Sex Investment Ratio and Split Sex Ratio in the Fission-Performing Ant Cataglyphis tartessica

Abstract

Inclusive fitness theory predicts that the sex investment ratio should be female biased in social insects with haplodiploidy-generated relatedness asymmetry (between females and males and also among females). However, this ratio should become male biased if related females are competing with each other for resources, as it is predicted by local resource competition hypothesis. This specific situation is expected to occur in ant species that reproduce by fission, where young queens leave their mother colonies on foot to found daughter colonies. We tested this idea using the ant Cataglyphis tartessica in Donana National Park (Spain). This species is monogynous (colonies headed by a single queen), monandrous (queen mate with only one male), and reproduces by fission. Given the similarity between foraging and colony fission distances in this species, a strong local competition for resources should be expected between mother and daughter colonies. In this study, we considered investment in females the sum of the investment in gynes (virgin queens) plus the investment in the worker force that leaves with them to found a new colony. Our results show a male-biased sex investment ratio in C. tartessica of 1:8 (numerical ratio gynes/males 1:56), suggesting queen control over sexual production. In addition, we found that, during its reproductive period, this species exhibits split sex (gyne-male) production as well as split male-worker production, without any evidence of variation in relatedness asymmetry between its colonies, at the population level. An interaction between local resources competition and colony size could explain our results.