The problem with Mammals, plus a Dino Digression

by Francesco Lami

So, if I recall correctly, I promised you an article about the stink of mammals.

Let’s get this party started.

Mammals are actually a quite impressive group. Although, back in the Triassic, dinos took over and forced them in the shades (more on that later), they managed to survive, and when the meteor went BOOM mammals had a whole new world all for them. What’s most amazing is that there’s such a huge variety of mammalian forms, when the number of their species is relatively low: only about 4000-5000 (compare that to the 8000 known snake and lizard species, or Squamata). In such a “small” variety, we find extremes like the Blue Whale (Balaenoptera musculus), the biggest animal known to have ever existed, perfectly adapted to live into the water from birth till death, and the Bumblebee Bat (Craseonycteris thonglongyai), weighing only 2 grams, and part of one of only four groups of animals in history (bats, birds, pterosaurs, insects) to have achieved flight. And what about the others? Some can run faster than any other land animal, some can dig, some can climb, some can jump. Most of them are covered in fur, in others the fur is absent or modified into body armour or quills. Some can make tools, like sharp sticks or computers. So, from a purely objective point of view, there’s a lot of interesting stuff to study and learn from the world of Mammalia.

Then what’s my problem with them? They stink.

Although bad smell could give the (probably absolutely wrong) impression that they are less clean than other animals, stink in mammals is physiological, since their skin contains different kinds of glands, some of which are precisely specialized in producing smelly substances (skin glands are a rarity among other amniotes – aka birds and reptiles). Smell is for many (probably most) mammals an important method of communication.

It’s not the only one. Some mammals use vocalization, even very complex vocalization (cough, Homo sapiens, cough), and some use visual methods of communication, from displays of beauty and strength in some ungulates to the wide array of facial expressions and hand gestures that characterize primates. All of this didn’t eliminate the importance of stink in most species. Stink could say everything from “I’m horny! Hump me!” to “This is MY territory. Stay the fuck away”.

I’ve studied nothing specific about the argument, but if I had to make a hypothesis, I think it would be pretty easy to guess the factors that made stink so important in mammal communication in the first place. The original mammals were small, rat-like creatures that probably hunted by night in order to avoid dinosaurs. Vocal and visual communication would have been too risky, so natural selection started favouring the mammals with the keenest olfaction– and the worst possible smell. This way, unnoticed in the night, the first mammals could communicate with each other, and even when dinosaurs disappeared and mammals could go out in the light of day, conquer Earth and develop new ways to communicate, smell remained an important legacy. Or at least, that’s what I lazily came up with.

And now a dinosaur digression still pertinent to the mammal subject: to the surprise of apparently everyone but me, a recent discovery officially confirmed that small predator dinosaurs were nocturnal. Seriously, people, didn’t you see that coming? Small, agile predators with big eyes, doesn’t that ring a bell (modern nocturnal animals)? Animals and ecosystems may have changed a lot in millions of years, but to occupy the same niche you often need the same general characteristics: it’s called convergent evolution. It’s great that we have the official confirmation, but they should at least have suspected it. Since predator dinos were identified as the main reason mammals would lead a nocturnal lifestyle back in the Mesozoic, now someone is suggesting that, since there were night dinosaurs after all, maybe mammals weren’t nocturnal at all at the time. I think they got it backwards: since mammals evolved to be nocturnal to avoid the diurnal dinosaurs, some dinosaurs started to evolve to be nocturnal to gain access to a food resource that was precluded to others. Once again, look at the modern world: small mammals today are not the same they were in the past, but they share the same basic characteristics: small, agile, omnivorous. And today, in spite of the fact that a lot of predators evolved to capture them at night, small mammals are almost universally nocturnal. Why? Because it is still easier to go unnoticed than it is to go unnoticed by day. Not to mention the other classic arguments in defence of the nocturnal origin of mammals: the mainly dull colours of their fur (they couldn’t use colour to communicate in the dark, while other diurnal animals are often brightly coloured), and the much higher number of night photoreceptors (rod cells) than day photoreceptors (cone cells) in the eyes of even fully diurnal mammals, like us. Considering this, I think that my “smell in the dark” theory is safe, for now.