Rodents and primates are periodically cited as some of the more intelligent animals on the planet, but it turns out that the large brains that these mammals possess have evolved more than once in their history. (Image Credit: Drow male, Arjan Haverkamp, ltshears, CC BY-SA 4.0, Image Cropped)

Encephalization and longevity evolved in acorrelated fashion in Euarchontoglires but not in other mammals (2018) DeCasien, Alex R., Evolution, DOI: doi:10.1111/evo.13633

The Crux

Some of the most striking footage from documentaries like the recent “Blue Planet II” involve organisms that display remarkable intelligence (the octopus that uses shells to disguise itself and hide from its shark predators was a particular favorite of mine). As humans, we sometimes assume that we have the best brains on the planet and have somewhat of a monopoly on intelligence, so it’s always fascinating and maybe even surprising to see other animals using their own brains to solve problems. In mammals, brains that are larger than expected have evolved more than once, which is somewhat of a surprise given how costly a big brain is. For example, your brain needs 20% of the oxygen that your body uses, so one out of every five breaths is exclusively for your brain.

Larger brains are also correlated with longer lives, relative to the group that the organism in question belongs to. Historically, studies on brain size and longevity have been dominated by primate species, so the concern was that this long life/large brain trend may only be a primate trend, instead of generalizable to all mammals. The authors of this study wanted to analyze this trend across more mammal groups, in addition to studying the relationship between larger brains and longer lives.

What They Did

The authors collected data on life span, brain weight, and body weight for over 600 mammal species, including carnivores, ungulates, whales and dolphins, rodents, tree shrews, primates, and others. As covered in a previous post, phylogenetic studies such as this one involve building multiple trees, and then considering which one is the “best” using a variety of statistical methods. Sharp-eyed readers may be thinking “how can you consider body weight and brain size when larger, heavier organisms tend to have larger brains relative to smaller, lighter organisms?” The authors took this into account, and used what’s called the “residual” of a regression. Basically what that does is control for body size, so this correlation wasn’t an issue in the analysis.

The first aim was to test for a correlation between large brains and longer lives, because although this is a pattern scientists have noted in the past, there may not be evidence for it in the data. The second aim was to examine the direction of the relationship, when such a correlation was found. For example, did a larger brain lead to a longer life? Or vice-versa? In order to figure out the direction of the relationship the authors used transition matrices, mathematical modeling that assigns likelihood values to certain transitions (long life to short life, large brain to small brain, and all the other combinations) and then computes the most likely transition.

Did You Know: Corvid Intelligence Although animals seem to make up the majority of large brained, longer lived organisms, another group of exceptionally intelligent animals are the corvids, the bird group that includes ravens, crows, magpies, and rooks. These birds not only use tools to get to their food (such as dropping snails or nuts in front of a bus, waiting for the bus to run it over, and then eating the juicy inside bits after the bus is gone), but they also have an incredible memory and facial recognition skills. Crows have been known to remember individual people (and enact retribution on humans they perceive as threatening or hostile), in addition to trading with or even rewarding humans for good behavior. Start a relationship with a crow by feeding it, and you may end up with a friend for life that will bring you shiny objects as a thank you.

What Did They Find Out?

Despite substantial variation with regard to brain size, body size, and lifespan, only the rodent and primate groups displayed correlations between larger brains and longer lives. For the second aim of this study, both rodents and primates exhibited various transitions between the small-bodied, small-brained and large-bodied, large-brained states. Despite this flexibility, the two groups had only one significant pathway each.

Interestingly enough, both primates and rodents showed different evolutionary histories to get to the large-bodied, large brained-state they are in today. Primates that are long-lived with large brains would have evolved from shorter-lived animals that already had large brains. Conversely, long-lived rodents with large brains evolved from smaller-brained organisms that already had long lives.

Problems?

When collecting data on average life span and maximum life span, the authors included data collected from animals both in captivity and in the wild. Animals living in captivity, such as those in a zoo, tend to live longer than their wild counterparts. This means that the data on life span may have been inflated for some species, and since zoos are a relatively new phenomenon when it comes to evolutionary history, these larger numbers of captive animals don’t hold much sway over the evolution of the other animals and should not have been taken into account.

So What?

This paper has shown that, although longer lives and larger brains are in fact common in mammalian species, they are only common in two groups. Not surprisingly primates are one of those groups, but it may come as a surprise to some that rodents are the other group.

The results of this paper represent a cautionary tale for research, in that one must be cautious of applying a general trend to a group that includes various disparate groups of organisms. The authors highlight that because most studies investigating longevity and brain size are mostly made up of rodents and primates, it isn’t surprising that these studies consistently find the long life/large brain correlation. Results like those in this study highlight the necessity of not only looking at the entire group, but running analyses on the subgroups themselves.