Why do animals grow bigger in hot places?

I mean: why do animals tend to be bigger in a hot, damp environment? Think about cockroaches: they’re tiny in S. Petersburg, while they are slightly bigger in Milan and huge in Bangkok or Sydney… I guess this might apply for every animal.

k.

What about polar bears, grizzly bears, Siberian tigers, moose, wooly mammoths, etc.?

Elephants and giraffes live in a hot, dry environment.

I think you’re making too much of a generalization from cockroaches. My best guess is that there are simply more insects and more insect biodiversity in hot, wet environments. Being so small, insects do better in warmer climates. (In cold climates, most insect species are not active during winter.)

Are Thais and Australians bigger than people from St. Petersburgh?

*waits for an Aussie dope to come in and assure us that they are.

sorry, Aussie doper. Not all Aussies are dopes, as far as I know.

Actually, the overall trend is for animals to be larger the further North they live. Mountain Lions and Wolves in Canada are larger than their Mexican subspecies. Ditto bears and foxes.

I’ve read it has a lot to do with body mass to surface area ratios and the fact that more dense animals preserve body heat.

Are the cockroaches the OP asks about all of the same variety? Different “breeds” (sorry, I’m drawing a blank on the techical term … kingdom, phylum, um…) might be of different sizes.

Density is not significant. The relationship is actually between body volume and surface area. Animal with less volume have considerably more surface area per unit of volume than do large animals. That’s more surface area from which excess heat can be dissipated through radiation, convection and evaporation (of sweat or other moisture). Large animals in hot climates, like elephants, actually face a significant challenge in avoiding overheating. Elephants have large, floppy ears to increase surface area and improve the dissipation of body heat, and have evolved behaviors like spraying themselves with mud to help lose heat through evaporation of moisture.

In more northern latitudes, body size can be an advantage, since it requires less physiological “work” (use of chemical energy) to regulate body temperature

You’re thinking of different species. There are literally thousands of cockroach species, with considerable variation between separate families. Size is a character that can vary considerably even between individuals of the same family, genus or species.

I was gonna guess “species” but it sounded wrong. (“Dog” is a species, right? That’s the level at which two organisms can interbreed?)

Anyway, it may just be fortuitous that a particular (small) species of roach is found in Moscow, and a larger species in Bora Bora, if that is, in faact, the case.

Right. This principal of larger size in colder areas is known as Bergmann’s Rule, and applies only to endotherms (’'warm-blooded" animals) - and strictly speaking, applies only within species, or between closely related species. Similar is Allen’s Rule, which states that endotherms in cold climates have shorter appendages than those in warmer areas, again because of heat-retention/dissipation issues.

Note that the fact that the largest land mammals (elephants, rhinos, hippos) today are found in the tropics is mainly an artifact of the megafaunal extinction at the end of the Pleistocene - there used to be woolly mammoths, rhinos, etc. even in Arctic areas thousands of years ago. The largest carnivores today - polar and Kodiak bears, Siberian Tigers - are from cold areas.

These rules do not apply to ectotherms (“cold-blooded” animals), such as lizards, crocodilians, insects, etc. In this case, the largest forms are often found in the tropics. For these animals, the problem is not so much losing heat but gaining enough in the first place. Large animals not only cool more slowly, they warm more slowly too. Since these ectotherms acquire their body heat mainly from the environment, it’s mainly in warm climates that large ones are able to gain heat quickly enough (and retain it) to be active efficiently. Another factor is that large ectotherms in cold areas often will not be able to find secure places to hibernate in the winter when they are forced to become inactive.

What you’re talking about is true for mammals which are “warm blooded” (with the correction made about density). I put “warm blooded” in quotes because it’s not a very precise term and isn’t used by biologists much anymore, but it still gets the meaning across. An insect may have a different response to thermal envirnomental factors than a mammal will since it uses a different method of regulating its body temperature. Perhaps one of your resident biologists will enter this thread and clarify how evolutoin molds the bodies of insects in different temperature zones.

You mean like all those Ice Age mammoths, mastodons, wooly rhinos, etc.?

And trust me, the cockroaches in St. Pete are NOT tiny in any sense of the word!