# Is my understanding of the Bergmann's rule correct? (mass is secondary)

According to Wikipedia, Bergmann's rule applies because animals living in colder areas have greater surface-area-to-volume ratio. If I understand that correctly, the advantage of the bulkier bodies of the polar animals isn't really about having a greater mass but about having a shape more closer to a sphere - which usually means having a greater mass.

Am I right?

• No, your argument seems to be flawed. Sphere’s surface-to-volume ratio is proportional to the radius, and a small sphere may have worse ratio than a bulky irregular body. Jan 12 '21 at 21:37
• @jmster That's exactly what I don't seem to get. Isn't size relative? Like how come the ratio is different for a small ball and a big one. The way I see it, the size only tells us comparison to human metrics like meters. Let's say there's a planet A with a ball 1 meter wide and there's a planet B which is twice as big so the humans who live there say the ball is 0,5 meter wide even though it's the exact same size as ball A. Can you please explain how come "size" matters as a seemingly arbitrary parameter? Jan 13 '21 at 19:23
• Heat transfer and many other physical processes depend on the molecular structure of the materials. The molecules on a big planet are not proportionally bigger than on a small planet, that's why you cannot expect everything that happens on those planets to scale up linearly. Jan 13 '21 at 22:27
• @jmster Right, thank you! Both of the occasions when I was explained the rule involved the presenter mentioning that surface of a sphere grows with square of the ratio which rightfully didn't make sense to me as the explanation. Jan 14 '21 at 11:44
• A couple of things. First, SA:V is lower in colder environments (see the similar "ecogeographic rule," Allen's Rule, Wikipedia. Second, SA:V and mass are two somewhat independent traits are the result of somewhat independent evolutionary mechanisms. Temperature affects mass, temperature affects SA:V, but SA:V does not affect mass. Feb 1 '21 at 15:37