Ontogeny refers to the ways in which the behavior develops across the course of the lifetime of the individual organism.

It Gets Better: Older Rats are more likely to be Dominant

Among male rates, dominant social status is not something a rat is born into, but rather acquires via their acquisition of age and body size. In a comprehensive study conducted by Berdoy et al., 1985, the social status of male rats born into both wild and laboratory conditions was tracked over the course of their lifespan. Something that became clear during their research was that, all else equal, a male with more body mass would likely have a higher ranking in the social hierarchy than a smaller male. This conclusion was perfectly in line with findings from other species, such as salmon (Huntington et al., 1990) and kangaroos (Miller et al., 2010), where larger body mass was associated an increased likelihood of the individual having a higher standing in the social hierarchy.

Larger body mass is correlated with higher dominance in the social hierarchy (1 = most dominant, increasing numbers indicate more subordiante). Reprinted from Berdoy et al., 1985.

However, upon further investigation Berdoy and colleagues found that although body mass seemed to be important for establishing dominance, the age of the animals being compared was an even greater predictor of who would end up being dominant. That is, the group found that even though increased body size tended to be associated with increased age, a younger but larger male would nevertheless likely end up subordinate to a smaller but older male.

Increased age is strongly correlated with increased ranking in the social hierarchy (1 = most dominant, increasing numbers = less and less dominant). Reprinted from Berdoy et al., 1985.

The finding that age was the single greatest predictor of standing in the social hierarchy reveals the importance of the development of various features in determining the rank of rats within the social hierarchy. That is, while body mass can be a somewhat reliable predictor for dominance if need be, other traits that require a prolonged period of development (e.g., fighting skill) are also necessary to reliably determine which individual among a pair will be dominant and which will be subordinate.