Strumpf, R.M., Boesch, C. (2005). Does promiscuous mating preclude female choice?
Femal sexual strategies in chimpanzees (Pan troglodytes verus) of the Tai National Park,
Cote d’Ivoire. Behav Ecol Sociobiol, 57, 511-524.
Summary by Aubrie DeBear, Neda Naimi, Cody Tyson
For Dr. Mills’ Psyc 310 Class, Spring 2012
Most scientists acknowledge that male reproductive success is dependent on
conceiving a child with the largest number of females possible, and female reproductive
success is dependent on choosing very few males to father their children based on their
ability to enhance the mother and child’s ability to survive. These survival benefits may
be derived from protection, increased resources, or simply a good genetic makeup. All of
these points seem even clearer when talking about female chimpanzees which share a
similarly drastic investment in their children and are unable to produce them in large
quantities.
Although all of this is expected and seems fairly intuitive female chimpanzees are
not all that selective. In fact female chimpanzees have an extremely high ratio of
participation in sexual intercourse to actual fertilization. Researchers predicted this could
be a tactic that has many benefits such as making males unaware of whether they fathered
the child or not. This would increase paternal support and discourage violent and
potentially deadly behavior towards the infant. This also instills an inherent sense of
competition and insures fertilization to occur.
The question that this study wanted to answer was whether a promiscuous
strategy would not allow females to also choose. Is it possible to be promiscuous and
choosy? If the two could coexist then it would be most beneficial for female chimpanzees
to act promiscuous outside of their periovulatory period(POP), time around ovulation in
which conception would be highly likely to occur. Then once their POP was near or
occurring they would become increasingly selective. Some interesting variations that
would be likely desired would be partner’s age or level of dominance. The hypotheses in
this study were: to find out if females had mate preferences for males, if so, whether
females are more mate selective at POP than Non-POP, whether females differ between
one and other in their preferences for mates, and what specific factors influence female
preferences.
The study used naturalistic observation in two different chimpanzee communities
located in Tai National Park in Cote d’Ivoire. Overall 2,600 hours of data were collected
over the course of two years (September 1998 – December 2000). These communities
included 62 and 32 chimpanzees, having 25 and 11 adult females and only 4 and 3 adult
males. Careful observation of 14 capable, but non-pregnant females were made,
especially around time of ovulation. Researchers recorded all behaviors before, during,
and following each sexual interaction between the two parties and made note of the
initiator and the following response. Social rankings of chimpanzees were made by
number of submissive vocalizations. This enabled researchers to group them into separate
ranks, followed by age categories.
In order to determine levels of fertility researchers collected urine samples and
measured hormonal levels to determine where the females were at in their cycle. The
periovulatory period(POP) was objectively defined as 3 days prior to and including
ovulation. Measures for female preferences were operationalized as female’s initial
behavior during sexual interaction, basically female initiated behavior and response to
male attempts. Males were placed in groups in terms of female preferences: preferred,
eschewed, or neutral. The tables below simplify that of table two in the current study
(Stumpf & Boesch, 2004).
Measure
Definition
Proceptivity rate
Number of female initiated sexual interactions over partners
time spent together during POP or Non-Pop.
Resistance rate
Number of times the female resisted over number of male
attempts.
Relative proceptivity
Difference between females proceptivity rate toward a specific
rate
male compared to that of males as a whole.
Relative resistance rate
Difference between females resistance rate toward a specific
male compared to that of males as a whole.
Categorical Term
Criteria
Preferred male
Female proceptivity rate towards a specific male is higher by
more than 25%, or resistance rate falls more than 25% below,
compared to her average towards males as a whole.
Eschewed male
Female proceptivity rate towards a specific male falls more
than 25% below, or resistance rate is higher by more than 25%,
compared to her average towards males as a whole.
Neutral male
Females proceptivity or resistance rate towards a specific male
does not deviate more than 25% in either direction when
compared to males as a whole.
These measures or terms are not standardized, but allow us to see clear
differences between certain behaviors. Analysis was later done to determine the
desirability of different males when comparing POP vs. Non-POP females. With this
analysis came a few new hypotheses, differences between female behavior in POP and
Non-POP are a product of corresponding differences in male solicitation or aggression
rates, increased male aggression is correlated with decreased female resistance or
increased proceptivity, and male solicitation rates are correlated with female resistance.
Aggression rates were derived by measuring number of aggressive behaviors towards a
female by a male over the interaction time. Male attempts for sexual encounters were
measured the same way as aggression.
The study found that all females do indeed demonstrate proceptivity and
resistance, though these are not significantly influenced by female rank or age. The
proceptivity rates were in fact, as predicted, lower during POP than Non-POP. Resistance
rates were also confirmed to be much higher during POP than Non-POP. Male behaviors
were not shown to influence this pattern (e.g., aggression rates in males were not
significantly correlated with female resistance rates). Researchers found that the male’s
categorical value (Preferred, eschewed, or neutral) moderated the rates of proceptivity
and resistance near ovulation in females. For instance, females resistance increasing
towards eschewed males during POP compared to Non-POP, while their proceptive
would decrease. Strangely enough this was actually almost reversed in Non-POP, the
females preferred the higher ranked and lower ranked males substantially more than the
two middle ranks. Female age and rank did not show a substantial influence on
proceptivity or resistance rates for the most part. Researchers did find slight variation in
Non-POP where chimpanzees of similar ranking were more inclined to be proceptitive
and have lower resistance rates. Male age was the strongest predictor of female resistance
rates during POP. Females had much higher rates of resistance towards older males.
