Why were prehistoric versions of modern mammals almost always larger?

[andrewsaurus rex]

There are numerous examples of mammals alive during the Pleistocene that were larger than their modern counterparts.  In fact any large mammal you can name today probably had a larger  prehistoric counterpart.  The European Hippo, most species of bears including polar bears, lions,  deer species and so on.....there was even a large cheetah in North America.  Was it the presence of humans that caused mammals to become smaller over time or were there environmental reasons instead or as well?

[CityRaptor]

Probably a mix of factors, but today the rule of thumb seems to be that larger versions tend to thrive in colder climates. Can at least be seen in bears and penguins today. Large animals tend to lose less heat over their surface area than smaller ones. Has too do with tham having proprortionally less surface are compared to their mass. And yes, I'm aware that the largest terrestrial mammals today are found in warm climates. Elephants at least compensate with their large ears. 

But yeah, as usual I would not outrule humans as a factor, too.

[Duna]

I disagree (in whale): 

[Faelrin]

Well that's only partly true. Some of the biggest record breaking mammals are still alive today. The largest cetacean, and animal, currently known to science is the blue whale. The largest carnivoran, and pinniped is the extant southern elephant seal. The extant moose (Alces alces) was on par with Megaloceros giganteus, and Cervalces scotti, with only Cervalces latifrons bigger. Though lots of extinct cervids did have much larger antlers in comparison. The extant tiger is on par with some of the largest extinct felids.

While much of the extinct mega-fauna that we know of certainly had record breaking species that have since gone extinct, likely to the arrival of early humans (or later colonizers), some also perished due to the changing environmental conditions at the time, if not a combination of factors.

Some of the largest mammals known went extinct before modern humans evolved, such as Paraceratherium (largest rhino), Epicyon (largest canid), and many other obscure ones.

Keep in mind the American cheetah, Miracinonyx, was in a different genus from the extant cheetah, and is really only called that because of occupying a similar niche and convergently evolved a similar morphology.

[GojiraGuy1954]

People

[Halichoeres]

CityRaptor is getting at part of it, which is Bergmann's Rule. Bergmann's Rule is the observation that, within a given lineage, the largest members tend to be found at higher latitudes, and smaller members are closer to the equator. That was true for elephants too, once upon a time. It doesn't mean you'll get rhino-sized rodents in the Arctic, because to get there rodents would have to go through an awkward intermediate phase where they are too big to hide or shelter effectively, but not large enough to thwart predators. The mechanism for Bergmann's Rule is as CityRaptor points out, a function of surface area relative to volume. So the last couple of million years, during the ice ages, if the whole earth is a little colder, then a lot of animals will average larger than when the earth is warmer.

The other side is humans as GojiraGuy 1954 points out. Every time humans colonized a new landmass the largest species went extinct sooner or later. It's hard to prove we wiped out any one species, since climate was also changing at the same time in lots of these places. But there are too many extinctions with just the right timing for it to be a coincidence. So a lot of those large animals would probably still be here if it weren't for us.

[andrewsaurus rex]

I've heard of Bergmann's Rule before but i'm not sure how useful it is because there seem to be a lot of exceptions to it.   Lots of arctic mammals are no larger than southern extant counterparts.  Arctic foxes, hares and voles to name just three.  It is certainly more useful to be larger in a cold climate, since bulk helps with thermoregulation but I think it is offset by the need to obtain much more food, both because of the increased bulk to support and for the higher energy demands of a cold climate, in an environment where finding food can be more difficult because of the cold.

So the animals that can find enough food will evolve bigger but those that can't will stay the same size as southern counterparts and need to find other ways to adapt to the cold..  So because Bergmann's Rule isn't universally applicable I don't see the point of it.

[Newt]

Biological rules and laws are just about always riddled with exceptions; biology is messy. That doesn't make the rules useless, as they are true often enough to be interesting. 

Anyways, there are some other, more fundamental issues here. One is that, while it is true that many lineages had larger members in the past, they also had smaller members in the past. Extinct elephants, for example, include not only the famous super-giants, but also tiny dwarfs (almost all on islands - island dwarfism is another "rule" with many exceptions, but still common enough to be interesting). Even ignoring these exceptional animals, many extinct continental elephants were similar to or smaller than the extant species. It really shouldn't be surprising that the three living elephant species show less size variation than the dozens of extinct species.

Time dilation is another issue. It is tempting to treat "the present" and "the past" as equivalent, but "the past" consists of many slices of time while the present is only one. There is no one slice of time in the past when all the animal lineages were gigantic (though the glacial maximums of the Pleistocene come close).

Some of those giant animals lived in ecosystems that no longer exist - the giant crocodilians, turtles, and rodents that infested Miocene South America lived in a pre-Amazonian mega-wetland system - swamps on a continental scale; no such ecosystem is around today. The mammoth steppes of the Pleistocene northlands are another example.

Yet another issue is that some of those extinct giants were not ecologically equivalent to their smaller modern relatives. The giant beaver Castoroides, for example, almost certainly did not fell trees and build dams like its modern cousins (with which it coexisted). It filled a role more like that of a tapir, and did not make it through the climate fluctuations at the end of the Pleistocene. 

