Compare the meerkats: Females literally weigh each other up in fight for title of reproductive queen

Female meerkats will eat more when they see a younger rival gaining weight so as to keep themselves in line for the role of reproductive queen. Researchers say this is the first evidence of competitive growth in mammals – where subordinates keep tabs on their rivals and adjust their own weight accordingly.

A team of scientists from the University of Cambridge studied a population of wild meerkats in the Kalahari Desert. In wild meerkat populations, reproduction is confined to a single dominant pair. Subordinate females that reproduce are banished by the alpha pair and the infants are killed.

Few female meerkats ever get to be dominant. But as they wait for the current queen to die or be usurped, a pecking order emerges. The title is normally passed onto the eldest and heaviest female in the group. However, younger sisters that outgrow their elders can jump the hierarchy queue.

To keep their place in the 'reproductive queue', females monitor the weight of their rivals closely, and gain weight when they see another, younger competitor getting heavier.

Scientists discovered this by identifying pairs of female littermates and feeding the lighter individual a hard-boiled egg twice a day for three months, so they would put on weight. They then compared the growth rate of this lighter meerkat to that of its heavyweight counterpart.

Findings, published in Nature, showed the larger meerkat responded to its rival gaining weight by eating more so as to maintain their place in line. They also found weight gain in the older corresponded to how much the younger sister grew i.e. the more weight the younger one put on, the more the larger put on.

They also subsequently found that a newly-crowned reproductive queen gained more weight so as not to be overthrown by a heavier rival. The amount of weight they put on in the first three months was greatest if the next female in line was of a similar weight.

"Size really does matter and it is important to stay on top," said senior author Tim Clutton-Brock. "Our findings suggest that subordinates may track changes in the growth and size of potential competitors through frequent interactions, and changes in growth rate may also be associated with olfactory cues that rivals can pick up.

"Meerkats are intensely social and all group members engage in bouts of wrestling, chasing and play fighting, though juveniles and adolescents play more than adults. 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."