These findings suggest that females exhibit both promiscuous and selective sexual
behaviors depending on where they are in their cycle. This allows them to use paternity
confusion to their advantage and increase male competition. This competition may weed
out any inferior males and give them a better chance of copulating with a superior male.
While the methods in this study were well thought out and significantly controlled it is
impossible to rule out male coercion indefinitely, although within primates it is rare to
see forced copulation. Also, female resistance rates were not correlated with male
aggression; in fact many females were consistently resistance towards one of the most
aggressive males in the study. All in all, it is clear that female chimpanzees do not mate
indiscriminately, but try to confuse males through sex when the chance of pregnancy is
very low, and become highly controlling of paternity when most likely to become
pregnant.
OUTLINE
Intro
A. Reproductive success
1. Males success lies in the most amount of females he fertilizes
2. Females success lies in the most offspring who grow to healthy and strong
B. Mothers
1. Extensive investment in offspring
2. Limited reproductive potential
3. These should result in them being very picky with choosing mates
C. Female chimpanzee’s aren’t selective
D. Pro’s to promiscuous mating for female chimpanzee’s
1. So many men, the father doesn’t know it’s his therefore no man will kill
the child
2. The strongest sperm will fight the other sperm and reach the egg
3. Makes sure the female get’s pregnant
E. Older males are more preferred because their ability to stay alive longer shows
their strength but the increased age also means lower sperm count.
F. Test in the study
1. If females like certain males more than others.
2. if so, are they more picky around their menstrual cycle or not
3. do the females have different preferences amongst the males
4. what is most liked about the males
G. Mating Patterns
1. Opportunistic mating
2. Possessive mating
3. Consorts
Results
A.
Females showed less sexual behavior during and around ovulation
(POP) than in non ovulation(non-pop) times.
B.
There was a difference in selectivity depending on time of ovulation for
female.
C.
Females resisted eschewed males more when they were ovulating than
not.
1. Females showed more resistance towards some males
2. And less resistance towards other males
D.
Studies from the North showed that the females didn’t like the alpha
male but preferred a male whose ranking was lower
1. In general, lower ranking females were more proceptive to lower
ranking males
2. And higher ranking females were more proceptive to higher ranking
males.
3. During POP females showed high resistance rates towards older males
CRITICAL REVIEW ITEMS
A. Interesting or informative points
1. Promiscuity does not rule out selectivity, and in fact female chimpanzees may
use both as a mixed strategy to attempt to influence which males will father their
offspring (p. 519). Males are unaware of when a female is actually in POP, this means
that they will not be sure of who sired the offspring. This hopefully leads to social
support and less male aggression.
2. The author indicated that female chimpanzees have a high proceptivity, or
desire to reproduce, with males that were ascending in the social hierarchy at a fast pace.
The fact that they have an ability to see this and predict it with great accuracy is
fascinating.
3. It was interesting to find out that both males and females initiate sexual
interactions. This stood out because in the human population this is true as well showing
a similar pattern.
B. Weak or unclear points
1. It was confusing that female chimpanzees had substantially higher
proceptivity rates toward either the highest OR lowest ranking males during
POP, excluding middle ranking males. This was just briefly mentioned, but
not clearly explained. If females were more selective and choosy during POP,
then why would they be just as proceptive to the lowest ranking males as
highest ranking over the middle ranked males?
2. The authors suggest that most pregnancies result from copulations between 13 days prior to ovulation and including ovulation. This was unclear because
females engage in over one hundred copulations so how can they be sure
which one led to pregnancy?
3. With such a small sample size I wonder if the significant results could
even really be representative of that species of primates as a whole. For
example, a few outliers could have had a drastic effect.
TEST QUESTIONS
1. What kind of mating does a dominant male actively attempt to prevent other males
from copulating with a female?
A.
B.
C.
D.
opportunistic mating
possessive mating
consorts
sexual mating
2. Which of the following is NOT a reason that female Chimpanzees may use sexual
promiscuity?
A.
B.
C.
D.
confuse paternity
ensure fertilization
discourage infanticide
to gain dominance
3. What was the most predictive quality in males for high proceptivity, and low resistance
rates in females?
A. Social Ranking
B. Physical Appearance
C. Age
D. Aggression Levels
4. It benefits females to be selective in choosing mates.
T/F
5. During Non-POP, higher-ranked females were more proceptive to lower-ranked males
T/F
6. Female chimpanzees are indiscriminant of their partners during sexual reproduction.
T/F
Answer key
1.
2.
3.
4.
5.
6.
B – possessive pg 512
D – to gain dominance pg 511-512
C - age
true
false
false