Thursday, July 26, 2018

Taxonomy Tuesday: Hyenas Are Not Dogs

The spotted,  or "laughing" hyena
Yes, I know it's Thursday. I got busy mid-week. Today's #TaxonomyTuesday is about hyenas, which, contrary to popular belief, are not related to dogs.

Hyenas are familiar African predators that are best known for scavenging, although they don’t actually scavenge most of their food. There are four living hyenas: the spotted or laughing hyena (Crocuta crocuta), brown hyena (Parahyaena brunnea), striped hyena (Hyaena hyaena), and aardwolf (Proteles cristatus). The striped hyena is also found in the Middle East and India. In general, hyenas are fairly large, dog-like animals with lengthy forelimbs and sloped backs (rather like a German Shepherd) and powerful, bone-cracking dentition. The aardwolf is the black sheep of the family, being much smaller than its cousins and subsists almost entirely on termites—apparently they won’t voluntarily eat meat.

The small, adorable aardwolf
Hyenas have an extensive fossil record going back to the Miocene. Fossil hyenas are impressively diverse—Chasmaporthetes appears to have been a cheetah-like cursorial hunter. The group’s bone-crunching contingent evolved some 10-12 million years ago and appears to have outcompeted the more basal “dog-like” hyenas but were kept out of North America by the successful bone-crunching borophagine dogs, who were already filling that niche. The largest of the bone-crunchers, Pachycrocuta, was a lion-sized carnivore that probably stole kills from machairodont cats throughout Eurasia and east Africa.

Pachycrocuta, by Mauricio Anton
It’s completely reasonable to assume hyenas are dogs. After all, the two groups share general body plans, non-retractable claws, and dental features. In particular, their skulls are similar to borophagine dogs—not surprising given their similar bone-crunching lifestyles.

Surprisingly, however, hyenas are actually on the cat family tree. They’re not cats per say, but if you think of dogs and cats as “end-points” on a diverging family tree, hyenas are around mid-tier on the cat branch. This relationship is given away by the morphology of their auditory bullae, or ear capsules. If you look at the bottom of a dog or cat skull, you’ll see two inflated areas down by the foramen magnum; this is where the ear bones live. In caniforms (dog branch), they have single-chambered bullae. Feliforms (cat branch) have double-chambered bullae, and it turns out that hyenas have double-chambered bullae.

You can see how the auditory bullae are divided in two here, from Wikipedia.
Hyenas are, thus, another good example of convergent evolution. Because they adopt a similar ecological role as dogs, hyenas also approach them in general morphology. This also underscores the importance of shared anatomical landmarks. Every monophyletic group is defined, or diagnosed, by one or more unique, shared features. Despite looking like dogs, hyenas have cat-type auditory bullae. In ceratopsians, it's the rostral bone (not horns or frills). In marsupials, it's a pouch (among other things). The thylacine, for instance, is (well, was) extremely similar to placental dogs like the red fox. However, it still had a pouch and reproduced the marsupial way. Another example is the similar body outlines of dolphins (which are mammals) and ichthyosaurs (which are reptiles). Turns out sharks (which are elasmobranchs) and tuna (which are actinopterygids) have similar outlines as well. None of them are closely related, but all four groups adapted to similar environmental conditions--that's just a very good shape for efficiently moving through and feeding in a marine environment.

But dolphins are clearly mammals, ichthyosaurs are clearly reptiles, sharks are clearly cartilaginous fish, and tuna are clearly ray-finned fish. They all solved the ecological challenges in different ways before coming, more or less, to the same conclusion.

This has gotten a little longer than I anticipated, but the point is that hyenas are not dogs; they're almost cats, and convergent evolution is super neat.

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