The Reasons of Infanticide in Primates - Term Paper Example

Argumentative Essay on Infanticide in Primates: Sexual Selection Hypothesis versus Social Pathology Hypothesis Introduction Infanticide is a strange phenomenon observed among various mammal species. The behavior which is common among primates is a subject of intense controversy among social biology scholars. Infanticide involves the killing of an offspring by a member of the same species as the offspring (Helmer and Van Schaik 27). In a more inclusive sense, infanticide refers to any behavior that contributes directly and significantly to the termination of the life of an offspring member of the aggressor’s own species. Though the behavior is rare within the species where it has been reported, it is however widely distributed in many animal species. Despite being a common behavior, its motivation and significance among the specie where it occurs remain unclear. Though several hypotheses have been suggested to explain infanticide behavior, two opposing theories; the Sexual Selection and the Social Pathology Theory are the most popular in scholarly circles. This paper arguers in favor of the sexual selection theory as the most appropriate explanation for infanticide behavior. As an explanation for infanticide, the sexual selection theory purports that infanticide is a male reproductive strategy as a result of reproductive rivalry among male species. The killing of an unweaned offspring by the male is assumed to confer reproductive advantage to the male aggressor given that; the infant was not sired by the male aggressor or when there is an almost zero probability that the offspring was sired by the male (Helmer and Van Schaik 27). The sexual selection theory also confers reproductive advantage to the male if the killing of the unweaned offspring would shorten the mother’s birth interval of the next offspring in the sense that the female would be fertilized earlier that if the offspring had not been killed (Helmer and Van Schaik 27). Lastly, infanticide is also assumed to be a reproductive strategy by males on condition that it would increase the male aggressor’s probability of siring the next offspring compared to the present one (Helmer and Van Schaik 27). Conversely, the social pathology theory holds a contrary opinion to the sexual selection hypothesis. Proponents of the social pathology hypothesis hold that infanticide is a behavior resulting from increased aggression by the males. The male aggression behavior is believed to result from increased social pressure due brought by unnatural habitats like in overpopulation, deforestation and destruction of natural habitats by fires among other factors (Borries et al 901). In this case, infanticide does not have any adaptive value and hence does not confer any reproductive advantage to the male aggressor. Under this theory, it is likely that the killed offspring happens to be a victim of circumstances and that the males do not select the offspring they kill. Under such conditions, the males may at times kill their own infants and this will not confer any reproductive advantage as a result. Infanticide has been found to be a relatively homogenous behavior. Males have been found to be the most likely perpetrators of infanticide and in majority of the cases, the males often kill infants that they have not sired without feeding on them (Helmer and Van Schaik 27). This excludes the possibility that infanticide is carried out to gain access for food. Following a protracted debate on the possible reasons behind infanticide, recent scholars have been inclined towards the male reproductive strategy in which the males increase their reproductive success through killing of the unrelated offspring where the death of the infant relatively shortens the receptivity period of the female and if the male has a higher chance of siring the female’s subsequent offspring (Helmer and Van Schaik 27-28). Of the two opposing theories, the sexual selection theory seems more convincing than the social pathology theory in explaining infanticide behavior among primates. As a result, the theory has been widely researched and corroborated by many scholars. Borries et al observes that following the killing of the offspring among primate species, the females engage in sexual activity earlier and give birth to another offspring sooner than their counterparts whose infants had survived (901).Though this raises the critical question as to whether the killed infants had been sired by the male aggressor, empirical studies have proved that the males seldom kill their own. In one case, it was observed that he infanticide males had recently immigrated to their new habitat, had not reached sexual maturity b y the time the offspring was conceived or were not observed to mate with the female during conception of the offspring (Borries et al 901). This implies that the males were not likely to have killed their own offspring. Further evidence suggests that infanticide behavior is a reproductive strategy among the males. The studies by Borries et al involving Hanuman langur monkeys (Presbytis entellus) established that infanticide is a reproductive strategy among the males of the same species. In most cases where the behavior was observed, the male aggressors remained in the same habitat where the infanticide acts have been committed. Further observations of the males’ behavior following infanticide revealed that the males are likely to have sired the next offspring and can therefore be said to have gained a reproductive advantage by committing infanticide (Borries et al 901). In such cases, the males were seen to mate with the female whose infant had been killed or were the only males within the group. In this case, another infant was born in four out of five cases where infanticide was reported (Borries et al 901). Examinations of whether the infanticide males fathered the next offspring indicated that the reported male aggressor of the previous infanticide was the probable sire of the next infant (Borries et al 902). This offers support that infanticide males engage in such behavior to increase their chances of mating with the female thereby siring the next offspring. The findings in the study indicate that the male’s probability of gaining a reproductive edge over the other males of the same species