Kevin Curry
From Drmills
Kevin Curry
Student ID# 5222
Infanticide: Male and Female Strategies
Although infanticide (the killing of newborn young) is by no means considered common in the animal world, there are a large number of example species that have been observed committing infanticide. Studies on infanticide have tended to focus mainly on males as a sneaky reproductive strategy, because infanticide of this nature has been observed most widely. Although the strategy is typically used by males, instances of infanticide by females have also been recorded. In humans, infanticide by women is actually much more prominent than infanticide by men, having been documented in the United States and even more extensively by Radha Hedge (1999) in South India. Through evaluation of the various evolutionary pressures, physiological and reproductive stresses, and other factors that lead to infanticide, some light may be shed on the differences between the occurrence of infanticide by males and females. By studying the purposes for infanticide in non-human species of the animal world, future conclusions may be made about infanticide in humans.
Infanticide as a male sneaky strategy has been noted in many mammalian species including an extensive list of primates, lions, jaguars, horses, marmosets, and even dolphins. However, infanticide has been documented most widely among Hunuman langur monkeys (Presbytis entellus). In a study done by Carola Borries and colleagues (1999) in Nepal, the occurrence of infanticide was recorded and DNA from the victim and the perpetrator were taken. The results showed that male Hunuman langurs only attacked and or killed infants that were genetically unrelated (not their own offspring) to themselves. This occurrence of infanticide in langurs appeared to be by intruder males as a sneaky reproductive strategy. Because mammalian females do not ovulate while lactating, infanticide has evolved as a means for males to stop female lactation and therefore initiate ovulation. Subsequently, the male langur uses infanticide to gain access to an estrous (fertile) female by reinitiating the females ovulation cycle, allowing the male to copulate more quickly and spread his genes more efficiently. Borries also found that in every case of infanticide that was recorded, the male langur copulated with the mother of the infant he had killed (Borries, Launhardt, C. Epplen, J. Epplen, and Winkler, 1999). Due to the fact that female langurs will readily mate with outsider males who have just killed one of the females’ offspring, infanticide has become a very successful, efficient, and stable evolutionary strategy for male langurs. In the same way, infanticide is used as a male reproductive strategy in the plains zebra (Equus burchelli). A study done by Jan Pluhacek and Ludek Bartos (1999) on a captive population of this species found that mortality rates of foals increased significantly after adding unrelated males to the population. Nine instances of infanticide by male plains zebras were documented. It was also noted that the males subsequently mated with the mothers of the foals they had killed (Pluhacek & Barton, 1999). Plains zebras and Hunuman langurs seem to have developed the use of infanticide as a sneaky reproductive strategy because of its immense reproductive value, illustrating behavioral convergent evolution of the two species.
Infanticide has also been documented among male Southern sea lions (Otaria byronia), but for a different purpose than that of the plains zebra and Hunuman langur. Claudio Campagna and his colleagues (1988) conducted a study in Santa Cruz, California on this species, observing 285 pup abductions by outsider males, most of which were followed by infanticide. A total of 1.3% of pups from each breeding season died from physical abuse. Campagna observed that many of the pup abductions and subsequent pup deaths were carried out by subadult males. These young males treated the pups in the same way that adult male Southern sea lions treat adult females, sequestering them, preventing their escape, and defending them against other males. In 9% of the abductions, males actually mounted the pups, although no intromission occurred (Campagna, Le Boeuf, & Cappozzo, 1988). These results combined with the fact that breeding is seasonal in this species (females who lose pups lose a whole breeding year) provide evidence that males do not receive an immediate reproductive benefit from pup abduction and infanticide. Therefore, the study concludes that “pups may be used as female substitutes by males that are sexually but not physically competitive with adults for mature mates”, further stating that “through practice with pups, ousted males may gain experience in controlling adult females” (Campagna et al., 1988). Ultimately, the infanticide committed by Southern sea lion males is a by-product of pups being treated as females for practice (forcefully and aggressively), and not a strategy for restarting ovulation of females, as in Hunuman langurs and plains zebras. However, infanticide in all three of these species is used in pursuit of a common goal: to successfully reproduce with a female (whether immediately or after maturity is reached).
