A review of the evolutionary advantages and disadvantages of infanticide as performed by Male & Female Mammals

Infanticide is the act of killing a young offspring. Here, the papers reviewed, it will be defined as the killing of a young member of the same species and will focused on infanticide committed by mammals. Infanticide may be well ingrained within mammals for many reasons. Sexual conflict (Stockley, and Bro-Jørgensen), access to resources (Stockley, and Bro-Jørgensen), and to return the female to a reproductive state more quickly (Lukas, and Huchard) are all reasons offered by these papers as to why infanticide occurs so regularly. The cited papers used a variety of methods to come to their respective conclusions such as genetic analysis, direct observation, reviewing a breadth of previously collected data, and comparative analysis.

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To provide a better understanding as to how the killing of a member of your own species could be advantageous, the concept of fitness and sexual selection must be introduced. Fitness, in essence, is the ability to pass along your genetic material in the form of offspring. To put this into context of the topic, some species of mammals will truncate the lifespan of young within their species so that the female, which is providing care for the young, will be able to return to a reproductive state quicker thus allowing the male to mate with and subsequently sire an infant with that female. Sexual selection has 2 faces, intrasexual & intersexual selection. Intrasexual selection is competition for a mate between members of the same sex while conversely, Intersexual selection is the competition for mating between members of opposing sexes. Both forms can take the form of outright aggression, dominance, or showiness, but in the context of this review, it will be in the form of the killing of infants or competition as a result of sexual selection.

I shall begin my review of these papers by giving a quick overview of their respective findings and how they relate. In the first paper, “The evolution of Infanticide by males in mammalian societies” (Lukas, and Huchard), they discuss how males, across taxa, attempt to kill infants in order to return the mother to being sexually active. It then discusses how in taxa where this behavior will not expedite the mother to resuming a fertile state, it is rare to observe infanticide. According to this paper, it is also a form of intrasexual selection where males can essentially increase the relative fitness of their offspring by killing those of other males, although this form primarily occurs in species where a small amount of males have multiple female mates and where the females can be sexually active year round. This shows that males, when advantageous for themselves, will destroy another males offspring so they can have the possibility of mating with the female and that females may create counter strategies preventing infanticide by having multiple mates. As mentioned in the paper, due to the paternity dilution hypothesis, infanticide is generally lost in population in which testis size has increased showing that infanticide can be eradicated where the females successfully end male sexual monopolization.

In the following paper, “Countering Infanticide: Chimpanzee Mothers Are Sensitive To The Relative Risks Posed By Males On Differing Rank Trajectories” (Lowe et al.), it is shown that female chimpanzees will develop counter strategies in order to prevent the infanticide committed by male chimps such as sexual promiscuity to confuse the males on whether or not they are the parents of the child, and the usage of males, generally the dominant one, who also had the highest possibility of being the father in order to protect their infant. During rank shifts, infanticide is more prevalent in the chimpanzees studied forcing upon mothers and infants a possible stressor to their own fitness so that the upcoming male can increase his own rank. This shows that female chimps (and possibly mammals) are quite inventive when it comes to developing strategies to protect their offspring in non-ideal conditions.

Next, in “Infanticide As Sexual Conflict: Coevolution Of Male Strategies And Female Counterstrategies” (Palombit) male strategies and the resultant female counter strategies are compared. Males, according to this article, will commit infanticide in order to return the female to a reproductive state, or achieve a higher rank in the social order, acquire a certain breeding site. Logically, the female will develop counter strategies in order to protect her infants. Among those mentioned in the article, females may develop strategies such as promiscuous sexual activity, being able to be fertilized by more than one male at a time, mating with the male who is most likely to kill infants which are not his, and being capable of reducing how many offspring they have until new dominant males are established. This simply strengthens and adds to the argument that females will formulate a multitude of strategies to protect their young, additionally it exemplifies the reasons behind the use of infanticide as a reproductive and fitness increasing strategy.

In the article “Female Competition and Its Evolutionary Consequences In Mammals” (Stockley, and Bro-Jørgensen) it is shown that males are not the only ones who commit infanticide. Females, for a variety of reasons, such as competition for limited resources, control of a location, and access to mates, may commit infanticide in order to increase both their own and their offsprings’ fitness. By destroying the offspring of others, they then reduce future resource competition thus improving their own and their offsprings likelihood of acquiring aforementioned resources later on. This is an interesting contrast and calls for the question: If females have been known to create counter strategies for the protection of their young, will males in the reverse scenario, also form counter strategies for the protection of their offspring? It would only make sense because the protection of your young is analogous to the protection of your own fitness. It would be a worthy topic to write over, but is outside the scope of this review.

In the final paper, “Infanticide In Brown Bear: A Case-Study In The Italian Alps – Genetic Identification Of Perpetrator And Implications In Small Populations” (Davoli et al.) we see that infanticide is so prevalent it even changes the overall demographics of the population. In this case-study, it was shown male bears willing to kill cubs which were not their own, which on occasion led to the subsequent of the mother male. Male bears sometimes undertake infanticide for the purpose of returning females to a reproductive state at a expedited rate so that the male can attempt to mate with her. This has caused the female brown bears to undergo an even more selective pressure than many other mammals where it not only selects against their offspring, but themselves as well. This has led to the development of a countermeasure i.e. sexual promiscuity. We can also understand from this paper that the father does not generally kill his own offspring since that would be a direct reduction in his own fitness.