is significantly increased through infanticide. Observations of infanticide behavior among mountain gorillas (Gorilla gorilla beringei) and common chimpanzees (Pan troglodytes schweinfurthii) indicate that the behavior confers reproductive advantage to the males. Over a period of 15 years, researchers investigating infanticide behavior of mountain gorillas residing in Rwanda observed nine cases of infanticide among the species (Hausfater and Hrdy 217-218). Among the observed infanticide cases, most episodes occurred during violent interactions among the groups when an adult male attempted to compete with another male for a mate (Hausfater and Hrdy 218). More specifically, the male aggressors were found to be young and in the process of establishing their reproductive niche. The behavior exhibited by the females following the killing of their offspring through infanticide reveals that infanticide had a significant effect on the reproductive outcomes in primates. Observations on the mountain gorilla species further revealed that infanticide increased the chances of migration of the infanticide victim’s mother to a different social ecosystem where in most cases, the mother was seen to join the same group with he infanticide male (Hausfater and Hrdy 218). This illustrates that infanticide behavior among gorilla species seems to be a reproductive strategy in which the males increase their chance of accessing the breeding female and consequently siring the next offspring. Infanticide behavior has been found to confer a higher reproductive significance among the mountain gorilla species compared to other primate species. Hausfater and Hrdy further observe that infanticide acts among members of the mountain gorilla species lead to victim’s mother gaining a considerably higher rank by being one of the earlier females to form a connection and subsequently mate with the infanticide male (218). This is majorly due to the fact that the ability of the female to become a dominant member is largely dependent on the order at which the female is acquired by the dominant males. Comparatively, the reproductive advantage acquired by the infanticidal males among the chimpanzee species is less obvious than that observed among the gorilla species. In a rather perplexing observation, chimpanzee s often cannibalise on their victims following infanticide acts which is not commonly observed in the gorilla species (Hausfater and Hrdy 218). Another possible explanation for the relatively low reproductive advantage from infanticide among the chimpanzees is due to their mating behavior which does not favor the males’ access to females following infanticide. Though circumstantial evidence and observation based studies support the sexual selection theory, it is also important to establish whether the benefits arising from the behavior outweigh the costs to facilitate natural selection. Helmer and Van Schaik suggested a formula for calculating the benefits (B) associated with male infanticide and purported that the benefits are the time gains obtained as compared to a situation where the male did not commit infanticide after correcting for the change in the paternity probability (28). In this case, given that a male can access a particular female and will only commit infanticide if its tenure will be enhanced by gaining access and reproducing with the female, the total reproductive lifespan of the male (T) can be determined by; Birth after infanticide (i) and k births without infanticide (n) after time intervals ti and tn hence T= ti +ktn (Helmer and Van Schaik 28). Assuming that the probability of siring the subsequent offspring after infanticide to be Pn after time interval tn and given the male’s reproductive lifespan as T, the benefit on the male as a result of infanticide will be; Bn=Pn (T/tn) (Helmer and Van Schaik 28-29). Otherwise, given the male’s probability of siring the subsequent offspring after infanticide as Pi after period ti and that the probability of the killed offspring to have been sired by the male aggressor as P, the benefit would therefore be; Bi= (Pi-p)+ [(T-ti)/tn]Pn (Helmer and Van Schaik 29) In this case, the net benefit achieved by the male after committing infanticide and establishing a reproductive control within its group would be; Bi-Bn=Pn (-ti/tn) +Pi-P (Helmer and Van Schaik 29). Assuming that Pn=Pi=P; that is the male’s habitat or dominance remains constant over the infanticide period, the male will have a positive benefit if [(tn-ti)/tn] P>P (Helmer and Van Schaik 29). Given the above condition, it is apparent that an infanticide male benefits since the only requirement is a reduced birth interval (tn-ti>0) on condition that P>P (Helmer and Van Schaik 29). Conclusion After examining infanticide behavior among primates with regard to the sexual selection theory and the social pathology theory, it is apparent that the sexual selection hypothesis offers a more convincing explanation for infanticide. Many studies among primate species reveal that infanticide as a reproductive strategy confers a reproductive advantage. Infanticide males increase their reproductive success by killing the unrelated offspring given that the death of the infant relatively shortens the receptivity period of the female and if the male has a higher chance of siring the female’s subsequent offspring. Observation studies on various primate species like mountain gorillas (Gorilla gorilla beringei) and Hanuman langur monkeys (Presbytis entellus) indicate that infanticide increases a male’s probability of gaining a reproductive edge over the other males of the same species. Analysis of the benefits and costs associated with infanticide reveal that infanticides confers reproductive benefits to the males since the only requirement is a reduced birth interval by the female after infanticide. Works cited Borries, Carola, Epplen Cornelia, Epplen Jorg, Launhardt Kristin & Winkler Paul. “DNA analyses support the hypothesis that infanticide is adaptive in Langur monkeys”. The Royal Society, London, vol. 266 (1999): 901-904. Hausfater, Glenn & Hrdy Sarah. Infanticide: Comparative and Evolutionary Perspectives. Piscataway, NJ: Transaction Publishers, 2008. Print. Helmer, Charles & Van Schaik Carel. Infanticide by Males and Its Implications. Cambridge: Cambridge University Press, 2000. Print. Read More