Differences in inhibitory capability among queens of the ant Solenopsis invicta

ABSTRACT. The abilities of various categories of queens of the fire ant, Solenopsis invicta Buren, to inhibit de-alation by virgin queens are compared by means of a standardized bioassay that detects the presence of an inhibitory pheromone. Highly fecund (high weight) queens of monogynous colonies and virgin replacement queens in queenless colonies have a significantly greater inhibitory capability than queens of lower fecundity (lower weight). In polygynous colonies, inhibitory effectiveness is also positively related to the fertility of individual queens, although no pheromone can be detected in queens that lay few eggs. Alate virgin queens that have overwintered in the parental nest, and sexually mature spring-reared virgin queens, either do not produce the pheromone, or produce too little to be detected. These results are discussed in relation to the degree of ovarian development of different queen categories and in relation to the social status of these queens.     

Alternative life-histories in a socially polymorphic ant

Social organisms vary greatly in the number of breeders per group; yet, the causes and consequences of this variation remain poorly known. Here, we show that variation in social structure is tightly linked with changes in several fundamental life-history traits within one population of ants. Multiple-queen colonies of Formica selysi were much more populous than single-queen ones. They also occurred in areas of higher nest density, had longer colony lifespan, produced smaller queens that presumably disperse less, and invested less in reproductive individuals relative to workers. These multiple changes in life histories are consistent with a shift in the mode of colony foundation and the degree of philopatry of queens. They may also provide various fitness benefits to members of multiple-queen colonies and are likely to play a central role in the evolution and maintenance of polymorphic social structures.     

Cooperation among unrelated individuals: the ant foundress case

Ant foundress associations are an example of cooperation among non-kin. Across a dozen genera, queens able to found a colony alone often join unrelated queens, thereby enhancing worker production and colony survivorship. The benefits of joining other queens vary with group size and ecological conditions. However, after the first workers mature, the queens fight until only one survives. The presence of cofoundresses, and their relative fighting ability, also affects the extent of cooperative investment before worker emergence. This reveals previously overlooked early conflicts among queens, which reduce the mutualistic benefits of cooperation.     

Physiological suppression eases in Damaraland mole-rat societies when ecological constraints on dispersal are relaxed

In many vertebrate societies, subordinate females exhibit down-regulated reproductive physiologies relative to those of dominants, a condition commonly termed physiological suppression. Research into the causes of physiological suppression has focused principally on the role of the subordinate's social environment (typically the presence of the dominant female and/or an absence of unrelated males within the group), while few studies have considered the additional role that the physical environment may play. Here we present new evidence from wild Damaraland mole-rats, Fukomys damarensis, revealing that physiological suppression among subordinate females eases markedly during the annual rains (a time when ecological constraints on dispersal are relaxed), despite the continued presence of the dominant female and in groups that contain no new immigrant males. Subordinate females showed substantially higher pituitary sensitivities to GnRH challenge during the wet period than the dry, a contrast that cannot be attributed to between-female differences (as it holds for paired within-female comparisons), associated changes in body mass (as our analyses control for this), or concomitant reductions in physiological stress (as their urinary cortisol concentrations were actually higher in the wet period). We suggest that our findings reflect selection for the maintenance of reproductive readiness among subordinate females during high rainfall periods, given the increased likelihood of encountering dispersal and/or mating opportunities with extra-group males when ecological constraints on dispersal are relaxed. These findings reveal new complexity in the processes that regulate physiological suppression, suggesting a key role in some species for changes in the physical as well as social environment.