Infanticide, at first glance, seems to be nothing more than a grisly phenomenon, showing merely the overwhelming potential for brutality present in the animal kingdom. It is, however a practice performed by many different species in many parts of the globe. If infanticide was localised to one group of one species, in one localised region, then the assumption could easily be made that it is indeed merely a localised practice, and perhaps merely an evolutionary misfiring – a non-adaptive strategy that would be quickly lost in the face of other, more useful ones. However, the fact that it is so widespread, and crosses not just group boundaries within species but is performed in many different species suggests that it is not just an act of violence, but is in fact an adaptive strategy with very real benefits for its practitioners.
From the point of view of a modern, western, and (most importantly) human society, infanticide is a reprehensible practice, both sickening and lacking in morality. However, when discussing evolution, reproduction, survival, and the strategies employed in these processes, there can be no pretence of morality. Nature does not care whether a successful strategy is disgusting by our standards, but merely that it is indeed a successful, and therefore adaptive strategy. Indeed, we must be careful not to anthropomorphise any facet of the processes of evolution, reproduction, and survival – there can be no question of judging the morality of evolutionary strategies, because the only thing that matters in evolutionary terms is what works. With this in mind, we must first recognise that despite our feelings towards it infanticide does indeed exist within the animal kingdom, and therefore examine the potential reasons as to why this practice continues.
Examples of infanticide in the animal kingdom
Historically, infanticide as a conscious strategy for selecting offspring by phenotype has indeed occurred in humans – not just as an act of warfare, or as an attempt to wipe out members of another group, but as a way of eliminating weak offspring, and removing the ‘weakest’ genes (i.e. those that contribute to creating these examples of weaker offspring) from the gene pool, in order to ensure the continued strength of the group – perhaps most famously known as a practice of the ancient Spartans. However, human incidences of infanticide can be problematic to study. Historical examples such as the aforementioned practice in Sparta cannot be used to form adequate models because we cannot verify the data attesting to the extent of both the usage and the success of the practice, not least because we are so far removed from that era. Furthermore, scientific studies regarding infanticide in humans are few and far between. As Hausfater (1982) states, ‘Infanticide has not received much study because it’s a repulsive subject…many people regard it as reprehensible to even think about it.’ Finally, it is important to remember that, even were we able to fully account for infanticide in humans, our findings would not necessarily hold true for the rest of the animal kingdom – for example, a strategy that is evolutionarily successful for humans need not be a beneficial strategy in other species. We should not think of humans as the pinnacle of evolution, as if we had somehow attained the grand prize in the game of life – in scientific terms, humans are just another species.
Infanticide is, of course, a practice not merely confined to Homo Sapiens. For example, Hrdy (1977) studied the selective killing of offspring in Presbytis Entellus, the Hanuman langur. Two troops of Presbytis Entellus were focused upon, named ‘Hillside’ and ‘Bazaar’. It was noticed that, on many occasions, a male from one group would switch to the other group and usurp control – the average tenure of a male leader in the troops in the studied area was found to be 27.5 months. Often, the new male would be found to attack the offspring of the previous incumbent. Offspring were also found to have been attacked by the companions of the new male. For example, during a power struggle between two males (Shifty and Mug), in which the offspring of a third was presumed to have been attacked and killed by the companions of Mug. As Hrdy (1977) states, ‘a daughter born to Pawless during the period when both Shifty and Mug were vying for control of Hillside troop was assaulted on several occasions by the five newer invaders; the infant eventually disappeared and was presumed dead.’ Hrdy theorised that ‘Given that the tenure of a usurper is likely to be short, he would benefit from telescoping as much of his females’ reproductive career as possible into the brief period during which he has access to them.’
Furthermore, the practice of infanticide is not confined solely to primates, nor to males. Cicirello and Wolff (1989) studied pairs of white-footed mice (Peromyscus leucopus), and found that both males and females of this species engaged in infanticide. Their study found that ‘cohabitation for 15 days without successful mating did not inhibit infanticide for males or for females, but cohabitation that included successful mating did inhibit infanticide at 4 days postpartum.’ This suggests that infanticide in this species is related to the male progenitor – when a successful pairing resulting in offspring occurred, cases of infanticide within that pair were reduced. The study also found that ‘more infanticidal females bred than did noninfanticidal females, and they bred in a shorter period of time’.
Is infanticide an adaptive strategy?
As stated in Hrdy (1977), early studies reporting on infanticide in langurs suggested that ‘all troop members…were functioning so as to maintain the fabric of the social structure…it was difficult to explain such behaviour in terms of group survival.’ The traditional view was that infanticide was a result of a breakdown in normal social conventions, suggesting that it could not be an adaptive strategy. The closest that early research came to providing an account for the advantages of infanticide was that it may have occurred as a result of overcrowding. However, this viewpoint focuses too much on the group as the basic unit of evolution. As explained by Dawkins (1976), neither the group or the individual can be considered the basic evolutionary unit – ‘…the fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity.’ Therefore, from an evolutionary point of view, the ‘needs’ of the group are insignificant – what really matters is the passing-on of genes. Genes can be considered ‘selfish’, because a successful gene is one that is reproduced. Likewise, a gene (or set of genes) that does not succeed in being replicated is unsuccessful. Therefore, an individual organism that passes on its genes can be considered to be successful – it has passed on its genes, and therefore fulfilled its purpose. Even though the individual male benefits from the practice of infanticide, the benefit comes from the successful passing-on of his genes.
In this way, we can see how the practice of infanticide can be an adaptive strategy, and be conducive to successful reproduction at the genetic level. A male killing the offspring of another male means that no resources are ‘wasted’ on that child, and increases the likelihood that the new male’s genes will be successfully passed on. The genes that promote the act of infanticide in the individual are therefore more likely to be successful than those that do not, because the individual male practising infanticide ensures that resources will be spent on rearing his offspring, and not on those sired by another.
 Hausfater, G, in Webster, B,’ Infanticide: Animal Behaviour Scrutinized For Clues To Humans’, The New York Times, 1982, Available: http://www.nytimes.com/1982/08/17/science/infanticide-animal-behavior-scrutinized-for-clues-to-humans.html?sec=health [Accessed 13 January 2010]
 Hrdy, SB, ‘Infanticide As A Primate Reproductive Strategy’, in American Scientist, Vol 65(1), Jan-Feb 1977
 Ibid, p23
 Ibid, p24
 Cicirello, D and Wolff, J, The effects of mating on infanticide and pup discrimination in white-footed mice, in Behavioral Ecology and Sociobiology Vol 26, No.4 (Berlin: Springer Berlin/Heidelberg, 1990), Available: http://www.springerlink.com/content/ng6w216wgrn06216/ [ Accessed: 13th Jan 2010]
 Ibid, p277
 Hrdy (1977), p.21-22
 Ibid, p22
 Dawkins, R, The Selfish Gene (Oxford: Oxford University Press, 2006), p11