Many animals live in groups governed by social hierarchies, but meerkats take social stratification to an extreme. Those small southern African carnivores live in communities of up to 50 individuals, but 90 percent of reproductive privileges belong to a single dominant pair—usually, the largest and more senior animals in the group. The pair’s children assist with raising young, and daughters queue up to assume dominance following their mother’s death, with older and larger ones typically taking the lead.
Younger individuals, however, sometimes offset that hierarchy when they grow faster than their older sisters, displacing them in the reproductive line. But as new research published in Nature shows, older sisters can respond in turn by ramping up their food intake and increasing their own size.
“Our data suggest that meerkats certainly are able to adjust their growth to the growth of their closest competitors,” says Tim Clutton-Brock, a zoologist at the University of Cambridge and senior author of the paper. “Individuals are actually growing competitively.”
Most previous studies of animal growth focused on external factors, such as food availability, temperature or population density. But this is the first evidence Clutton-Brock and his colleagues know of showing that individuals adjust their food intake—and thus their growth rates—to those of their rivals.
This finding built upon a foundation of over two decades of meerkat research, conducted at field sites in the southern Kalahari Desert in South Africa. There, Clutton-Brock and his colleagues follow 15 to 20 different breeding groups, totaling around 300 meerkats at any one time (all are chipped with transponders for identification). They have habituated the animals to close observation, and even trained them to climb onto electronic scales using crumbs of hard boiled eggs or drips of water as enticement. This gives the researchers “unusual access” to their wild study subjects, Clutton-Brock says. “They’re entirely used to us and on board with us being there.”
To arrive at their latest findings, the team singled out pairs of same-sex, same-age juvenile or young adult littermates from 14 of the meerkat groups. They fed the smaller of the two animals half of a hard-boiled egg twice a day for several weeks—roughly equivalent to half of a full day’s intake of food—and did not feed the older sibling. Other pairs served as controls, with neither meerkat receiving egg treats. After three months, they compared the growth of the larger, unfed meerkats in both groups. They found that meerkats whose littermates enjoyed the egg benefits had also significantly increased their own weight more—by eating more--when compared to their control group counterparts.
Youth isn’t the only time at which meerkats grow, however. When a dominant female dies and is succeeded by her daughter, the new matriarch undergoes a growth spurt. The researchers wondered, however, if the new dominants might also increase their growth rate in relation to the size of their rivals. As predicted, they found that the newly dominant individual’s weight gain corresponded to the next-in-line meerkat’s size: if the dominant meerkat’s younger sister was nearly the same size, the older sister grew more. But if the next-in-line was significantly smaller, the new dominant’s growth spurt was more modest.
“This finding that meerkats can adjust their growth rate in response to the body mass of their closest competitors is a well-documented, fascinating discovery by the authors, with far-reaching implications for all social mammals,” says John Hoogland, an ecologist at the University of Maryland who was not involved in the study.
Clutton-Brock and his colleagues also suspect that competitive eating and growth may be widespread in other species as well, especially ones that intensely vie for breeding opportunities. Other researchers agree. “Such ‘keeping up with the Joneses’ should be broadly expected and will encourage me to look for it in other species,” says Daniel Blumstein, an ecologist at the University of California, Los Angeles, who was not involved in the study. “Digging down and understanding more about how social stress influences animals’ abilities to maintain mass and grow in these situations would be a good follow-up.”
Indeed, Clutton-Brock and his colleagues are pursuing those questions, including investigating why individual animals don’t always opt for maximum growth to beat their opponents. Research on captive rats and fish, at least, suggest that there may be hidden costs of accelerated growth: reduced telomere length, increased aging and shortened lifespans. “We don’t know yet if that’s the case, but we’re now investigating whether meerkats that engage in competitive growth spurts suffer similar costs in their later years,” Clutton-Brock says.
He adds that the surprising insights that came of this study—and the ones that will likely follow—were only made possible through decades spent in the field, observing and habituating meerkats. “Long term studies can get at questions that no one else can answer, and produce dramatically new findings.”