Eating poop usually ranks near the top of people’s list of most disgusting things. But this isn’t so for the small animals called naked mole rats. A recent study reports that eating the queen’s poop may be one mechanism that causes subordinate females in a group to care for their queen’s children.
Naked mole rats are pink, hairless rodents found in East Africa that live in underground burrows. Their wrinkled skin and two teeth that stick out make them look like tiny walruses. Although they look strange, they are somewhat of superheroes of the animal world. Studies have found they rarely get cancer, can survive without oxygen for up to 18 minutes and don’t seem to age. They live in colonies of 60-80 rats, each ruled by a queen. The queen is the only one in the colony that can bear young, just like in colonies of ants and bees. The subordinate rats do not mate and usually help forage for food, defend the colony and help care for the queen’s offspring.
This behaviour of letting someone else care for your young, called alloparenting, is not unique in the animal kingdom. Subordinate females in wild dogs and wolves that have never borne young sometimes develop enlarged mammary glands and wet-nurse another female’s offspring. Sometimes warthog females that have lost their own young ones suckle piglets from other litters. There is still debate about why such a seemingly altruistic behaviour may have evolved. Some theories suggest it helps kin selection, which allows the reproductive success of one’s relatives, while others think it may have benefits such as practising to be a mother and increased chance of survival of the brood.
This behaviour intrigued Kazutaka Mogi, an animal researcher at Azabu University in Japan. Mogi and his team have been studying this behaviour in mice. In particular, how social interactions and specific hormones help. When they came across similar behaviour in naked mole rats, they were curious to understand how subordinate rats came to care for their queen’s pups.
“It has long been debated whether naked mole rat queens can somehow affect the development of their group members, and if they have the capacity to somehow manipulate the behaviour of others in the group to make them better helpers,” said Markus Zöttl of the University of Cambridge, who studies animal behaviour. However, there is no definite answer yet.
The subordinate females have no mature sex organs nor do they produce gonadal hormones, such as estradiol and progesterone in females and testosterone in males, which responsible for sexual characteristics and reproduction. However, the nipples of the subordinate females have been observed to enlarge after the queen gives birth. So the team wanted to find out what drove their alloparenting behaviour.
“We thought that they must get gonadal hormones from somewhere,” said Mogi. It is known that gonadal hormones can be detected in the faeces of many species, he said, so they decided to focus on the queen’s poop and a curious habit among naked mole rats: to eat each other’s poop.
Though this habit, called coprophagy, may sound revolting, some animals do it routinely. Rabbits and hares do so to better digest the tough plant materials they eat. Insects like flies and the dung beetle eat faeces of herbivores to get to the semi-digested food in it. Sometimes baby elephants, koalas and hippos may eat poop to acquire bacteria needed to properly digest food.
To test their hypothesis, Mogi and co. studied the naked mole rats they bred, housed in plastic boxes joined by transparent tubes. They fed female subordinates faecal pellets from pregnant queens, non-pregnant queens and pellets with estradiol and progesterone. Speakers with recorded pups’ calls were played on one side of tubes and the team measured how long it took the subordinates to enter tubes with pups’ sounds and how long they stayed there.
An analysis of the subordinates’ urine and faecal samples indicated an increase in estradiol concentration during the queen’s pregnancy. Their response to the pups’ calls increased after she gave birth for the rats eating the queen’s faeces or the pellets with the hormones.
What Mogi found surprising was that estradiol, a hormone, was being used as a pheromone. Pheromones are commonly used by insects such as bees and ants for changing the behaviour or physiology of others in the colony. For example, ants use pheromones to lay down a trail for their fellow ants, and the queen bee uses it to prevent worker bees from reproducing. Insects also use pheromones to sound an alarm.
However, alloparenting using hormones has not been observed before. Naked mole rats do not have well-developed odour-sensing organs. In addition, their underground tunnels do not have good ventilation. “Thus, naked mole-rats may have evolved a unique means of communication that does not rely on the odourants, but rather on a hormone through coprophagy,” said Mogi.
“If you eat somebody else’s hormones, [that] it can be active in your body is not surprising and unique,” said Zöttl. But that it might be used to affect another individual’s behaviour could be something new. And the study shows that it is possible that the naked mole rat queen is able to affect her subordinates’ behaviour after they eat her poop. “But the paper does not show whether naked mole rats really do that [in the wild],” he clarified.
Mogi says it has been observed that some females always remain in the nest, where the queen remains for increasingly longer periods during gestation. So it is possible in the wild that these subordinates eat the faeces of the pregnant queen and become alloparents. In addition, other nearby subordinates may eat the faeces of the alloparents and get the hormones themselves.
According to Zöttl, this is something that is really important to find out – but it will not be easy to do because these rats live in a complex maze of underground tunnels. In addition, other questions that need to be answered are if this really leads to the pups receiving more help, and a link between this mechanism and the survival and development of pups.
The study was published on August 27, 2018.
Lakshmi Supriya is a freelance science writer based in Bengaluru.