Abstract: Colony-level predictions about sex-ratio optimization in eusocial Hymenoptera are different from the ones that apply to the population level. A recent empirical study on the sweat bee Halictus rubicundus has revealed a distinct pattern in the colony sex ratio of the summer brood: eusocial colonies produced more female-biased sex ratios and non-eusocial colonies produce more male-biased sex ratios. These data are consistent with theoretical hypotheses as put forward by Trivers and Hare and several later authors. When interpreted in the light of these theoretical contentions, the sex-ratio variation in Halictus rubicundus appears to be adaptive for workers, replacement queens and – under reasonable additional assumptions – also for foundresses.

Abstract: Animal societies provide perfect conditions for the spread of infections and are therefore expected to employ mechanisms that reduce the probability of transmitting pathogens to group members [14]. Death in nature rarely results from old age but commonly results from diseases. Leaving one's group to die in seclusion might be an efficient way of minimizing the risk of infecting kin. Anecdotal observations of moribund individuals deserting from their groups exist for several species, including humans (e.g., [5]), but have rarely been substantiated by quantitative analysis. Furthermore, to confirm that dying in solitude has evolved because of its altruistic benefits requires refuting the alternative explanation of pathogen manipulation of host behavior. Here we show that workers of the ant Temnothorax unifasciatus dying from fungal infection, uninfected workers whose life expectancy was reduced by exposure to 95% CO2 [6, 7], and workers dying spontaneously in observation colonies exhibited the same suite of behavior of isolating themselves from their nestmates days or hours before death. Actively leaving the nest and breaking off all social interactions thus occurred regardless of whether individuals were infected or not. Social withdrawal might be a commonly overlooked altruistic trait serving the inclusive fitness interests of dying individuals in social animals. s Ants actively leave their nests before death and break up all social contact s Social withdrawal is not due to manipulation through pathogens or parasites s Instead, it appears to be an altruistic act of the ants themselves

Abstract: In insect societies, workers cooperate but may also pursue their individual interests, such as laying viable male eggs. The case of obligatory slave-making ants is of particular interest because workers do not engage in maintenance activities and foraging. Therefore, worker egg laying is expected to be less detrimental for colony efficiency than in related, nonparasitic species. Furthermore, as slave-making workers usually do not perform brood care and thus might have little power in manipulating sex allocation, they might be more strongly selected to increase their direct fitness by producing their own sons than workers in nonparasitic species. In this study we investigated worker reproduction in four natural colonies of the slave-making ant Polyergus rufescens, using highly variable microsatellite markers. Our results show that workers produce up to 100% of the males. This study thus presents the first direct evidence of an almost complete takeover of male reproduction by workers in ants.