The Law That Forces Mongooses to Give Birth on the Same Day... Or Else

Plenty of social animals have ways to police each other's behavior — so that if an animal enters the wrong territory or reproduces when it's not supposed to, it's punished. But researchers have found that banded mongooses have a more repressive system, which forces all females to give birth at the same time. Or else.

We talked to the lead researcher behind this study about the reason behind this amazing synchronicity — and what it teaches us about how coercion and policing began.

Threats Versus Punishments

Plenty of social animals police each other's behavior — but unlike human laws and penalties, animal policing doesn't seem to have any kind of preventative function.

Take, for example, honeybees and other insects in the Hymenoptera order. Queens are the only ones in the colony that are supposed to be laying eggs, so when a worker bee "selfishly" lays eggs of her own, the queen and other workers rush to destroy the eggs. This policing behavior serves primarily to reduce the potential damaging effects the extra eggs would have on the fitness of the colony.

Yet, time and again, workers will spit out eggs even when they know the eggs will almost certainly be destroyed, suggesting that policing doesn't act as a deterrent to unwanted behaviors.

Studies have suggested that the levels of policing and transgression in these animals are genetically "hard-wired." But in a system like ours, which works via threats, the levels of policing and transgression are not so predictable, because they depend on each individual's reading of the situation — that is, people will generally try to break the law only if they think they won't get caught.

"Evidence for insects being socially responsive and evaluating whether they can sneak an egg in is not well supported," Michael Cant, an evolutionary biologist at the University of Exeter in the UK, tells io9.

But we do know that some animals can evaluate their social situations and adjust their behaviors accordingly — so some scientists have suspected that certain animal societies do police using threats, rather than just imposing consequences after the fact.

But there's an inherent difficulty in proving that animals do this. To reveal that such threats exist, researchers would need to be able to do an experiment where they shift the status quo, and break the rules underlying the threats.

And Cant and his colleagues learned that they could do just that with banded mongooses (Mungos mungo).

A Peculiar Reproductive System

In Uganda, banded mongooses live in mixed-sex groups of 20 adults, which breed about four times a year. Older, dominant females run these groups and rule over several younger subordinate females.

"There is no aggression between the females, but we call them dominant and subordinate because the older females will throw out the younger females when the group gets too large," Cant tells io9.

In a previous study, Cant and his team discovered that banded mongooses have a very peculiar reproductive system, in which the females will give birth to a communal litter, usually on the same day. This seemingly egalitarian society is quite strange from a reproductive stand point — if a female really wanted her pups to come out on top, she'd give birth first, so that her kids are older and more fit than the rest of the litter.

But upon closer inspection, the researchers found something even odder. When females do give birth on different days, the pups born earliest almost always die within the first few days. Turns out, the other females will slaughter the first-born pups.

The researchers hypothesized that the females synchronize their births to escape from this threat of infanticide. The idea is that the when all the pups are born around the same time, murderous females can no longer use temporal or spatial cues to maternity, so they don't kill any of the pups. "Only when the females can be certain that the litter does not contain any of her pups will they kill the litter," Cant explained.

To test this theory, Cant and his colleagues decided to blow the system all to hell.

The Law That Forces Mongooses to Give Birth on the Same Day... Or Else

Messing With the System

Using contraceptive treatments (hormone injections), the team altered who could and could not give birth in a breeding period.

In their first experiment, they stopped all of the subordinate females from breeding and only allowed the dominant females to have pups (at the same time). They found that all of the pups survived their first week of life, showing that subordinate females don't commit infanticide. In the second experiment, they stopped all subordinate and all but one dominant female from breeding. In this case, her litter died within a couple of days — the other dominant females killed her pups.

In the final experiment, the researchers switched off the breeding for all of the dominant females. "It was a disaster for the litter," Cant said. That is, all of the subordinate females' pups died within the first couple of days. What's more, few of the subordinate females even dared trying to reproduce in the next breeding season. The threat hit home, and they understood what would happen if they broke the law.

The results suggest that the banded mongooses live in a system based on a very simple and scary threat imposed by dominant females: "Do not have children, or I will kill them." To escape this threat, females synchronize their births to slip their own offspring safely into the communal litter.

Cant thinks that this type of coercive social system may be more widespread in social animals than people believe. Additionally, the role of threats in enforcing cooperation in animal societies has probably been underestimated quite a lot — especially in societies that seem egalitarian or harmonious on the surface.

Presumably, scientists haven't detected these interactions before, because the mechanisms of coercion and control may be far subtler than anyone realized.

"It's a bit like if you were an alien observer to our planet," Cant said. "It may take a long time to realize that our system is ruled by threats, because you would only see it when an individual breaks the law."

Check out the study in the journal PNAS.

Top image via Andy Young. Inset image via Steve Slater/Flickr.