Role of resource availability on sex, caste and reproductive allocation ratios in the Argentine ant Linepithema humile

Résumé 0

1. Resource availability has long been recognized to influence reproductive decisions in eusocial Hymenoptera. We analysed how protein availability affects the number and the weight of worker, male and queen pupae, as well as its effect on sex, caste and reproductive allocation ratios, in the Argentine ant Linepithema humile Mayr. Colonies were maintained on diets with three levels of access to proteins: no protein, intermediate and high levels of proteins. 2. There was no significant difference between the intermediate and high levels of protein in the number of queen, male and worker pupae produced. Similarly, the intermediate vs. high levels of protein treatments did not differ with regard to the weight of pupae nor the relative investments between the three castes. This suggests that, in L. humile, there could be a threshold over which additional amounts of protein have no more effect on reproductive allocation or the size of the individuals produced. 3. The main effect of protein supplementation was to increase the number of sexual pupae (queens and males) produced. By contrast, the number of workers produced remained unaffected by the level of proteins. The higher investment in sexuals in nests supplemented with proteins resulted in a significantly higher proportion of queen pupae among females, as well as higher proportion of males among all individuals produced in these nests. However, the proportion of males among sexuals (numerical sex ratio) was not significantly different between supplemented and unsupplemented nests. 4. Workers reacted to a higher protein diet not only by rearing more sexuals, but also by producing larger individuals of the three castes. 5. The proportional investment allocated to queens among females was higher in the protein-supplemented treatments. Protein-supplemented nests also invested proportionally more into sexual production than unsupplemented nests, However, whereas increased sexual production is generally associated with a more female-biased sex investment ratio in ants, sex allocation ratio was not significantly different between supplemented and unsupplemented nests. 6. Overall, our results support the view that protein availability influences the proportion of brood that is culled, with workers eliminating a significant proportion of both the male and female sexual brood in unsupplemented nests. 7. The increasing amount of experimental data showing that brood culling is an important part of the biology of ants and other social insects emphasis the need to combine kin selection and life-history approaches to study the outcome of queen-worker conflicts on the dynamics of colony growth and sex allocation.