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 / DefinitionProceptivity 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 rate / Difference between females proceptivity rate toward a specific 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 1-3 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. opportunistic mating
B. possessive mating
C. consorts
D. sexual mating
2. Which of the following is NOT a reason that female Chimpanzees may use sexual promiscuity?
A. confuse paternity
B. ensure fertilization
C. discourage infanticide
D. 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. B – possessive pg 512
2. D – to gain dominance pg 511-512
3. C - age
4. true
5. false
6. false