I’ll wager that mole rats are (apart from humans) some of the weirdest mammals on which you’ll ever lay eyes. Nevertheless, these odd little rodents have a lot to teach us about the evolution of our own tendency for cooperation or sense of morality. How? Let me back up a bit…
The other day, my wife and I were enjoying a pleasant stroll through a Tennessee riverside park. Searching for a sufficiently large stick to serve in our dog’s favourite game of save-the-log-from-drowning, I overturned a promising branch, and found its underside crawling with small black ants and fat white termites. Immediately, the science geek in me exclaimed, “Cooool!” and my wife was immediately subjected to a short but enthusiastic rant about eusocial insects. Eusociality is a term used to describe the highest level of social organization in animals, in which societies are arranged into well-developed hierarchies, often with a single reproductive pair and a large number of workers. These societies are wonderfully complex and integrated. Have you ever squinted at a swarming ant nest and imagined it to be a single large creature, spreading amoeba-like tentacles to draw in food particles? Have you noticed how coordinated is the protective response to intruders such as the stick with which you poked at the nest? But what does this have to do with desert rodents? Or human morality, our apparent ability to distinguish between “right” and “wrong”?
We all know that ants, bees, wasps and termites show an unusually high degree of social organization. But how many of us know that there’s a group of mammals that have similarly complex societies? No, I’m not thinking of our own humble species this time. Enter the mole rats, a group of about a dozen species of small, burrowing sub-Saharan African rodents with big incisor teeth that are located practically outside of their mouth so that they can dig with them without having to eat dirt. Their eyes are tiny, nearly vestigial organs, for vision is not a great asset in dark burrows. Cool adaptations to subterranean life.
But the really neat thing is that two of the species, the Damaraland mole rat (Cryptomys damarensis) and the naked mole rat (Heterocephalus glaber), have eusocial societies remarkably similar to those of ants. Now this is REALLY cool. Naked mole rats are the weirdest of the two, and have the most ant-like colonies. They’re also the downright ugliest. As their name suggests, they are practically hairless, and they look for all the world like a cross between a newborn mouse and a sharpei dog. Although they are mammals, they are incapable of regulating their body temperature physiologically the way we do, so in order to warm up, they huddle together in warm parts of their burrows.
Naked mole rats live in groups of around six or seven dozen finger-sized individuals, most of which are sterile workers in two size classes: smaller ones mainly gather food (tubers) and dig burrows (spanning more than a mile in cumulative length); the larger ones fill the protective role of soldiers, again just like in ant nests. There’s even a queen, a single enlarged female whose role it is to breed with two or three males and produce copious workers for the colony. And, just like ants, these workers have a very low genetic diversity — they are extremely closely related, like what you get in a community with extensive inbreeding. In fact, in light of this fact, it’s a marvel that they are so devoid of genetic disorders that result from inbreeding. On the contrary, naked mole rats hold the title of the longest-lived rodents, with a life-span of 28 years! Their unusually stable genetic makeup is currently being investigated; the humble naked mole-rat is the latest addition to the list of animals whose genes have been fully sequenced.
What could have driven a mammal to develop a complex eusocial structure so similar to distantly related ants? The answer actually embodies a nice example of the predictive power of evolutionary theory. Prior to the discovery of naked mole rats, a biologist by the name of Richard Alexander used ecological principles to predict that a hypothetical eusocial mammal should evolve from a colonial subterranean rodent exploiting a widely distributed food source from an expandable underground nest . His description nearly perfectly matched the nature of the eusocial naked mole-rats that were discovered soon thereafter.
What spurred Alexander to make such a prediction? Recall the literary description of wild nature as “red in tooth and claw”. This adage alludes to the selfishness that is predicted to evolve in creatures by natural selection. The better an individual can compete for resources, the more resources it can acquire and the higher will be its fitness. If the traits that lead to this increased competitiveness are genetically encoded, then they may be passed on to the creature’s offspring. Its greater access to resources means that it can afford to produce more offspring. This will lead to an increase in the proportion of ‘competition-enhancing’ genes in the population from one generation to the next.
Unfortunately, there’s an all-too-popular misconception that continues to circulate: it’s the idea that increased competitiveness can only arise from the selfishness of individuals. This oversimplified interpretation has led to the erroneous belief (usually by proponents of creationism or intelligent design) that evolution can’t explain the existence of cooperation or the apparent presence of a “moral code” in humans. How could just about every human society on earth possess an innate knowledge of what constitutes right and wrong, if it wasn’t instilled in us by a creator? Since many animals do not hesitate to kill each other (also a bit of an oversimplification), why do we seem to realize that killing or stealing is wrong? Surely this is evidence that these altruistic tendencies were bestowed on us divinely, right?
