Juvenile hormone interacts with multiple factors to modulate aggression and dominance in groups of orphan bumble bee (Bombus terrestris) workers

Abstract

Juvenile hormone (JH) is a key regulator of insect development and reproduction. Given that JH commonly affects adult insect fertility, it has been hypothesized to also regulate behaviors such as dominance and aggression that are associated with reproduction. We tested this hypothesis in the bumble bee Bombus terrestris for which JH has been shown to be the major gonadotropin. We used the allatoxin Precocene-I (P-I) to reduce hemolymph JH titers and replacement therapy with the natural JH to revert this effect. In small orphan groups of workers with similar body size but mixed treatment, P-I treated bees showed lower aggressiveness, oogenesis, and dominance rank compared with control and replacement therapy treated bees. In similar groups in which all bees were treated similarly, there was a clear dominance hierarchy, even in P-I and replacement therapy treatment groups in which the bees showed similar levels of ovarian activation. In a similar experiment in which bees differed in body size, larger bees were more likely to be dominant despite their similar JH treatment and ovarian state. In the last experiment, we show that JH manipulation does not affect dominance rank in groups that had already established a stable dominance hierarchy. These findings solve previous ambiguities concerning whether or not JH affects dominance in bumble bees. JH positively affects dominance, but bees with similar levels of JH can nevertheless establish dominance hierarchies. Thus, multiple factors including JH, body size, and previous experience affect dominance and aggression in social bumble bees.

Convergent selection on hormone signaling shaped social evolution in bees

“…JH is essential to reproductive maturation in solitary insects, but this signaling system has also been frequently co-opted during major life history transitions 43,[46][47][48] , including eusociality 42 . In relatively small eusocial societies like halictids 49,50 , paper wasps 51 , and bumblebees 52,53 , JH has maintained its association with reproductive maturation, but has also gained a new role in mediating aggression and dominance. In the much more elaborate eusocial colonies of honey bees, further modifications to JH signaling have also resulted in a secondary decoupling of JH in workers independent of its ancestral, reproductive role 42 .…”