Although infanticide by females is not as common as infanticide by males, this behavior has been documented in females from a decent number of different species. Rather than for some immediate reproductive benefit as seen in males, female infanticide in the wild is typically driven by factors that promote survival and are less closely related to reproductive success (with a few exceptions). In the ultimate scheme of things, these factors are also promoting reproductive success simply by facilitating the individual’s survival (the longer an individual lives, the more opportunity to reproduce). Sarah Hrdy (1979) discusses the different reasons for female infanticide, identifying four different classes: exploitation, resource competition, parental manipulation, and sexual selection. In the first, female infanticide occurs as a means of exploiting the infants as a resource either for direct consumption (cannibalism) or in rare occasions as a prop for “play mothering”, which is seen in some species of primates. Infanticide by females that actually consume the infant, such as that among ground squirrels, hyenas, sometimes lions, and even some fish and insects, is a means of attaining a food resource and avoiding starvation. Infanticide resulting from resource competition (wild dogs, primates, elephant seals, Belding’s ground squirrel), occurs when females kill the young of female conspecifics in order to free up more resources. Due to the fact that in most animal species young are known to use up a large amount of resources, a female may gain access to more resources by killing the young of a nearby, unrelated female, ultimately reducing resource competition. Female infanticide as a means of parental manipulation occurs when the female kills her own young (filicide) in order to terminate parental investment to that particular offspring (Hrdy, 1979). By doing this, the female increases her own chances of survival as well as the survival of any other offspring she may have. This type of infanticide is also seen in human females, who are unable to economically or physically care for their children (Gauthier, Chaudoir, and Forsyth, 2002). Hrdy proposes that the last purpose of female infanticide results from sexual selection. Even though infanticide of this nature is the main cause of infanticide by males, it is rarely seen in females. Infanticide by females due to sexual selection occurs in species with reversed sex roles, where females compete for access to males. In the button quail (Turnix sylvatica), a species with reversed sex roles, the male cares for the young, so females compete for male services. This often leads to the destruction of a female’s eggs by a female conspecific as a means of monopolizing the male’s services (Hrdy, 1979). This, in turn, increases the guilty female’s own reproductive success in addition to that of her young.
By studying the infanticide committed by both males and females of different animal species, the various implications of infanticide have come to light. Male infanticide is strongly correlated with a direct reproductive benefit, that appears to have evolved as an ultimate cause, with some exceptions (Southern sea lion). However, infanticide by females has both proximate and ultimate causes including exploitation, resource competition, parental manipulation, and sexual selection. Female infanticide is more strongly related to survival rather than to reproductive gain (as in infanticidal males) although there are exceptions in species with reversed sex roles, like the button quail. This sexual dimorphism in infanticidal behavior that exists between males and females (in non-human species) may share evolutionary roots with human infanticide, providing further explanations for the sexual dimorphism of the trait that is also present in humans. Further investigation is imperative, as it has implications in reducing homicide and child abuse rates. Hopefully one day research in infanticide will be able to provide an explanation for this trend of human infanticide by females rather than males, while the rest of the animal kingdom shows the opposite trend.
Works Cited
Borries, C., Launhardt, K., Epplen, C., Epplen, J., & Winkler, P. (1999). DNA analyses support the
hypothesis that infanticide is adaptive in langur monkeys. The Royal Society Publishing, 266,
901-904. Retrieved April 19, 2008, from
http://journals.royalsociety.org/content/21ytfnl0l4dpanby/
Campagna, C., Le Boeuf, B., & Cappozzo, H. (1988). Pup Abduction and Infanticide in Southern
Sea Lions. Behavior, 107, 44-60. Retrieved April 19, 2008, from
http://www.ingentaconnect.com/content/brill/beh/1988/00000107/F0020001/art00004
Gauthier, D., Chaudoir, N., & Forsyth, C. (2002). A sociological analysis of maternal infanticide in the United States, 1984-1996. Deviant Behavior, 24, 393-404. Retrieved April 25, 2008, from http://www.ingentaconnect.com/content/routledg/udbh/2003/00000024/00000004/art00005
Hedge, R. (1999). Marking Bodies, Reproducing Violence: A Feminist Reading of Female Infanticide in South India. Violence Against Women, 5, 507-524. Retrieved April 25, 2008, from
http://vaw.sagepub.com/cgi/content/abstract/5/5/507
Hrdy, S. (1979). Infanticide Among Animals: A Review, Classification, and Examination of the
Implications for the Reproductive Strategies of Females. Ethology and Sociobiology, 1, 13-40.
Retrieved April 19, 2008, from
http://0-www.sciencedirect.com.linus.lmu.edu/science?_ob=ArticleURL&_udi=B6X2B-4CB01TY-D&_user=945462&_coverDate=10%2F31%2F1979&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000048964&_version=1&_urlVersion=0&_userid=945462&md5=efb0c954617e8744534bd369c52f7e41
Pluhacek, J. & Bartos, L. (1999). Male infanticide in captive plains zebra, Eqqus burchelli. Animal Behavior, 59, 689-694. Retrieved April 19, 2008, from
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W9W-45FK6K4-43&_user=945462&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000048964&_version=1&_urlVersion=0&_userid=945462&md5=d2f71d619db6f7fa78f0a158bc5441e1
Additional Web Sources:
http://www.gendercide.org/case_infanticide.html
http://www.infanticide.org/history.htm
http://www.karisable.com/crinfant.htm
http://www.youtube.com/watch?v=mnmtKLQRh6g (video)
http://www.youtube.com/watch?v=mnmtKLQRh6g (video)
http://www.youtube.com/watch?v=mnmtKLQRh6g (video)
Rating scale:
Ratings ( 1 - 5):
- found appropriate research references relevant to a topic 5
- discovered, integrated and synthesized relevant information about the topic 5
- writing is of high quality: interesting, flows, analytic, organized 5
- used APA style referencing appropriately (including reference list) 5
- inlcuded as a list of web links to the original articles cited (if available), andincluded a list of additional resources relevant to the topic on the web 5
Total points: 25
