Most animal behaviours belong to one of four categories, the four ‘F’s: feeding, fighting, fleeing, and…reproduction. Of these, probably the most critical in terms of natural selection is the last. If an animal doesn’t reproduce, and therefore fails to pass on its genes, it becomes an evolutionary dead-end. Richard Dawkins espoused the most extreme version of this view in The Selfish Gene (back when he wrote about science). He wrote: “we are survival machines – robot vehicles blindly programmed to preserve the selfish molecules known as genes.” According to a gene-centric view of evolution, animals are elaborate machines constructed for the purpose of passing on genes – a virus is a fairly simple machine, a rhinoceros is a little more complicated. The end goal is always reproduction.

It’s even in textbooks.

But what about species where individuals forgo their own reproduction in order to help another individual raise their offspring? This behaviour, called cooperative breeding, is a hallmark of certain animal societies – including one of the most ugly, the naked mole-rat (Heterocephalus glaber).

Why, hello there.

The naked mole-rat is an African rodent that lives underground, in colonies of up to 300 (but averaging around 80). Their burrows are large, with tunnel systems that can range over kilometres, and most of them will live their entire lives without ever setting foot on the surface. Other than the colony size (and that distinctive visage), these traits don’t particularly separate them from other subterranean rodents. What makes them unique is that they are the only truly eusocial mammal.

Schematic of a naked mole-rat colony.

Eusociality is a particular type of social organization expressed by a few groups of animals, most notably the Hymenoptera – ants, bees, and wasps. Most readers will be familiar with it at least in passing. Eusocial organisms live in large colonies, hives, or nests and are organized like a despotic monarchy.

The hierarchy of a naked mole-rat colony.

In eusocial societies, one individual – the queen – reproduces. The entire colony is composed of her children, which are kept sterile either by the nature of their biology, or by hormonal suppression. Her children develop into different castes – workers, soldiers, gatherers, etc – that spend their entire lives selflessly serving the colony. They never reproduce.

Naked mole-rats are characteristic eusocial societies. A mole-rat colony has one queen, who reproduces with two-to-three males. The rest of the colony, both males and females alike, are sterile. The sterile individuals are divided into two castes. Workers are smaller, and have large front teeth, for expanding the burrow. Soldiers are bigger and are used for defence – against snakes who hunt in the burrows, and in the event they encounter a different colony. The majority of naked mole-rats will live and die without ever reproducing.

For a long time, biologists had a difficult time explaining this behaviour. If reproduction were the ultimate ‘goal’ of evolution, why would natural selection lead to a species where 99% of the individuals die without pass on their genes?

The answer is kin-selection. In the 1930s, the biologist JBS Haldane said, “I would lay down my life for two brothers or eight cousins.” He was, jokingly, acknowledging the degree to which we share relatedness with close family. My sister and I share 50% of our genes by descent. My cousins and I share 12.5% of our genes. If Haldane died, but saved his two brothers, each 50% identical to him, the evolutionary pay-off is equal to what it would be if he lived, but they both died.

In the 1960s, the biologist WD Hamilton formalized this general understanding into a mathematical rule:

rB > C

In Hamilton’s rule, ‘r’ is the relatedness between two individuals (0.5, in the case of full siblings). B is the reproductive benefit gained by the relative if you help them raise their offspring, and C is the evolutionary cost you incur by not reproducing yourself. If rB is greater than C, it makes more evolutionary sense to forgo your own reproduction and help your family raise their offspring – the shared genes that are passed on will be more than the genes you can pass on yourself.

Eusociality: the solution when everyone is too damn ugly to mate with. Credit: Jonathan Rosenberg.

This is the foundation for eusocial societies. Ants achieve high levels of relatedness via a quirk in their biology that causes sisters to be more related to one another then they are to their own offspring. But naked mole-rats have to achieve this high level of relatedness the old-fashioned way: they inbreed.

Naked mole-rats are the hillbillies of the animal world. They have the highest recorded level of inbreeding in any known animal species. Even half-siblings in a naked mole-rat colony are likely to share 45% of their genes – and full-siblings can share close to 75% of their genetic material. When you’re that related to one another, and one of your brothers is getting lucky with the queen (who also happens to be your mother) it’s okay if you don’t reproduce. And maybe it’s better that you don’t because lets be honest – it’s weird.

For naked mole-rats, inbreeding is compounded by the difficult environment that they live in. They eat roots and tubers, but live in arid habitats where food is concentrated in small patches. Even if a revolutionary mole-rat demanded freedom, it would have nowhere to go – dispersing to try and start a new colony would likely be fatal. Instead they stay at home, and try and fight their way up the ranks.

Considering they’re an ugly little thing, smaller than a teacup, naked mole-rats are surprisingly vicious. Like hyenas, they have a linear dominance hierarchy. Every female naked mole-rat has a position on the totem pole, determined by their aggression and testosterone levels. The higher, the better, because it increases the chance you’ll succeed to the throne when the old queen dies (or is killed. Regicide is not uncommon.).

When queens die, the highest ranked females fight tooth-and-claw to succeed her. Often the battles are fatal, but eventually a new queen is crowned. The new queens body undergoes physiological changes, triggered by a rush of victory hormones. Her vertebrae elongate, stretching her spine, and increasing the size of her uterus to prepare her for bearing young. And they bear a lot of young. Naked mole-rats have one of the highest litter sizes of any rodent, popping out up to 24 individual pups every 80 days.

The stretched lumbar vertebrae of a mole-rat queen.

Oh, and baby naked mole-rats eat feces while they’re young. Yeah, they’re kind of strange animals.

Most babies are cute. These are not.

The queen rules the colony with an iron fist. Any female that begins to behave like a queen, or to produce hormones that may lead to reproduction, is ruthlessly attacked. She may rule for decades, they can live for up to 30 years, and almost nothing will slow them down. Naked mole-rats are virtually immune to cancer. They don’t suffer from age-induced damage and stress like other animals do. They don’t even feel pain. Be glad that they’re so small (and maybe too inbred to do anything smart). Otherwise we’d likely be spending our days slaving away for our mole-rat overlords.

Neil Griffin

Literature Cited

Burda H, Honeycutt RL, Begall S, Locker-Grutjen O, Scharff A. 2000. Are naked and common mole-rats eusocial and if so, why? Behavioral Ecology and Sociobiology 47: 293-303.

Clarke FM and CG Faulkes. 1997. Dominance and queen succession in captive colonies of the eusocial naked mole-rat, Heterocephalus glaber. Proc. R. Soc. Lond. B 264: 993-1000.

Dawkins, R. 1976. The Selfish Gene. New York City: Oxford University Press

Jarvis, JU. 1981. Eusociality in a mammal: Cooperative breeding in naked mole-rat colonies. Science 212: 571-573.

Reeve HK. 1992. Queen activation of lazy workers in colonies of the eusocial naked mole-rat. Nature 358: 147-149.

Reeve HK, Westneat DF, Noon WA, Sherman PW and CF Aquadro. 1990. DNA “fingerprinting” reveals high levels of inbreeding in colonies of the eusocial naked mole-rat. PNAS 87: 2496-2500.