What Happens When Mongooses Don't Know Their Babies

When females in banded mongoose groups give birth, they all do it at the same time in an underground den. The interesting result is that none of the parents know which pups belong to them.

This creates a fair society based on what researchers call a "veil of ignorance," a new study finds. In this case, that means they give care to the babies based on which ones need it most, not based on which ones are related to them.

In order to test this theory, researchers gave extra food to half of the pregnant mothers in groups of wild banded mongooses so that their pups would be larger than those born to the rest of the mothers.

“To create the imbalance we fed half of the pregnant females 50 grams of egg a day (approximately a 33% increase in their daily energy intake) whilst leaving the other half of the pregnant females unfed,” lead author Harry Marshall of the University of Roehampton in the United Kingdom tells Treehugger.

“Once the pups were born and moving with the group, the females who we had fed during pregnancy directed more of their care towards the pups of the unfed mothers. These pups from unfed mothers were smaller than the pups from fed mothers initially, but the extra care they received meant they caught up by the end of the care period.”

This is much different than is typical in nature, where most mothers and fathers favor their own young.

“In some social species, offspring will be cared for by adults who are not their parents—these are known as cooperative breeders. However, in these cooperatively breeding species it is usually the case that only one dominant pair breed and all other members of the group act as helpers,” senior author Michael Cant of the University of Exeter in the U.K. tells Treehugger.

This helping behavior is not selfless, he points out. Helpers benefit personally because they are related to the babies in some way or are able to stay as part of the group until they are able to breed themselves.

“Similarly, our study shows that fed mothers directing their care towards the pups of unfed mothers is not selfless but is the best strategy to increase their own personal gains. This is because they do not know whose pup is whose, so they care for the smaller pups in case they are their own.”

Understanding Synchronized Birth

In earlier work, the researchers observed that there's a reason that pregnant females in a group nearly always give birth on the same night.

"Previous work on our study population (The Banded Mongoose Research Project) has shown that when females don’t give birth in synchrony like this then the resulting litter is much more likely to fail," Marshall says

In particular, some previous work that Cant led showed that older, dominant females controlled the timing of birth.

"The reason for this synchrony seems to be that if a female gives birth too early then other females will know these pups are not theirs (since they are still pregnant). These pregnant females will then try to kill the new pups as they would compete with their unborn pups," Cant says.

"However, if a female gives birth too late then their pups are less developed than their older littermates and so will be at a disadvantage when they compete for adult carers (called ‘escorts’) when the litter emerges from the den at around 30 days old. The resulting pushes to be neither too early or too late produces the extreme synchrony where all females give birth on the same night."

The Benefits of Impartiality

For the new study, researchers examined seven groups of banded mongooses in Uganda. They predicted that this “veil of ignorance'' would cause new mothers to direct additional care on pups that needed it most.

And that’s what they found. The results were published in the journal Nature Communications.

“We were really pleased there was a good match between our data and our theoretical model about how parental care under such a veil of ignorance over parentage should be distributed,” Marshall says.

“However, we could have equally imagined it going the other way—so that pups that had the best start in life went on to receive more care, amplifying the initial disparities in weight. The fact that we found the opposite confirms that the veil exists—it’s the only plausible reason why females would allocate extra help to the most needy.”

This impartiality helps level those initial size disparities and equalized the chance that pups would survive to adulthood. This benefits all the pups, including their own.

“It shows for the first time that the veil of ignorance operates in essentially the same way to achieve fairness in both human and a non-human animal society,” Cant says. “It is confirmation that, from behind a veil of ignorance, self-interested agents make decisions for the good of society since, being a member of this society, these decisions also benefit them personally.”