Phylogeny refers to the study of the relationship between different taxonomic groups over the course of evolutionary history. A phylogenetic tree, as seen in Figure 1, is a diagram that represents the evolutionary relationships between species on the basis of morphological and genetic relatedness.

Phylogeny of Rats and Other Species

As seen in Figure 1, many of the listed species are separated by millions of years of evolution: the divergence of mammals from fish occurred 450 million years, with the most recent common ancestor of non-human primates and rats occuring about 75 million years ago.

Figure 1. Phylogenetic tree showing distance of rats from other animals, including zebrafish and non-human primates. Reprinted from Gao et al., 2009.

Despite the enormity of this evolutionary distance, species such as zebrafish, rats, and non-human primates have a number of things in common: they all display social behavior to at least some degree, and the formation and maintenance of social dominance hierarchies has been documented in each species. This surprising degree of conservation for social dominance behavior suggests that examining the facets and consequences of social hierarchies among zebrafish and non-human primates may offer some insights into dominance hierarchies among rats.

Differential Reproductive Sucess of Dominant/Subordinate Zebrafish

Zebrafish, despite several hundred millions of years of divergence, display dominance hierarchies in many ways similar to rats. As Paull et al., 2010 demonstrated in an innovative study, zebrafish fall into two distinct behavioral phenotypes relative to one another, with the dominant phenotype displaying aggressive social behaviors towards the subordinate phenotype. Interestingly, the authors also tracked the reproductive success of each phenotype and found that dominant males produced significantly greater offspring than subordinate males (there was no significant difference between dominant and subordinate females in offspring production). This is a clear demonstration that being a dominant male may confer significant benefits to reproductive fitness relative to being a subordinate male, which is a reasonable explanation why dominant behaviors would be conserved across hundreds of millions of years of evolution between zebrafish and rats.

Higher frequency of aggressive behaviors in dominant zebrafish compared to subordinate zebrafish (left figure) and increased reproductive fitness of dominant male zebrafish relative to subordinate male zebrafish. Reprinted from Paull et al., 2010.

Hierarchies in Baboons

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Baboons, which share a more recent common ancestor with rats than rats do with zebrafish, are similar to rats in that they possess fairly complex social hierarchies where individuals compete for dominance.

As Robert Sapolsky demonstrated through field research in the 1980's, baboon dominance hierarches are generally associated with aggressive social behaviors from dominant males directed at subordinate males (Sapolsky 1982). This similarity in the behavior in question, the aggression associated with dominance hierarchies, between three species that widely differ in almost every other respect, suggests that the formation and maintenance of dominance hierarchies within social species is highly conserved across evolutionary time.