Variation in male body size and reproductive allocation in the leafcutter ant Atta colombica: estimating variance components and possible trade-offs

Abstract

Remarkably little is known about the traits that determine reproductive success of males in eusocial insects. Their window for mate choice decisions is very short, the actual mating process is very difficult to observe, and their small body sizes have likely prevented systematic studies in many species. In 2008 and 2009, we revisited a Panamanian population of Atta colombica leafcutter ants to partially repeat and complement a study of more than 15 years ago. We compared within- and between-colony variation in male body size (mass and width of head, mesosoma and gaster) and sperm characteristics (length, number and survival after exposure to saline buffer with and without added accessory gland secretion). We also measured the size of accessory glands as the main contributor of seminal fluid and the accessory testes containing all mature sperm, but we found few correlations between these variables. We also obtained little or no evidence for expected trade-offs between sperm number and sperm length and between mesosoma mass and sperm complement, although this could be due to limited sample size and unknown variation in larval resource allocation that was beyond our control. However, we found an interestingly bimodal distribution in broad-sense heritabilities (intra-class correlations) among the variables that we measured. Low heritabilities suggest that mesosoma size (mass and width), accessory testes size, sperm viability (measured as % survival in saline) and probably also accessory gland size are traits directly correlated with reproductive success. However, the much higher heritabilities for total body mass, gaster mass, head width, sperm length and sperm number suggest that these traits are less likely to make direct contributions to male fitness.