DANTAisms

by Lisa Barrett

Sexual conflict is the clash between evolutionary interests of males and females. It stems from anisogamy, the discrepant sizes of the male and female gametes (sperm and eggs, respectively). Think: eggs are bigger and more costly to produce; sperm is cheap and plentiful. These gametic differences lead to (sometimes) opposing reproductive interests and strategies for the sexes– a topic about which I have been fascinated ever since my undergraduate career in Evolutionary Anthropology with fellow “DANTAisms” blogger, Sam!

 

Anisogamy explains the intriguing differences in sexually-selected strategies for male and female nonhuman primates: males are limited by the number of females with which they can mate, whereas females are limited by resources to support egg production, gestation, and lactation. Traditional views of female response to male strategies include cooperation and passivity by females. Interestingly, much of this early research was conducted by male researchers who tended to view things from a male’s perspective.

But we now know that females exhibit several counterstrategies.

Sexually-selected Infanticide

Did you know that incoming or new males may kill a female’s dependent infants, because it results in a faster return to estrus? This gives the incoming male a higher chance of him fathering an offspring with the female than if he had not committed infanticide.

For females, infanticide results in the death of her offspring (and a decrease in her reproductive success). So females may mate with multiple males to confuse paternity and reduce risk of infanticide. In other words, if a male mates with a promiscuous female he cannot be sure who fathered the female’s offspring, and since it may be his own offspring he cannot risk committing infanticide.

Females may also form female-male friendships. Evidence from baboon populations suggests that a male friend (who is also a potential father) protects the female’s offspring against infanticidal males.

A third counter-strategy to infanticide involves a termination of pregnancy following exposure to an unfamiliar male. This is known as the Bruce Effect and has been documented in wild Gelada baboons. In this scenario, it will be less costly to terminate a pregnancy than to waste investment on an infant that will be killed.

Conclusion

In the past, female counter-strategies to infanticide were relatively under-studied compared to male behavior and strategies. Perhaps this is because much sexually-selected male behavior is overt (after all, they were committing infanticide!). Or maybe the gender or social upbringing of the researcher created biases. Evolutionary feminists, such as Sarah Blaffer Hrdy and Barbara Smuts, have made important contributions to our understanding of sexual conflict from a female’s perspective.

Related: Parents: To Care or Not To Care?

References

Bell, Graham. The evolution of anisogamy (1978). Journal of Theoretical Biology, 73, 2, 247-270.

Borries, C., Laundardt, K., Epplen, C., Epplen, J. T., & Winkler, P. (1999a). DNA analyses support the hypothesis that infanticide is adaptive in langur monkeys. Proceedings of the Royal Society B, 266, 901-904.

Borries, C., Laundardt, K., Epplen, C., Epplen, J. T., & Winkler, P. (1999b). Males as infant protectors in Hanuman Langurs (Presbytis entellus) living in multimale groups – Defence pattern, paternity and sexual behaviour. Behavioral Ecology and Sociobiology, 46, 5, 350-356.

Brennan, P.L.R., Clark, C.J., & Prum, R.O. (2010). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia. Proceedings of the Royal Society B., 277, 1686, 1309-1314.

Bruce, H.M (1959). An exteroceptive block to pregnancy in the mouse. Nature, 184, 105.

Chapman, T., Arnqvist, G., Bangham, J., & Rowe, L. (2003). Sexual conflict. Trends in Ecology & Evolution, 18,1, 41-47.

Eberle, M. & Kappeler, P.M. (2004). Selected polyandry: Female choice and inter-sexual conflict in a small nocturnal solitary primate (Microcebus murinus). Behavioral Ecology and Sociobiology, 57(1), 91-100. doi: 10.1007/s00265-004-0823-4.

Halliday, T.R. (1983). The study of mate choice. In: Mate Choice (Ed. by P. Bateson), pp. 3-32. Cambridge, Massachusetts: Cambridge University Press.

Hrdy, S.B. (1979). Infanticide among animals: a review, classification, and examination of the implications for reproductive strategies of females. Ethology & Sociobiology, 1, 13-40.

Knott, C.D, Emery Thompson, M., Stumpf, R.M, & McIntyre, M.H. (2010). Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion. Proceedings of the Royal Society B, 277(1678), 105-113. doi:10.1098/rspb.2009.1552.

Palombit, R.A., Seyfarth, R.M., & Cheney, D.L. (1997). The adaptive value of ‘friendships’ to female baboons: experimental and observational evidence. Animal Behaviour, 54(3), 599-614.

Parker, G.A. (2006). Sexual conflict over mating and fertilization: An overview. Philosophical Transactions of the Royal Society B: Biological Sciences, 361.1466, 235-259.

Roberts, E., Lu, A., Bergman, T., & Beehner, J. (2012). A Bruce effect in wild geladas. Science, 335, 6073, 1222-1225.

Smuts, Barbara B. (1985). Sex and friendship in baboons. Aldine.

Soltis, J., Mitsunaga, F., Shimizu, K., Yanagihara, Y., and M. Nozaki.  (1997) Sexual selection in Japanese macaques I: female mate choice or male sexual coercion?. Animal behaviour,  54.3,  725-736.

Stumpf, R.M., Martinez-Mota, R., Milich, K.M., Righini, N., & Shattuck, M.R. (2011). Sexual conflict in primates. Evolutionary Anthropology: Issues, News, and Reviews, 20,2, 62-75.

van Noordwijk, M. A., and C. P. van Schaik. 2000. Reproductive patterns in eutherian mammals: adaptations against infanticide. In: Infanticide by males and its implications (Ed. by van Schaik and Janson). Cambridge: Cambridge University Press. pp. 322-360.