The Mara and the Hare
Mara are large rodents found in Argentina. Their scientific name is Dolichotis patagonum. Darwin doesn’t mention coming across Mara per se in The Voyage of the Beagle; instead he talks about encountering agoutis on the plains of Patagonia. However, there seems little doubt that what he was referring to was Mara.
Darwin’s fascination with Mara stemmed from the fact that he recognized that in the absence of any hares, these large rodents were their ecological equivalents. In fact, he called them, “Patagonian hares.” While a Mara is considerably bigger than a hare, it is true that there is a good deal of physical similarity between these phylogenetically quite distinct creatures. It is an example of what would become known as convergent evolution, where unrelated animals placed in quite similar circumstances come to resemble each other because natural selection leads to similar adaptations or outcomes. It’s why Thylacines resemble wolves and auks do a pretty good imitation of penguins.
Some might argue that was probably the most crucial take-home message a young Darwin could extract from these rabbit-like rodents. Yet, to me, the most amazing thing about Mara is that they are not like normal mammals at all. They certainly don’t breed like rabbits: they are one of the few monogamous mammals for one thing. And while in his later years Darwin would come to puzzle over the implications of males being inherently polygynous (big words for saying simply that males like to get it away with as many females as they can), the exciting question is not why evolution made men and other mammals like that, but why it made Mara the way it did. It is the exceptions that, so often in biology, prove the rule.