Natural History

I know I posted this video last year.  I find it extremely fascinating. So here it is again:

Source.

This lecture is a discussion of the parallel evolution of the bone-crushing jaws in hyenas and an extinct North American subfamily of canids called the Borophaginae.  Canidae (‘the dog family”) has historically had three subfamilies.  The earliest dogs were  Hesperocyoninae (“the dawn dogs”).  The earliest known dog species, which was in this subfamily, superficially resembled a ringtail. The Hesperocyoninae were generally smaller animals, and many of these animals were capable of climbing trees.

It is from these primitive dogs that the two other subfamilies evolved.  Most early canid evolution happened in North America, and it is here that all three subfamilies first evolved.

As this lecture discusses, the borophagine dogs evolved to eat bones from very large prey species. They became massive creatures, the largest dogs the world has ever seen.

They thrived in North America f0r 33 million years.

But the big borophagine dogs became hyperspecialized to living on large carcasses.

And as they became more specialized, they became much more vulnerable to extinction. All it would take is for the relative abundance of certain large prey species to drop a bit, and their populations could collapse.

The extant subfamily of Canidade, the Caninae, are much more generalist in their diet. Although Ethiopian wolves live almost exclusive on a species of mole-rat, they can effectively hunt other things, and the only extant dog species that has a truly specialized diet is the bat-eared fox, which lives almost exclusively harvester termites. All other dogs are capable of varying their diet quite a bit.

And that’s probably why there are about 35 species of Caninae in existence today.

And there are no large borophagine dogs left. Some of the smaller, less specialized Borophaginae might still be around, but these animals became extinct relatively early on, as did all the Hesperocyoninae.

Now, let’s make things really confusing.

Modern hyenas are a mere remnant of what was once a fairly diverse family of carnivorans.

They evolved for the same bone-crushing abilities as the Borophaginae.

And it is likely that their hyperspecialization resulted in their demise as well.

Today, there are only four species of hyena left. One of these (the aardwolf) is a termite-eating specialist– just like the bat-eared fox. Brown and striped hyenas are mostly scavengers. Only the spotted hyena, which just so happens to live on the only continent that has anything like megafauna left, is a major bone-crushing predator.

And just as there have been the parallel evolution of bone-crushing in both dogs and hyenas, there has also been an evolution of the cursorial “wolf avatars” in both families.

There were once dog-like hyenas and hyena-like dogs. Only a single species of dog-like hyena still exists. It is the aardwolf, the little termite-eating hyena that I mentioned earlier.

Chasmaporthetes, the hyena that Tseng mentions in this talk, was also a dog-like hyena. It is the only hyena to have ever made it into North America. It was also the most northerly distributed.

There are lots of questions about why this hyena made it in North America.

One hypothesis is that it actually had less of a hyena-like ecological niche, and therefore, it would not have had to compete with the borophagine dogs.

Tseng mentions that his research found that Chasmaporthetes had the ability to crush bones as well as a modern spotted hyena.

So maybe it wasn’t that different from the borophagine dogs.

Of course, the reasons why it thrived in North America are likely quite complex, and because it was a hyena trying to be a wolf in a continent filled with wolves trying to be hyenas, it still may have had very little competition after all.

So the story of dogs and hyenas is complex.

Now, it should be mentioned that dogs and hyenas are not closely related. The last time they shared a common ancestors was 43 million years ago, when their common ancestor would have been a Miacid.

Canids belong to the suborder Caniformia, which includes mustelids, skunks and stink badgers, bears, procyonids, the red panda, the earless seals, the eared seals, and the walrus.

Hyenas, despite their similarity to dogs living and extinct, are actually more closely related to cats. They are in the suborder Feliformia, which includes the cat family (Felidae) and then a whole bunch of other small carnivorans whose exact taxonomic position is still being worked out. Hyenas are most closely related to the civet family (Viverridae).Mongooses and meerkats are also relatives in the family Herpestidae, and closely related to them are the Malagasy carnivorans, which are all in the family Eupleridae. Several species of Malagasy carnivorans look like mongooses, and they are still referred to as mongooses. However, there are others, like the Malagasy civet and the fossa, that were classified in the civet family until recently. There are also African and Asiatic linsangs that were  also classified as civets, but recent genetic evidence has revealed that African linsangs are civets but the two Asiatic species are actually very primitive relatives of the cat family.

So just as hyenas are not dogs, mongooses are not weasels.

Carnivoran evolution is a bit confusing at times.

It’s very hard to keep the relationships between families straight.

Caniforms are more diverse than feliforms, which with the exception of cats and hyenas, have tended to remain small and to be found almost exclusively on the African and Asia mainland. Caniforms vary from the tiny least weasel that weighs only 2 or 3 ounces to the southern elephant seal that can weigh 8,800 pounds.

I hope this clear up some misconceptions about hyenas and dogs and of carnivoran taxonomy in general.

Most people can tell the difference between a cat and a dog.

But when someone tells you fisher cats, polecats, and ring-tailed cats are not cats and hyenas aren’t dogs, it is difficult to understand.

Evolution makes for strange convergences.

It’s one of the weird and marvelous things about it.