A few days ago, I had a discussion on Facebook with someone about Sumatran rhinoceroses and why they were still hairy. I had posted one of Frans de Waal’s photos of a Sumatra rhino calf, and as calves go, it was very shaggy.
The discussion got to a really interesting question, which I think points out to a misunderstanding of the mechanisms by which evolution happens. The discussion went something like this:
“Sumatran rhinos are hairy. Therefore, there must be some reason they are hairy. Otherwise, there would be selection against having lots of hair in jungles of Southeast Asia, Borneo, and Sumatra.”
The answer I had was that the Sumatran rhino is the closest living relative to the extinct woolly rhino. Woolly rhinos were quite well-furred out, and it’s possible even now for animals to retain primitive traits. You hear people talking all the time about living fossils, but there is usually no understanding that a 20 million-year-old shark that looks very similar to living species most likely is incapable of reproducing with that living species (if the two were made to live contemporaneously with each other.)
The error is in assuming that a trait an animal possesses is always a benefit to it. This is a corollary of thinking the evolutions happens through natural selection alone, when it is actually only one way that it happens. (And it’s actually not the main way).
The truth is the Sumatran rhino is nothing more– or less– than the last survivor a lineage of rhinos that retained their furry bodies of their tapir-like ancestors. (And Sumatran rhinos aren’t even that hairy!).
And it should be noted that there are plenty of mammals in the Sumatran rhino’s range that have much furrier bodies.
Evolution isn’t planned out. It can only work with what genetic material is available in the population. Assuming that Sumatran rhinos retain their excessive fur only because it has some advantage to them is putting the cart before the horse.
Now, there is a possibility that there is some advantage for these rhinos to retain the very furred up bodies. I’ve not seen any literature that explores this possibility, but the fact that this animal is so closely related to the woolly rhino kind of provides the best explanation. This animal retains a primitive trait solely because that’s what its ancestors were. It just can’t magically evolve a relatively hairless body.
So always be careful in assuming that a trait exists solely because there is some advantage to it. That’s a real problem in our popular understanding of evolution.
Be very careful of putting the cart before the horse. Sometimes a trait is advantageous. It’s often just neutral.