Meerkats can be model citizens, helping out their communities with services like babysitting, grooming and guard duty. Yet despite their famously cooperative — and adorable — family groups, meerkat society can also be surprisingly competitive.

Known as "mobs," meerkat colonies number up to 50 individuals, with reproduction almost totally limited to one dominant pair. That couple's offspring help with child-rearing and other chores, waiting for a chance to inherit the throne and become parents themselves. Subordinate females are ranked in a hierarchy based on age and weight, forming what scientists call a "reproductive queue."

When a dominant female dies, her eldest, heaviest daughter is normally next in line. Sometimes, though, a younger daughter outgrows her older sister and bypasses her in the breeding queue. And a new study offers a fascinating glimpse of how these sibling rivalries play out: They turn into eating contests.

The study, published this week in the journal Nature, shows that young meerkats somehow know to adjust their diet — and thus manage their own growth rate — to outgrow close rivals. Their eating contests are actually growing contests, providing the first evidence of competitive growing in mammals, the researchers say.

To reveal this, the authors relied on an unusual relationship with a group of wild meerkats at the Kuruman River Reserve in South Africa's Kalahari Desert. Since 1993, University of Cambridge zoologist Tim Clutton-Brock and his colleagues have been following more than 40 meerkat mobs in the region, totaling thousands of individuals who are recognizable by dye marks. The meerkats were habituated to humans so researchers can study them up close, and most were even trained to climb onto electronic scales for regular weigh-ins (pictured below).

compare the meerkat A researcher weighs wild meerkats in the Kalahari Desert. (Photo: Tim Clutton-Brock/University of Cambridge)

First, the researchers identified pairs of sisters within a meerkat group. They then artificially boosted the growth of each pair's younger sister, feeding her three times a day with hard-boiled egg. The younger sisters were weighed daily for three months, as were the older sisters who didn't receive any hard-boiled egg.

The extra food made younger meerkats gain weight, but it also had an indirect effect on their older sisters: They started eating more food per day on their own, and gaining more weight, in an apparent attempt to outgrow their upstart sisters. Other meerkats whose siblings weren't being fed by scientists didn't do this.

"Tellingly, the extent to which the older sister increased her weight was greater when her younger sister's weight gain was relatively large than when it was slight," according to a statement released by the University of Cambridge. In other words, the elder meerkats weren't just eating more food — they were specifically recalibrating their diets in a race to fatten up faster than the rest of their family.

Once a meerkat is dominant, however, the competitive eating still isn't over. Her tenure at the top is longer, and her breeding success is higher, if she remains heavier than her heftiest underling, the researchers report. And for three months after being coronated, dominant females keep gaining weight to bolster their new status from potential usurpers, even if they're already adults. Also, their degree of weight gain is higher if the heaviest subordinate of the same sex is close to them in weight.

The researchers studied male meerkats, too, although their social ladder works a little differently. Males leave their birth group around the age of sexual maturity, then try to displace males in other mobs. But body weight is also important for them, as the study revealed a competitive weight-gain strategy among subordinate males, with the heaviest male often becoming dominant.

So how do meerkats keep tabs on each other's weight gain? "Meerkats are intensely social and all group members engage in bouts of wrestling, chasing and play fighting," Clutton-Brock says. "Since they live together in such close proximity and interact many times each day, it is unsurprising that individual meerkats are able to monitor each other's strength, weight and growth."

meerkats playing Meerkat mobs spend a lot of time together. (Photo: Shutterstock)

If weight gain is so important, why don't meerkats just eat as much as possible, gorging themselves into obese overlords? It's not that simple, the researchers write — adding more weight than necessary could pose unknown health risks, and being too obsessed with foraging could leave meerkats vulnerable to predators.

"Allocation of additional resources to growth by challenged individuals may depress immune function and reduce longevity as a result of increases in oxidative stress and telomere shortening," they write, "while increases in time spent foraging may raise predation risk, which is high in meerkats."

This study is part of the ongoing Kalahari Meerkat Project, so future research may shed more light on the costs of meerkats' competitive eating. The researchers also hope to learn more about the role hormones play in this process, noting that "the hormonal profile of dominant meerkats is distinct from that of subordinates." Plus, they add, since size and weight affect breeding success in many social mammals, this kind of competitive growth may also occur in other social species, "possibly including domestic mammals, non-human primates and humans."

Meerkats are obviously very different from humans, but there are some similarities. Like us, meerkats have complicated relationships with their siblings. They are each other's closest rivals for resources, but also each other's best allies in times of danger. And not much sums up that dynamic quite like a meerkat play-fight:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.