Why Aren’t There Giant Animals Anymore?

Since I moved to Alaska and learned about the state’s Pleistocene history, I’ve bemoaned the absence of the ice-age mega-megafauna. Here in the interior, 20,000 years ago, mammoths, wooly rhinoceros, giant ground sloths, Smilodon, dire wolves, short-faced bears, American lions, and other now sadly absent animals, roamed. While a few ice-age mammals persist (think musk oxen, bison, and moose), most are long gone.

This giant (by today’s standards) Katmai Brown Bear, is about as big as land mammals now get in Alaska (approx 400 kg).

Long before the Pleistocene, another group of giants once dominated the Earth: dinosaurs. While I don’t bemoan their loss as much as the mammals, I have wondered what happened to all these giant animals of the past.

When it comes to land mammals, the largest extant species are elephants. Very few others are anywhere close to as large. Really big land critters, with a rare few exceptions, are absent from our planet. Why?

I got a partial answer to that question as I browsing a recent articles on PLOS Biology. Scrolling through the titles, one stood out: “Where Have All the Giants Gone? How

Animals Deal with the Problem of Size”. Published in mid-January of 2017, the article sheds some light on the subject, by reviewing the literature on animal locomotion and the musculoskeletal system.

The largest land animals to ever live, were some dinosaurs that may have reached a mass of 50,000 kg. That makes a 6000 kg African Bush Elephant seem dwarfish by comparison. And the dinosaurs were successful too, dominating Earth’s fauna for 165 million years. That is a staggering level of success.

To get at why land animals no longer reach such size, the author of the paper first looked at the demands (and costs) of great size. The bigger a creature, the more demands are placed on its muscles and bones. To a point, this can be counter-acted by increasing muscle size, bone density, and decreasing the length of bones in proportion to thickness. But at some point, increasing mass outstrips the ability of the musculoskeletal system to compensate. A more upright posture too, may help ease the strain on bones and muscles, but not all animals adopt that change (think lizards and cats). The authors use the example of the Komodo dragon which reaches 100kg without the aid of an upright posture. In some cases, animals like the Komodo change the architecture of their muscles to provide more strength to support (easing pressure on the bones), while sacrificing some strength of locomotion.

But at certain point, even combining a bunch of techniques for maintaining large size, something has to be sacrificed, and that something is speed.

The author found that speed increases with size…up to a point. In Varanids (monitor lizards), speed tops out at about 1 or 2 kg, in non-felid mammals speed is greatest right at 121 kg, while the fastest cats tend to be between 80 and 90 kg. Indeed very few species of mammals (only about 3.8% of all species) are larger than the optimum speed to mass ratio.

The take-home message is that really big animals, like dinosaurs and mammoths, had to be slow to make up for their great mass.

And that may be the key to why giants are no longer common: something in our ecosystems changed to favor speed over size. Perhaps predators are now faster and more agile, meaning size alone is not enough to prevent predation. Giants also take time to evolve, and the world may be changing too fast since the ice ages, meaning the stability needed for such an evolution just isn’t present. As the authors conclude, unless we see new techniques evolve to cope with the speed-size trade-off, or the world’s ecosystem changes again to favor great size, it is unlikely that giant species will ever appear again.

I guess I won’t be seeing a return of the Pleistocene fauna to Alaska.

Bummer.

Reference:
Dick TJM, Clemente CJ (2017) Where Have All the Giants Gone? How Animals Deal with the Problem of Size. PLoS Biol 15(1): e2000473. doi:10.1371/journal.pbio.2000473