When the queen of an Indian jumping ant colony dies, the female workers duel each other in ritual fights to establish dominance. The resulting cadre of worker queens undergo major physical changes, which a new study says is thanks to the neurohormone dopamine.
With social insects, dominance status is used to determine a reproductive division of labor: A few colony members reproduce while most remain functionally sterile. Changes to status need to be coordinated. While the tournaments among Harpegnathos saltator can be fierce, the battles rarely result in physical injury. (Watch this video of them dueling below.) Eventually, a dozen or so workers will become a group of worker queens, called gamergates. They may look like ordinary workers, but the gamergates undergo extreme internal changes: brains shrink by 25 percent, ovaries expand to fill the abdomens, and life expectancy jumps from about six months to several years at least.
The ants undergo all these changes in physiology without any related changes to their DNA -- social and environmental factors must be determining when certain genes are turned on or off. A team led by Clint Penick from North Carolina State University wanted to know what was responsible for these physical changes.
They took a subset of workers from a colony (let’s call them Colony A) and separated them from their gamergates. These workers effectively formed their own colony (Colony B) and began fighting to establish dominance. Whenever workers in Colony B began to get the upper hand, the researchers removed them from the colony. As it turns out, these dominant workers had already begun to produce elevated levels of dopamine: two to three times higher than other workers, but less than full-fledged gamergates.
When they placed these dominant workers back into Colony A, the regular workers recognized their changes and started to police them: They’d hold down the dominant ants so they couldn’t move. Within 24 hours, their dopamine levels dropped back to normal, and they became regular workers again. The policing behavior seems to regulate neurohormone levels -- and subsequently physiology -- to make sure the workers don’t reproduce.
“The very act of winning these ritual battles increases dopamine levels in H. saltator, which ultimately leads to the physical changes we see in gamergates,” Penick explains in a news release. “Similarly, losing these fights pushes dopamine levels down.”
The work was published in Journal of Experimental Biology this week.
Images: Clint Penick