Meerkats Mysteriously Know to Outgrow Rivals

For meerkats, being just a tad larger could mean living longer and getting the chance to mate.

Meerkats are icons of cooperation. These mask-eyed mongooses, so beloved of natural history documentaries, live in colonies of up to 50 that groom each other, keep watch for predators, fend off invading snakes, and babysit each other’s young.

But their societies are also marked by competition and inequality. In any colony, almost all the sex takes place between one pair of dominant individuals. Their rank, as well as that of all their subordinates, is determined by their age and weight. Females either wait for the dominant matriarch to die or try to displace her, whereupon the next oldest and heaviest individual takes her spot. Males try to displace the heads of other groups, but the result is the same: The biggest challenger takes over from the deposed.

So for meerkats, size is fate. A few extra centimetres or pounds can make the difference between siring the next generation and going unnoticed by history. This means that individuals continuously size up their peers. They, yes, compare the meerkats as in the viral advertising campaign.

Elise Huchard from the University of Cambridge demonstrated this by selectively feeding certain meerkats in a colony. She found that individuals notice if their competitors are nudging ahead in the great social race, and shift their own growth up a gear.

Other scientists had seen signs of similar growth leagues in other species. For example, in coral gobies (a kind of social fish), subordinates will go on diets to stop themselves from approaching dominant individuals in size; that way, they avoid conflict. “We wondered if similar processes operate in mammal societies,” says Huchard. “You could also imagine the reverse, whether competition boosts growth.” That’s why she looked at meerkats.

For the last 24 years, Huchard’s supervisor Tim Clutton-Brock has been studying wild meerkats in the Kalahari desert of southern Africa. His team know every individual in more than 60 groups and have trained them to climb onto electronic scales three times a day. From 14 of these groups, Huchard singled out pairs of littermates (who were born at the same time) and fed the lighter one with half a hard-boiled egg, twice a day for three months.

Huchard found that the unfed littermate responded to the fortunes of its companion by also eating more food and growing faster, to a greater degree than other meerkats that weren’t part of the experiment. It’s unclear whether they track their competitors through sight, sound, smell, touch, or other cues, but whatever the method, it’s a sensitive one. The unfed individuals grew in proportion to their fed peers, which suggests that they weren’t just gluttonously wolfing down whatever they found. Instead, they were fine-tuning their own appetites according to the status of their challengers.

“That was a surprise,” says Huchard. “Many people assumed that wild animals eat as much as they can when they’re foraging, but this experiment shows that this isn’t the case, and they have some flexibility.”

So why not just eat more all the time, and constantly grow as fast as possible? Huchard notes that “meerkats are very vulnerable to predators like birds of prey,” and if they spend a lot of time foraging and digging, they may also become unwary—and thus, eaten. There could also be hidden costs to fast spurts of growth: perhaps it suppresses the immune system, or slashes lifespan in the long run?

Huchard and her team can check for these costs in future experiments. They also want to measure the meerkats’ hormone levels to see if unfed individuals experience bodily changes that affect their growth regardless of how much they eat.

She already found some hints of this: After subordinate meerkats finally nabbed the dominant ranks, they also grew faster in the following months, even though they didn’t eat any more food. Tellingly, this growth spurt was particularly pronounced if the new rulers’ nearest rivals were close to them in weight. It seems like a way of distancing themselves from any future usurpers.

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