Heating, Eating, and Thinking

This is the fifth in a series of posts on my book, The Four Realms of Existence. Read posts one, two, three, and four.

Source: Harvard University Press/used with permission.

The key premise of my book is that everything about a human being, biologically and psychologically, can be subsumed within four fundamental realms of existence: the biological, neurobiological, cognitive, and conscious.

In writing about the cognitive realm, I came across a fascinating theory about why mammals and birds have cognitive capacities that seem to be lacking in other vertebrates and invertebrates.

The story starts with the divergence of the ancestors of mammals and reptiles from their common ancestor around 300 million years ago. Early mammals evolved a new feature lacking in both their vertebrate ancestors and in reptiles—legs positioned directly under their trunks that could move parallel to their vertebral column. This allowed them to breathe and run simultaneously when escaping predators or capturing prey. But this adaptation required an uptick in metabolic energy expenditure, which meant they had to consume more food and take in more oxygen in order to make more energy through metabolism.

Higher metabolic activity, in turn, allowed them to maintain body temperature internally. That is, mammals were warm-blooded, or endothermic. Other vertebrates and invertebrates are, for the most part, cold-blooded, or ectothermic, as they use external sources to control body temperature (for example, reptiles use sunlight and shade). A few non-mammalian vertebrates (some snakes and fish) and some invertebrates (insects such as bees) switch from exothermic to endothermic responses in some situations and/or in some parts of their bodies. Only mammals and birds are full-time, full-body, endotherms.

Source: eva_mask/Shutterstock

So what? Well, according to the theory, to obtain the amount of food required to support high metabolic rates and endothermy necessitated that more time be spent foraging, which itself requires energy expenditure. What made this radical change worth the effort? The hypothesis is that advantages accrued to animals that could make plans about when (time of year and time of day) and where (locally and distally) to forage for what (perishable or durable food) in relation to predictions about how much energy was worth being expended and about the prospects for obtaining more energy resources in the near- and long-term. In other words, planning, predicting, and deciding capacities may have arisen symbiotically with endothermy in mammals.

If this is correct, it would help explain why evidence of robust cognition is difficult to obtain in non-mammalian vertebrates and invertebrates. But there is one exception. Besides mammals, the only other true endothermic animals are birds, which are also the only animals that rival mammals, including primates, in cognitive capacities.

Birds evolved from cold-blooded reptiles. Because there is no known warm-blooded common ancestor of mammals and birds, they must have evolved endothermy and cognition independently. Yet, while all birds are warm-blooded, not all have sophisticated cognition. Consequently, it is likely a combination of warm-bloodedness and foraging pressures that resulted in the evolution of the capacity to plan, predict, and decide in mammals and certain birds known as corvids.