PAUL SHERMAN: I've studied the animal that I'm going to talk to you about today, the naked mole rat, for 29 years. Now, let me start by showing you a naked mole rat. I have one right here. This is a female. This is a 27-year-old female. And I want you to take a good look at this little animal to see her remarkable physical features.
The first thing you notice is she's almost hairless, hence, the name naked mole rat, or scientifically, heterocephalus, other headed, as you'll see the head, and naked, no hair. She has tiny little indications of eyes. She can't see out of them, but she can see light from dark. She has no external ear pinna, just little holes for the ears.
And she has a very loose skin. You can see how loose it is. And if I hold her by the back, she can actually flip around, and suddenly, I'm holding her stomach.
Now, what do all of these unusual physical features have to do with? Naked mole rats live in Kenya, Ethiopia, and Somalia in Africa, and totally under the ground. They never come above the ground. The only time they open their burrow systems is to eject soil that they've dug while burrowing.
That far under the ground in eastern Africa, the climate is warm year round. The animals don't need hair to thermoregulate. Since they live in totally closed subterranean tunnels, they don't need eyes to see. Their ears, if they had pinna, would pull against the sides of the tunnels. And these animals can run as fast backwards as forwards because of their aerodynamic body design. They can also squeeze through tiny little cracks in tunnel systems, and little holes underground.
So our research here at Cornell has been to understand and discover how the colony works. We house them in plexiglass tunnel systems and under conditions which are as close to subterranean Kenya as possible. That is, we're trying to make them feel comfortable, as if they were back home in nature.
And we do this by keeping them in warm, dark, quiet, humid rooms in tunnel systems that are the same diameter as their tunnels in nature, and with nest boxes that are the same size as their nest boxes in nature. We also feed them tubers, and we ask them to go and get those tubers and bring them back to the nest as they would in nature.
I've been particularly interested in these little animals not only because of their unusual looks and physiology, but also because of their remarkable social structure. These animals live in vast colonies under the ground. They never are found alone. The colonies can be up to 300 animals. The average colony size is about 80.
And even more remarkably, within these colonies, there is only a single reproductive female and one or two reproductive males. All of the other males and females in the colony are workers, that is, they are non breeders, and they work to extend the colony system, find food, and help the reproductives rear more young.
Now, you're probably thinking, gee, that sounds a lot like bees, and ants, and wasps. And that's exactly what is so interesting about these animals. Here, we have a mammal which has a convergent that is similar evolutionarily social system to bees, wasps, and ants, which are phylogenetically of course, very far removed from these little animals.
The interesting thing is that the complex social systems of the social insects are therefore not unique to insects per se, but rather, have something to do with a shared ecology that these mole rats are sharing with the social insects. Now, what could that be? Our studies in Kenya have shown that the animals live in extremely hard soil, so hard you can hardly dig it with a pick.
They dig with their front teeth, unlike our north American gophers and moles, which dig with their paws. These animals dig with their teeth. And their lips close behind their teeth to keep the dirt out of their mouths when they're digging.
They dig in groups almost like a moving conveyor belt, with a single animal at the front digging the dirt, a single animal at the back kicking the dirt out the volcano hole, and all the other individuals, sometimes 15 or 20 in a line between them picking up soil from the digger, kicking it behind them as they go backward toward the opening, with the other animals straddling them above and moving forward. They feed on widely separated very large tubers sometimes the size of pumpkins, sometimes, almost the size of bushel baskets.
In order to find these tubers, the animals burrow blindly, that is, the tubers don't give off any chemical smells, of course, cannot be seen. So the animals burrow blindly. But the tubers are far enough apart and the soil is hard enough that a single mole rat trying to find them alone would expire before it found the next one.
By living in a colony, these animals-- and digging cooperatively-- these animals can find these bonanza tubers. And once they find one, they have enough food to last the colony for up to a month. Then they continue burrowing to find another one. But if they had to do it alone, they would never make it before they ran out of food completely.
Another aspect of their group living is that they defend against predators, especially predatory snakes in groups. The snakes attempt to enter the burrow systems through the volcano holes that the mole rats open to eject the loose soil. But when a snake does enter, the mole rats attack it en masse and are able to dispatch even the most dangerous poisonous snakes that live in the area by group attacking and biting with these very large incisors.
This group digging for the tubers and group defense against predators is apparently the basis of the mole rat sociality. And since they're all one large family, just as the bees, and wasps, and ants are, it is not as much of a puzzle why certain individuals would forgo reproduction. They are simply helping raise little brothers and sisters.
The queen mallrat is quite a remarkable animal too. This is a female, but she's not pregnant right now. But if we had a pregnant female here, she would be much larger and longer. When a queen mallrat becomes the breeder, her body length increases by about 30% as she adds bone to her vertebral column.
This enables her to carry very large litters. An average litter of mallrats is about 18 pups, and that's pretty big for an animal the size of a of a gerbil. But the largest we've had born here at Cornell was 33 pups in a single litter.
Now, how could she possibly feed all those young? Well, she has only 16 to 18 nipples. But since the queen's needs are provided for by the rest of the colony, she can stay in the nest all day long and do nothing but nurse. She is cleaned, groomed, and fed by other colony members bringing the food in. And so therefore, she can lie in the nest and feed the pups. Each pup nurses as long as necessary, drops off, another pup comes on.
But she doesn't have to do what other mammalian mothers do, go out and forage. defend against predators, et cetera. She can stay right there, and therefore, she can raise extremely large litters successfully. All of this is because of the cooperation of the non-breeding workers, her sons, and daughters from previous litters who are helping rear their brothers and sisters in the current litter.
And for many years, 100 of years, we thought that the social insects were somehow unique in this regard, that they lived in these large colonies with single breeding females. And we thought it was something about insect phylogeny, or insect ecology, something special. The discovery of the naked mole rat, which I think is the sociobiological find of the 20th century, is these social systems are similar in mammals and insects. And it's not just something associated with being an insect, but rather, something associated with ecology, that is, these ecological factors that force the animals to work together to find food and kin selection, the idea of being a family working together for the benefit of one's own genes as represented in the mother, and then in brothers and sisters.
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Naked mole-rats are native to Kenya, Ethiopia and Somalia. They live in extensive underground tunnel systems and subsist on large tubers which they find by burrowing. Though a colony consists of up to several hundred individuals, only one female and a few males reproduce--the rest of the animals support the colony. As Cornell professor Paul Sherman explains, this is remarkably similar to the societies of the social insects like bees and wasps.
Sherman's research focuses on the fragile balance between cooperation and conflict in various mammalian and avian societies.