The general theme of all these papers can be distilled down to a very simple statement; males in a population may attempt infanticide in order to increase their mating opportunities or fitness, female may attempt infanticide in order to increase their own fitness or acquire better resources, and when females encounter selective pressure such as infanticide they may develop countermeasures to protect themselves and/or their offspring. This is certainly in alignment with the general nature of the wild environment. The lack of altruism and the central aspect of animalia mindsets being the increase in their own fitness and the passing along of their genes to the following generation.

To properly review these papers, it would be careless to not compare the similarities and contrast their differences. As shown by a number of these papers, females will formulate counterstrategies to infanticide (Davoli et al.) (Lowe et al.) (Palombit). If the females were to be amenable to infanticide, their ensuing fitness would likely be minimized because even though a male commits infanticide, that does not guarantee he will copulate and replace the amount of offspring which he had killed. It would not only decrease the females’ fitness, but that of the entire species if the females did not create these counter strategies.

An interesting contrast to the stereotypical expectation of only males performing infanticide is discussed in Female Competition And Its Evolutionary Consequences In Mammals(Stockley, and Bro-Jørgensen). This paper demonstrates the variety of reasons for females to kill the offspring of other females such as access to resources and better mating opportunities. An interesting implication to this paper is that simply being female does not necessarily entail a caring nature. We can see that these females only care for their young and in a way which, as inferred from the paper, is more similar to the protection of property than that of the protection of one cared for. Which is logical considering their offspring are a direct determinant of fitness. I would like to comment on this with an example made in Countering Infanticide: Chimpanzee Mothers Are Sensitive To The Relative Risks Posed By Males On Differing Rank Trajectories, in this paper it is shown that association with a more dominant male will lead to a higher likelihood that the offspring will survive into adulthood (Lowe et al.). Once again, we see the offspring being treated as social capital. The relevancy of this is evident by the likely fitness disadvantage presented by this sort of treatment of the offspring. With offspring being treated as a form of social capital, even animals can see, and they do, that if you want to become more dominant you have to destroy other males offspring and have more of your own. This is made evident by males attempting to or are in the process of improving their rank by destroying young which are not their own (Lowe et al.). With such considerations, we can see that offspring face an onslaught of potential fitness disadvantages for a variety of reasons.(expand on this last point)

    Yet so far, we have only talked about what can occur but not why they occur. Primarily, it is due to sexual selection in the form of intrasexual selection which drives the use of infanticide(Palombit) since males can theoretically kill an infant thus ending the mother’s lactational amenorrhea allowing the male to mate with said female. Intrasexual selection also occurs in females whereby the females compete for access to the males (Stockley, and Bro-Jørgensen). Other reasons which cause infanticide are the close proximity of others creating competition for resources (Stockley, and Bro-Jørgensen) and the drive to become more dominant in the social structure(Lowe et al.)(Stockley, and Bro-Jørgensen). All of the previously listed reasons make sense due to the possible increase in one’s own personal fitness that may occur if they were to commit infanticide. It seems that areas with limited resources, over-population, and a social hierarchy are where infanticide is most likely to occur.

In conclusion, we can see that the act of infanticide is a very powerful tool, although a double edged sword. It provides the opportunity to reproduce(Lukas, and Huchard), increase ones rank(Lowe et al.), or even acquire better resources (Stockley, and Bro-Jørgensen). The consequence of infanticide related to fitness is that, among other things, it decreases the fitness of others in the population (Lukas, and Huchard) (Davoli et al.), and can kill a member of the sex one can copulate with (Davoli et al.) which subsequently reduces likelihood of matong and thus fitness. The subject of infanticide is a very broad concept and more prevalent than would be expected. Many questions can still be asked and considered regarding the subject at hand, and these would hopefully provide insight into the multiple reasons and situations which lead to this act.

Bibliography

Davoli, Francesca et al. “Infanticide In Brown Bear: A Case-Study In The Italian Alps – Genetic Identification Of Perpetrator And Implications In Small Populations”. Nature Conservation, vol 25, 2018, pp. 55-75. Pensoft Publishers, doi:10.3897/natureconservation.25.23776. [in text:(Davoli et al.)]

Palombit, Ryne A. “Infanticide As Sexual Conflict: Coevolution Of Male Strategies And Female Counterstrategies”. Cold Spring Harbor Perspectives In Biology, vol 7, no. 6, 2015, p. a017640. Cold Spring Harbor Laboratory, doi:10.1101/cshperspect.a017640. [in text:(Palombit)]

Lukas, D., and E. Huchard. “The Evolution Of Infanticide By Males In Mammalian Societies”. Science, vol 346, no. 6211, 2014, pp. 841-844. American Association For The Advancement Of Science (AAAS), doi:10.1126/science.1257226. [in text:(Lukas, and Huchard)]

Stockley, Paula, and Jakob Bro-Jørgensen. “Female Competition And Its Evolutionary Consequences In Mammals”. Biological Reviews, vol 86, no. 2, 2011, pp. 341-366. Wiley, doi:10.1111/j.1469-185x.2010.00149.x. [in text:(Stockley, and Bro-Jørgensen)]