Photo by: Alan and Sandy Carey 35

Determinants of Parental
Investment in Vertebrates
Mode of fertilization


Internal fertilization – primarily female care

E.g., birds, mammals
External fertilization – primarily biparental or male care

E.g., fish
Chapter 18 36

Determinants of Parental
Investment in Vertebrates

Certainty of Paternity Hypothesis

Reliability of paternity assumed to be greater when eggs are fertilized externally (e.g., fish) rather than internally (e.g., mammals, birds)
Chapter 18 37

Determinants of Parental
Investment in Vertebrates
Association Hypothesis




Proximity of adults to offspring
Internal fertilization – females closer
External fertilization – both sexes close, but male/biparental care more common when offspring associated with a male’s territory
E.g., males defend territories, little extra energy to protect eggs already on territory
(e.g., fish)
Chapter 18 38

Determinants of Parental
Investment in Vertebrates
Phylogenetic Constraints:

Birds

Eggs develop externally

Incubation, feeding, guarding by both sexes

Mammals

Internal gestation


Lactation
Males can provide little care
Chapter 18 39

Determinants of Parental
Investment in Vertebrates


Phylogenetic constraints
Fish



External fertilization
Both sexes free to desert, or
Guard nest site
(territory)
Chapter 18 40

Determinants of
Parental Investment in Vertebrates



Reproductive Effort: Total energy expended in reproducing, which includes mating effort & parental care
May direct all reproductive effort into mating (e.g., codfish - lay eggs, fertilize them, swim away), OR
May expend much energy in parental care (e.g., primates – 25% of offspring’s lifespan)
Chapter 18 41

Reproductive Effort
Male
MALE
Female
FEMALE
Mating
System
ME
PE
PE PE
ME
PE
PE
PE
ME
PE
PE
ME
Polygamous/
Promiscuous
Chapter 18
Monogamous
42

Determinants of Parental
Investment in Vertebrates

How Do Ecological Factors Affect
Parental Investment?

K- & r-selection theory

“Bet-hedging” theory
Chapter 18 43

r - & K- Selection Theory
Stable environment

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Larger body size
Slower development
Longer lifespan
Have young at intervals
( iteroparity );
K-selected (at/near carrying capacity of environment, K) few young that receive much care
Fluctuating environment





Small body size
Rapid development
Have young all at once
( semelparity ), r-selected (at/near reproductive rate of population, r) many young that receive little/no care
Chapter 18 44

“Bet-hedging” Theory

In environments where survival of offspring is low and unpredictable, parents may “hedge their bets” by putting in only a small reproductive effort each season
 spread their reproductive effort out over time rather than all at once
 e.g., California Gull: Lives 15+ yrs. As adults age, appear to increase effort (lay more eggs, feed chicks more food, defend more vigorously) due to their own decreasing reproductive potential.
Chapter 18 45



Parent-Offspring Recognition
Misdirected parental care can be costly (waste of energy, reduced fitness)

Fostering in seals
Recognition appears to evolve in species in which mixing of unrelated offspring is likely
Chapter 18 46


 colonial ground-nesting gulls & terns:
 precocial chicks mobile at ~5 days,
 adults seem to develop recognition for own chicks at that time kittiwakes
 cliff-nesters;
 chicks fledge at 3 weeks

Parents do not seem to recognize offspring
Chapter 18 47

Parent-Offspring Conflict


Offspring demand more investment from the parent (usually mother) than the parent is willing to give
Trivers (1974): Natural selection acts differently on mother and offspring

Based on coefficient of relatedness and parental investment
Chapter 18 48

Parent-Offspring Conflict



Mother - should invest a certain amount of energy into first offspring, “wean” him, then invest in producing second offspring, when the cost of raising #1 starts to exceed the benefits of having #2…
Offspring - should “demand” investment from mother until such point that the mother’s fitness starts to decline (&, thus, so does his inclusive fitness, as he shares half his genes with her).
when cost of raising #1 becomes 2x the benefit to mother, offspring's inclusive fitness 
Chapter 18 49

Parental Reproductive Success

How can parents increase their reproductive success and fitness?

Manipulation of dependent young
Chapter 18 50

Parental Manipulation


Parents may selectively provide care to certain offspring at the expense of others
Brood adjustment

Coots - selectively adjusts brood size by killing chicks (European) or feeding specific chicks (American)
Chapter 18 51

Parental Manipulation


Hatching asynchrony - milder form of brood adjustment: first-to-hatch have advantage, get more food, larger, can push others out of nest
(i.e., siblicide)
It is in parents’ interests to create inequalities in size of chicks: WHY?
 biggest, strongest offspring survive and are more likely to reach reproductive age
Chapter 18 52

Sibling Rivalry

Pigs: “runt” gets least preferred teat (little milk); piglets have sharp teeth and fight for access to best nipples

 young predatory birds (ex: boobies, eagles, egrets) kill younger siblings soon after hatching, may be because the second egg serves only as parental insurance in case the first fails to hatch.
Chapter 18 53

Case Study: Spotted hyenas
Chapter 18 54