[andrewsaurus rex]

very interesting post, Newt.  Thanks.   It's an especially good point about 'the past' being treated on equal terms as 'the present', when in fact the past is a vastly greater amount of time.

[Remko]

very interesting post, Newt.  Thanks.   It's an especially good point about 'the past' being treated on equal terms as 'the present', when in fact the past is a vastly greater amount of time.

Not only that, but lots of "present" animals (not just mammals), were around in the "past" as well. Some even before modern humans evolved.

Giant beavers and their modern counterpart were already named. But there where others too that lived alongside now extinct species like whoolly rhino's and mammoths, such as the Saiga antilope.

And don't forget about Steller's seacow. It's only recently extinct, but was the largest if it's kind. Much bigger than it's still living cousins.
But, to be fair, the species was already heading into extinction well before it was discovered.

And what about horses? Modern horsebreeds can be about as big, or even bigger, than the extinct Equus giganteus.

[Thialfi]

R @Remko valid points, but I am not sure if we reasonably can compare modern horse breeds to natural evolution.

[Halichoeres]

So because Bergmann's Rule isn't universally applicable I don't see the point of it.

Your observation about extinct relatives of modern species is not universally applicable. Does that mean there is no point to it?

[andrewsaurus rex]

Good point, maybe there isn't. 

Of course, mine was just a question by a schmo, nobody has named it Andrewsaurus's Rule.   lol

Bergmann's Rule doesn't seem like a 'rule' to me.  It seems like something that sometimes happens.

[Newt]

Good point, maybe there isn't. 

Of course, mine was just a question by a schmo, nobody has named it Andrewsauruses Rule.  lol

Bergmann's Rule doesn't seem like a 'rule' to me.  It seems like something that sometimes happens.

One study I read found about 65% of the mammal species they looked at conformed to Bergmann's rule. Other studies criticized their selection of species and suggested the real number is probably somewhat lower. I can try to locate those studies if you want to check them out.

That's not universal, but it's also not nothing - there's something real going on there. Just like if half of the patrons who ate at Big Jim's Barbecue Shack got sick afterwards, you might think there's something worth investigating there even though not every single patron got sick.

[andrewsaurus rex]

Interesting.   But maybe instead of Bergmann's Rule it should be called Bergmann's Guideline. 

[Halichoeres]

Good point, maybe there isn't. 

Of course, mine was just a question by a schmo, nobody has named it Andrewsaurus's Rule.   lol

Bergmann's Rule doesn't seem like a 'rule' to me.  It seems like something that sometimes happens.

Ha ha, I wasn't trying to denigrate your observation, but rather argue that even a non-universal principle can have some use. Kind of a half-full/half-empty way of looking at it. The reason it is called Bergmann's Rule in the first place is to be clear that it isn't a law, like the Law of Conservation of Energy or Boyle's Law. It's the same sense of "rule" as in the sentence, "As a rule, Halichoeres is forgetful." It implies that I sometimes remember things, maybe a great many things, for example the home phone number of my high school girlfriend.

Anyway, thanks Newt about the warning re: Big Jim's; I will be patronizing his competitors.

[andrewsaurus rex]

That's an interesting distinction between Rule and Law.  I was interpreting the term 'Bergmann's Rule' as an absolute ie something that's always true.  But I gather from this conversation that 'rule' is meant to mean something that is true more often than not.  If the way i was interpreting were correct, it would be written as 'Bergmann's Law'.

[Newt]

It would be nice if that were true. Unfortunately "laws" in biology are also exception-prone. Cope's Law, the observation that animal lineages increase in body size through time, is one that comes to mind and is relevant to our current conversation. I think many scientists did formerly believe it was close to universal, so they enshrined it as a law, but it is no more universal than Bergmann's Rule. The "law" moniker stuck though.

I think it is safest to treat all biological "rules" and "laws" as interesting tendencies, and not worry too much about the nomenclature. A lot of these things were formulated in the late nineteenth and early twentieth centuries, when biologists were suffering from physics envy, advanced Platonism, residual teleology, and a host of other conceptual diseases that made them prone to developing rules. But organisms are more headstrong than particles and don't like to follow our rules and laws.  

@Halichoeres - Try Dirty Dave's Pork Pit, the ribs are on point. Dave may be dirty, but the restaurant is clean...ish. I mean, you don't want to eat barbecue at a place that sparkles, you know? My grandfather had an observation - I will now formally formulate it as Buckner's Rule - that the quality of a barbecue joint's food was inversely proportional to its health department score. 

[Halichoeres]

Huh, I actually didn't know Cope's Rule had ever been referred to as Cope's Law.

Buckner's Rule: words to live by.

[stargatedalek]

Smaller animals are inherently more adaptable, being able to survive off of less food. This makes them more likely to survive changing climate, invasive species, and human encroachment/habitat loss, both on the scale of different species and of individuals within a given species.

We are living in a period of abnormally rapid extinction, so animals are going to skew abnormally small.