why are there fewer species of mammals than there are of reptiles, birds, or bony fish?

Hi Joey,

This is a very deep question.

The simplest (please note: and very incomplete) answer is that evolution takes time. Although mammals have been around since the Mesozoic, they didn't diversify until after the dinosaurs became extinct, about 65 million years ago. That's not a long time, in biological terms. Compare that with, for example, birds: They started all the way back in the Jurassic! Archaeopteryx, widely considered the first bird, is about 150 million years old...

Hope this helps,


Joey, another point is that mammals went through a period where they all were very small. Small animals (rat-sized, very roughly) often goes along with living in complexly structured environments, especially for very active animals: forests, mountains, etc.... other factos add to this, including small size per se, and you end up with proportionally more mammals unliekly to fossilize.

To be fair, mammal species diversity is not really that much smaller than that of the squamates (5,500 compared to 6,700) and they are certainly in the same ball-park. There are nearly twice as many birds - but they have the ability to exploit a wider range of niches - feathers and low body mass mean more than flight - they enable the exploitation of the surface of water bodies, as one example.

The average body size of mammals is much greater than the average body size of living squamates or birds, so you would expect species diversity to be smaller, since larger animals have longer generation times (slowing the rate of evolution). You'll notice that the most diverse mammals are the rodents and bats, which are mostly very small.

Although bats have a very long generational time (almost polar opposites to rodents - a bat may have one pup, at best, very year but often only one every two or even three years), so it may be that their diversity stems from powered flight, unlike birds, which, as Paolo says, have more cards up their sleeves than just flight to thank for their diversity.

Hi Joey,

As Neil pointed out in this post:
http://www.askabiologist.org.uk/answers … p?id=2874,
We must consider not just number of
species, but different taxonomic levels: classes, orders, families,
genera. At the species level mammals may seem less diverse then other groups but from the family level mammals are much more diverse. There have been about three times more mammals families then reptiles families in recent times and one and a half times more mammal families then bird families.
An explanatory digaram can be found here:
Figure 2 of http://rsbl.royalsocietypublishing.org/ … 4/544.full