Meerkats, those sociable weasellike mammals of the Kalahari, better beef up if they want to become moms or dads. That’s the conclusion of a study that reveals a previously unknown contest among mammals: competitive growth. Meerkats (Suricata suricatta, shown above) are small carnivores that live in large social groups of as many as 50 individuals. One pair monopolizes the reproduction. All other members are subordinate, and help feed and babysit the dominant pair’s pups. Young males eventually move away to try to displace the guys in another group. But lower ranked females stay put, forming a hierarchy based on age and weight while they wait for the dominant female to die. At that point, usually her oldest and heaviest daughter steps out from the front of the reproductive queue. She then packs on a few more grams. Scientists have suspected that her additional weight gain is a strategy to both increase her fertility and firm up her social position. To find out, the researchers identified pairs of sisters, and fed the younger members of each pair some hard-boiled eggs three times a day. The scientists weighed the sisters daily for 3 months. Their analysis showed that the younger sisters’ weight gain stimulated the older females to eat more so that they could grow faster than their rivals, the scientists report online today in Nature. Females in the reproductive queue apparently keep track of each other’s weight during rough-and-tumble play bouts. The researchers found similar results among males, leading them to conclude that when it comes to staying on top, size really does matter to meerkats.