There are few greater tragedies for an ant colony than the death of a queen. For some species, a deceased queen means game over for the whole colony. For other species, such as the Indian jumping ants (Harpegnathos saltator), the power vacuum will cause certain forager ants to vie for power in an aggressive duel. The winner will earn the promotion of becoming "royalty", but it will come at a grave cost: around a fifth of their brain.
Scientists at New York University School of Medicine and Arizona State University have recently described the unusual ability of Indian jumping ants to reduce their brain size if they grab the reins of power to become a reproductive worker, known as a gamergate (yeah, not that gamergate), which is effectively like a temporary queen. As reported in the journal Proceedings of the Royal Society B: Biological Sciences, gamergates exhibited a 19 percent reduction in brain volume compared to the foragers, as well as other clear significant changes to their ovaries, behavior, and venom production.
The everyday life of a forager can be cognitively taxing (for an ant, that is), involving relatively complex tasks such as navigating in and around the nest and retrieving live prey items. By comparison, a gamergate doesn't need to perform many tasks that require higher cognitive processing – but they do need to conserve as much energy as possible for the purposes of reproduction. Limiting their brainpower, therefore, is an understandable pay-off.
Perhaps most curiously, the researchers noted that the ants were able to revert back to their former selves and bloat up their brains again within eight weeks. The researchers suspect this is the first insect known to both increase and decrease their brain size. For instance, the authors of the paper note that the brain size of honeybees increases when nurse workers transition to foragers, but their brain doesn't shrink back when foragers reverted back to nurses.
It's thought this flexibility in Indian jumping ants can be found because they live in relatively small colonies, usually with no more than 100 individuals. In this cozy setting, each worker represents a more valuable resource to the whole colony, so it pays to be highly flexible and non-expendable. On the other hand, honeybees may live in a colony of 50,000, meaning the foragers can easily be replaced by new workers.
The idea of shrinking and regrowing brains might sound distinctively alien-like, but it's something that's been documented in vertebrates many times before. Gambel’s white-crowned sparrow is a species of songbird found in North America whose neurons will die off then rebloom with the seasons. Each mating season, when the bird will recite songs to woo mates, they will be enriched with a flurry of new neurons, especially in the "song center" of their brains. By winter, many of these neurons have died off, and won't be replaced until the following spring when the cycle continues.