Screenshot of The Express webpage.
Screenshot of The Express webpage.

This week in the Metro: “Giant rats bigger than sheep could roam the earth in future, experts warn”. The Daily Mail declared “Rats could one day be bigger than COWS” and the Express helpfully photoshopped a rat into a herd of sheep so that we could better imagine this “terrifying vision”.

These stories all refer to a press release from the University of Leicester, the general gist of which is that while many large mammals are headed for extinction, the adaptable rat is a good bet for longterm survival. With fewer big animals around, rats might evolve to make use of habitats left vacant by extinct species. This speculation is hardly worth the hyperbolic headlines it generated, but giant rodents really did roam the Earth millions of years ago. Let’s see what they were like.

Castoroides

Castoroides, by Charles R Knight,1904
Castoroides, by Charles R Knight,1904

Castoroides is basically a giant beaver that inhabited the swamps of North America during the Pleistocene. The largest species, Castoroides ohioensis, may have weighed up to 100kg – (about the size of a panda) and became extinct at the end of the last Ice Age about 12,000 years ago.

They were a lot like modern beavers, but instead of sharp, chisel-shaped incisors for chewing through bark, they had blunter incisors up to 15cm long. Studies analysing the composition of their teeth suggest that they mainly ate aquatic plants like pondweed and sedges.

100kg is not so huge in the grand scheme of things – it’s only a bit bigger than the largest rodent alive today, the capybara, which can weigh up to 80kg. But we have a lot of fossils of Castoroides. The first fossils were found in the late 17th century, and they’re still being discovered. This complete picture means that there’s been a lot of work done on the animal, and techniques estimating the weight of Castoroides can be applied to other rodents, such as:

Phoberomys

I can't find a copyright-free image of Phoberomys, so here's a capybara with baby.
I can’t find a copyright-free image of Phoberomys, so here’s a sceptical-looking capybara with baby.

Phoberomys has been estimated to weigh in at up to 700kg – roughly equivalent to a beef bull – though it’s hard to say for sure. It had robust hindquarters and back legs, and relatively slender forearms that it may have used to manipulate food. It inhabited swampy areas like today’s capybara. A fairly complete skeleton was discovered in Venezuela in 2000. With a body length of three metres long and a tail adding another metre to that, it was the largest rodent known for several years, until:

 

 

 

 

 

Josephoartigasia

Reconstruction of Josephoartigasia head, with scale. The smaller animal is a pacarana - its closest living relative. From Rinderknecht & Blanco, 2008
Reconstruction of Josephoartigasia head, with scale. The smaller animal is a pacarana – its closest living relative. From Rinderknecht & Blanco, 2008

Published in 2008, weight estimates put Josephoartigasia at 1000kg – around the weight of a black rhinoceros, and by far the largest rodent ever found.  But despite being described in the press this week as a “horrifying creature” and a “giant rat”, it’s unlikely that Josephoartigasia looked much like a rat.

As an animal gets larger, you can’t simply scale up. A rat weighing a tonne would collapse under its own weight, and couldn’t digest the volume of food required for such a large body. Josephoartigasia had a broad muzzle, and is very likely to have had pillar-like legs and a stocky body, like today’s capybara but more so. Tooth studies have shown that it ate soft vegetation such as water plants or fruit, though it may have been able to give a nasty bite with its incisors if threatened. It probably lived in an environment with river estuaries and forests, and may have been hunted by sabre-toothed tigers.

As these real rodents of unusual size show, environment shapes biology. You can’t fill the ecological niche of a large herbivore whilst still looking like a rat. Instead, if you evolve to live like a cow, you’re gonna end up looking a lot like a cow.

So, scary giant rats? More like vegetarian hippo-rats. As one commentator on the Mail website said: I, for one, welcome our new ungulate-sized rodent overlords.

Reconstruction of Josephoartigasia, by Nobu Tamura
Reconstruction of Josephoartigasia, by Nobu Tamura. Isn’t it kind of cute?

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