Nguyen, C. T. Y., & Saltzman, W. (2021). Sex differences in offspring discrimination in the biparental California mouse (Peromyscus californicus). Journal of Ethology, 39(3), 309–318. https://doi.org/10.1007/s10164-021-00705-2

The researchers of this study focused on the California mouse or the  California deer-mouse, and wanted to look into whether or not there are sex differences regardings discrimination amongst kin. The study presented a total of 12 mated pairs (N=24). Each parent was presented with two pups ( kin v. non-kin) during postpartum days (referred to as PPD in the study; days examined 3,7, 16, and 28) and each pair were presented the same. They did this to test and see if there was any kin discrimination occuring between the sexes. They primarily believed that mothers would have higher rates of discrimination in comparison to the father. The presentation of the pups to the parents were randomized, however the pups appeared to be relatively close in size and shape. The idea was to see if the parents would go to their “kin” or not. When the data was compared regarding the pups (kin and non-kin), no major differences were found when analyzed. The only significant finding during this analysis was that fathers would spend less time sniffing or being around the pup that was not related to them. A lot of observed behaviors that the male mice or fathers conducted were inconsistent and were not of any significance.

This study attempts to show cooperation levels as a function of relatedness independent from a species biology. Meaning they tried to display a result that isn’t influenced by the species biological or genetic ability to differentiate between members of the same species. They also assumed the cost and benefit functions of cooperation were the same across the board as well as the personal-fitness effects of a given amount of cooperation for the actor and recipient being independent of their relatedness to one another. With these variables held constant, the study did indeed show through both the linear and convex models that kin discrimination is shown to lead to increased or at least equivalent levels of cooperation when compared to the levels expected in the “veil of ignorance” scenario. (Faria & Gardner, 2020)

Thompson and fellow authors state that traditionally when kin selection is possible, animals will typically act more favorably toward individuals that are closely related genetically to themselves, and more aggressively towards those who’re less related. In opposition to the norm however is a study of the wild banded mongoose, or Mungos mungo. In this species, females who’re closely related to the dominant individuals of a group are often targeted for eviction which results in injury and sometimes death in extreme cases. The negative kin discrimination displayed here in this species is restricted almost entirely to eviction behavior towards females capable of resistance, and the dominant mongoose also doesn’t exhibit kin discrimination when attempting to evict younger females or when carrying out infanticidal attacks on defenseless young. This is behavior is in contradiction to the normal model of kin discrimination and the behavior can’t be explained by inbreeding avoidance or local kin competition. So, this research team used game theory to show how negative kin discrimination can be explained by selection for unrelated targets to invest more effort in resisting eviction. Through an examination of a two-step game in which the first step asses the decision made by the aggressor, and the second step involving the resistance decision, the variables were cost of selfish act by aggressor, cost of resisting by evictee, benefit of aggressor, benefit of evictee, and a relatedness coefficient between aggressor and evictee. With all of these factors in mind and the results obtained, the research team concluded that it seems relatedness does indeed play a factor in this decision-making process. Which is to be expected in cases where kin discrimination applies, however, for mongooses the relatedness coefficient between the evictee and dominant females of the group was more important than almost any other factor. This study shows that in some species, kin selection theory doesn’t fully explain preferential altruistic behavior towards individuals of higher genetic relatedness. So, further exploration of subsequent social interactions proceeding selfish behavior will most certainly be of great benefit for a better understanding of the “why” behind this behavior. It seems aggressive and cooperative behavior within animal groups can be more subtle, variable, and nuanced than capable of explanation by traditional kin selection theory.