Mole rats, had they the facility of speech, would beg to differ. They (and myriad other animals) demonstrate that a species can achieve competitiveness by the evolution of cooperation rather than selfish fighting among individuals, essentially extending the unit of selfishness to the group rather than the individual. (Remember that squinty-eyed view of the ant nest as an amoeba? Yes, think along the lines of a superorganism, and you’re on the right track.) The key factors in the case of mole-rats turn out to be (1) relatedness and (2) the distribution of food sources. (Keep these factors in mind for human evolution too…)
Remember that the mole rats in a colony are extremely closely related to each other. Their colonies are large families consisting of many, many siblings, just like ants. Now, aside from the occasional sibling rivalry that crops up in families, brothers and sisters tend to get along quite well, and usually favour each other over strangers when it comes to food and fortune. Why? Ecologists explain this by pointing out that siblings share a large proportion of their genes, for they all acquired their genetic material from the same two parents. Thus, if you help your brother or sister to gather resources (and therefore to better afford to have kids), you are helping to pass along a portion of your own genetic code almost as well as if you have children of your own. Exercising a set of strategies that help one’s relatives survive (even at a small cost to one’s own fitness) is referred to as “kin selection”. So really, evolutionary theory predicts that a closely related group should express some cooperative behaviour to each other. And, the more similar the genetic makeup of individuals, the more likely they are to promote the survival of their own genes by helping each other; so the closer the familial relationship, the higher the predicted degree of cooperation. (We tend to help our brothers and sisters more readily than our second cousins.) The extremely low genetic diversity among mole rats helps to explain their excessively high cooperative behaviour (remember, the workers are sterile, foregoing reproduction entirely in order to gather food and maintain the burrows).
OK, so why live in family groups at all? If selfish individuals are just as likely to succeed as selfish family groups, then why should individuals band together at all? After all, cooperation does require one to give up some of one’s own resources. Well, have you observed how families stick together most strongly in the face of adversity or lean times? Mole rat societies demonstrate wonderfully well how a patchy distribution of food can stimulate cooperative behaviour. Naked mole rats live in arid regions of Africa that receive little rain. They feed on tubers (enlarged portions of roots that are rich in nutrients, serving as storage structures for plants in harsh environments), which are widely spaced due to the paucity of nutrients and water for plant growth in these deserts. Although a single tuber could last an individual mole rat for a long time, a single mole rat would have a very hard time finding another tuber before succumbing to hunger and death. It would be very advantageous for this mole rat to be able to be in many places at once while searching for its next meal in order to cover more ground. The best way for it to do this is to make many copies of itself and share the burden of the search among its copies. That’s very close to what naked mole rats do. They set up cohesive colonies and produce a lot of extremely closely related offspring that can broaden the search and increase the chance of finding tubers. Kin selection then encourages them to cooperate by sharing their finds rather than keeping it for themselves. In some ways, then, you can almost see the closely knit colony as a “superorganism”, a kind of extension of the individual to increase survival by spreading the workload to overcome some of the environment’s unpredictability.
So then, harsh environments with scarce and difficult-to-find resources can promote the emergence of cooperative groups, and kin selection helps to increase the cohesive bond of these groups. This phenomenon is taken to the extreme in mole rats, but how does it relate to human evolution and the rise of our cooperative tendencies or our apparently innate sense of right and wrong? The following ideas have been presented by many investigators prior to this explanation, but the argument goes something like this:
Like mole rats, our ancestors also arose in Africa. The arid savannah presented an environment in which food was widely dispersed, giving groups of hominids a better chance of survival than solitary individuals . Group living also made more eyes and ears available for detection of predators, further increasing chances for survival through cooperative vigilance and alarm-calling. It became evolutionarily advantageous for early hominids to exhibit cooperative behaviour, and for the group to frown on behaviour that was destructive to this strategy (e.g. killing others of the group, stealing resources, etc.). Interestingly, these are the very same strategies that make up what many of us refer to as human morality. Groups exhibiting more extensive cooperation would have had a better chance of surviving, and would therefore have passed on their genes more numerously to the next generation. Thus any genes that promoted altruistic behaviour would have increased in proportion from one generation to the next, setting the basis for the evolution of behaviours that we label as morally right.
So there we have it. From mole rats to morality. I have certainly skipped over many details in human behavioural evolution, including the influence of society on behaviour in non-genetic ways. I’ve also left out many mechanisms by which cooperation can evolve, either within or between species, and the evolution and persistence of cheating, but those are the subjects of another fascinating story… For now, my intent was to briefly describe one plausible explanation for the emergence of cooperative behaviour in humans by invoking natural selection and by comparison to the mole rat model. Who knew that mole rats had so much to teach us of the evolutionary routes taken by our own ancestors?
1. PW Sherman, JUM Jarvis and RD Alexander. (1991) The Biology of the Naked Mole-rat, Princeton University Press, Princeton, New Jersey
2. Jeffrey A. Kurland and Stephen J. Beckerman. (2009) Optimal Foraging and Hominid Evolution: Labor and Reciprocity (pages 73–93) DOI: 10.1525/aa.1985.87.1.02a00070
Evolution of cooperation and prosociality