Of course the title is clickbait, but it does express a new finding: that, among Rodentia (yes, beavers are rodents), whose phylogeny was till now a bit unclear, we now learn that beavers are more closely related to kangaroo rats than to squirrels. For a long time, beavers had been thought to be closely related to squirrels (the “sciurid rodents”) because of the similar arrangement of their masseter muscles—the muscles that close the jaw. Recently, there was some slight but not completely convincing evidence, however, that beavers may be more closely related to kangaroo rats: those cute hopping mice in the family Heteromyidae. (Heteromyids also include pocket mice, kangaroo mice, and spiny pocket mice.) The molecular evidence was based on a similar piece of DNA in beavers and heteromyids: a single “retrotransposon,” a “jumping gene” that moves around the DNA by being transcribed from its RNA and then stuck in different places in the genome.
So we have a muscle similarity coming up against a single molecular similarity. Well, a new paper in Nature Scientific Reports by Liliya Doronina et al. (reference below; free download), using a lot more molecular data, shows that the kangaroo-rat affinity wins. This is based on a phylogeny constructed from both DNA sequences as well as the presence and position of retrotransposons.
It turns out that beavers, compared to other groups of rodents, share seven new retrotransposons with the kangaroo rat, and none with other groups of rodents. This shows that beavers and kangaroo rats are monophyletic: they have a common ancestor that is not a common ancestor with any other rodent. Below you can see what the new rodent phylogeny looks like, and you can also see, along the right, the similarity of the muscles between squirrels and beavers.
Note that they used the Eurasian beaver (Castor fiber), rather than its good old New World counterpart, the North American beaver (Castor canadensis). But that doesn’t matter, for the two species of beavers—there are only two and they diverged about 9 million years ago—are more closely related to each other than to kangaroo rats or any other rodent.
Before I give the reveal, here are the animals:
A Eurasian beaver:
A North American beaver (much cuter!):
A kangaroo rat (Dipodomys sp.):
It turns out that a similar arrangement of the masseter muscle evolved three times independently in rodents, so that’s not a good character to use for making evolutionary trees; it’s an “evolutionary convergence” that doesn’t tell us much about ancestry. DNA is much better, and here’s the final tree:
At last we can rest easy, knowing that the beaver is not a close relative of the squirrel. The similarity of their muscle configuration undoubtedly comes from their similar habits of gnawing tough stuff, which led to a convergent arrangement of strong jaw muscles.
h/t: Matthew Cobb
Doronina, L., A. Matzke, G. Churakov, M. Stoll, A. Huge, and J. Schmitz. 2017. The beaver’s phylogenetic lineage illuminated by retroposon reads. Nature Scientific Reports 7, Article number: 43562 (2017) doi:10.1038/srep43562