Penelope F. Kukuk

Emergence of division of labour in halictine bees: contributions of social interactions and behavioural variance

A central question in the study of insect societies concerns the emergence of division of labour. Among the different models of division of labour, the threshold model postulates that it can emerge from initial interindividual differences in the propensity to perform a task. The requirements of this variance-based model are likely to be present at the origins of sociality and may thus contribute to the emergence of division of labour in early social evolution. However, social interactions among members of the incipient groups are also likely to shape the genesis of division of labour. In this study, we compared task performance and social interactions during nest construction in forced associations of the solitary halictine bee, Lasioglossum (Ctenonomia) NDA-1, and of the communal Lasioglossum (Chilalictus) hemichalceum. We recorded and compared individual behaviours during nest construction for pairs of solitary or communal bees. These data were compared, in turn, to the excavation behaviours of the same bees while alone to determine how the presence of a social group influenced task performance. We additionally measured social interactions of pairs outside the context of nest construction. Pairs of solitary bees displayed higher levels of division of labour than communal pairs. A numerical model implementing behavioural rules derived from experiments suggests that in communal bees, division of labour can be primarily explained by differences in task propensity. In solitary bees, both behavioural asymmetry and social interactions contribute significantly to division of labour. These results are discussed in the framework of social transitions in halictine bees.

Mutualistic Benefits Generate an Unequal Distribution of Risky Activities Among Unrelated Group Members

Abstract.Recent studies provide a new challenge to the adequacy of theories concerning the evolution of cooperation among nonrelatives: some individuals perform high-risk activities while others do not. We examined a communal hymenopteran species, Lasioglossum(Chilalictus)hemichalceum, to determine why group members engaged in demonstrably risky activities (foraging) tolerate the selfish behavior (remaining in the nest) of unrelated nestmates. Experimental removal of adult females indicated that their presence is required for the protection of brood from ant predators. Nonforagers ensure the continued presence of adults in the nest if the risk-taking foragers die, thereby safeguarding the survival of forager offspring. This results in an unequal distribution of risky activities within social groups in which avoidance of risky activities by some group members is ultimately beneficial to risk takers.

Division of labour and socially induced changes in response thresholds in associations of solitary halictine bees

Division of labour is a recurrent property of social groups. Among the different models proposed to explain the origin of division of labour, response-threshold models have garnered strong theoretical and empirical support. These models postulate that task specialization can arise spontaneously from interindividual variation in thresholds for responding to task-associated stimuli. Consequently, individuals with lower thresholds for a given task are more likely to become specialists. Self-reinforcement models expand this hypothesis by proposing that the successful performance of a task lowers an individuals threshold, increasing the probability that it will perform that task again. Although an important component of many models of division of labour, self-reinforcement can be difficult to test in real-world contexts. Here, we asked whether social experience modulates the individual response thresholds of normally solitary individuals. We focused on task performance during the early stages of nest construction in forced associations of the normally solitary halictine bee Lasioglossum (Ctenonomia) NDA-1. Within each pair, a strong behavioural asymmetry arose, with one bee specializing in excavation and her nestmate specializing in guarding the nest entrance. Individual performance of excavation by each bee was compared before and after being paired with a conspecific. After experiencing a social environment, individuals substantially increased their excavation performance. However, bees excavating more frequently in groups did not excavate differentially more afterwards, as would be predicted by self-reinforcement. The social context experienced by bees seems to promote behavioural differentiation leading to task specialization and to modulate response thresholds for excavation.

Quantitative genetic analysis of natural populations.

Quantitative genetic studies in natural populations have been rare because they require large breeding programmes or known pedigrees. The relatedness that has been estimated from molecular markers can now be used to substitute for breeding, allowing studies of previously inaccessible species. Many behavioural ecologists have a sufficient number of markers and study species with characteristics that are amenable to this approach. It is now time to combine studies of selection with studies of genetic variation for a more complete understanding of behavioural evolution.

Maintaining low intragroup relatedness: evolutionary stability of nonkin social groups

Communal sociality, involving cooperation among unrelated group members, occurs in all of the six families of bees. However, most communal species also occupy nests that persist over many generations. Why, then, do females not remain in their natal nests with their relatives? We examined how sex-biased dispersal contributes to maintaining low intragroup relatedness by documenting dispersal using microsatellite data in the communal halictine bee, Lasioglossum (Chilalictus) hemichalceum. The composition and genotypes of brood from nests excavated late in the reproductive season (late-season) were compared with those of adults from nests excavated from the same aggregation in early winter (post-season). Females left their natal nests if nests were large and if their brothers were present, demonstrating female-biased dispersal to avoid inbreeding in a social insect for the first time. Thus, the occurrence of within-nest mating in communal species, combined with strong selection against inbreeding in the Hymenoptera, appears to select for female-biased dispersal. This in turn results in a population-wide decrease in intracolony relatedness, thereby maintaining cooperation among nonkin in communal Hymenoptera.

Increased group size promotes task specialization in a normally solitary halictine bee

Division of labour is a fundamental property of animal groups; how it is generated and evolves is, therefore, of central importance to sociobiology. Self-organizational models suggest (1) that division of labour can emerge spontaneously at the origin of group living and (2) that increased group size further promotes task specialization. We examined the emergence and scaling of division of labour in an evolutionarily incipient social system: forced associations of the normally solitary halictine bee Lasioglossum (Ctenonomia) NDA-1. A division of labour between nest excavation and guarding arose in pairs and in groups of four bees, with individuals in larger groups exhibiting higher degrees of task specialization. Task differentiation may be facilitated by intrinsic behavioral variability and/or spatial organization. Our results support the hypotheses that division of labour can self-organize at early stages of social evolution and that greater individual specialization is an emergent consequence of increased group size.

Quantitative genetic analysis of natural populations: old wine in a new but defective bottle?

Quantitative genetic analysis of natural populations: old wine in a new but defective bottle?