Multiple breeders, breeder shifts and inclusive fitness returns in an ant

In social insects, colonies may contain multiple reproductively active queens. This leads to potential conflicts over the apportionment of brood maternity, especially with respect to the production of reproductive offspring. We investigated reproductive partitioning in offspring females (gynes) and workers in the ant Formica fusca , and combined this information with data on the genetic returns gained by workers. Our results provide the first evidence that differential reproductive partitioning among breeders can enhance the inclusive fitness returns for sterile individuals that tend non-descendant offspring. Two aspects of reproductive partitioning contribute to this outcome. First, significantly fewer mother queens contribute to gyne (new reproductive females) than to worker brood, such that relatedness increases from worker to gyne brood. Second, and more importantly, adult workers were significantly more related to the reproductive brood raised by the colony, than to the contemporary worker brood. Thus, the observed breeder shift leads to genetic benefits for the adult workers that tend the brood. Our results also have repercussions for genetic population analyses. Given the observed pattern of reproductive partitioning, estimates of effective population size based on worker and gyne samples are not interchangeable.

Evidence for geographic variation in colony social organization in an obligately social sweat bee, Lasioglossum malachurum Kirby (Hymenoptera; Halictidae)

The expression of altruism and colony eusociality are both a matter of degree in social sweat bees. Even in obligately social species, variation in these traits may be observed across a species' range. Lasioglossum (Evylaeus) malachurum (Kirby) is an obligately eusocial sweat bee found across Europe. In western Europe, L. malachurum exhibits north-south clines of increasing colony size associated with the production of more worker broods, and worker production of males, but societies conform to the model of a classically eusocial hymenopteran insect. A population of L. malachurum studied from 1994 to 1998 at Agios Nikolaos Monemvasias in southern Greece exhibits a startlingly different type of social structure. Dissections of female bees collected while foraging on flowers or from excavations of nests showed that the majority of mid- to late-summer workers are mated and (or) have developing ovaries, indicating that some workers are highly reproductive. Nest excavations indicated that in many or most colonies, the queen has disappeared by midsummer, before ovipositing the final, reproductive brood. In orphan nests, workers become the major reproductives, which suggests that males and gynes in the final brood are the offspring of workers. The very long breeding season in southern Greece may explain why colonies often outlive their queen. The result is the expression of a multivoltine colony cycle and a behavioural switch from eusocial to semisocial colony organization.

The social organisation of Lasioglossum (Dialictus) laevissimum (Smith) in southern Alberta

An aggregation of Lasioglossum (Dialictus) laevissimum was studied in Calgary, Alberta, in the summer of 1988. This species was weakly eusocial, with an average of less than 2.5 workers per nest, 43% males in the worker brood, 63% of workers with well-developed ovaries, 35% of them mated, and a mean queen–worker size dimorphism of 7%. Based upon its average rank for these variables, in comparison with eight other species, L. laevissimum is the most weakly eusocial member of the subgenus Dialictus. Nonetheless, reproductive-brood production averaged around 25 per nest, and this species is clearly well adapted to short-summer environments. There was little evidence that any worker-brood females entered early diapause rather than functioning as workers. A few spring nests were initiated by more than one overwintered foundress. These pleometrotic nests often had worker-brood productivities that exceeded average reproductive-brood size. Brood mortality was low, infection of provision masses after rainfall being the major factor.

The phenology and social biology of four sweat bees in a marginal environment: Cape Breton Island

Augochlorella striata, Lasioglossum (Evylaeus) cinctipes, Lasioglossum (Evylaeus) comagenense, and Lasioglossum (Dialictus) laevissimum were studied on Cape Breton Island, Nova Scotia, throughout their flight season in 1987. The weather during the summer was unusually good, with above-average temperatures and hours of sunshine but very low rainfall. Conversely, the previous summer had been very poor for bees, with comparatively few days suitable for foraging, particularly in July. Augochlorella striata was basically solitary but some nests produced one or, at most, two workers, thereby becoming eusocial. In other localities, L. cinctipes is known to be eusocial with well-developed morphological and physiological caste differentiation. However, most foundresses observed in 1987 were extremely small, smaller than usual for workers elsewhere, and none of the more than 100 nests produced adult workers or a reproductive brood. Most foundresses were either survivors of the worker brood from the previous year or unusually small reproductive brood females produced as a result of the bad weather in 1986. Lasioglossum comagenense was solitary or semisocial with one to four females occupying a nest. Lasioglossum laevissimum exhibited significant levels of pleometrosis and an extended period of worker foraging in summer. A comparison of the productivities of these four species indicates the importance of a flexible social system in a marginal climate.

The effect of Bombylius pulchellus (Diptera; Bombyliidae) and other mortality factors upon the biology of Halictus ligatus (Hymenoptera; Halictidae) in southern Ontario

Two factors were important in causing brood mortality in a southern Ontario population of Halictus ligatus: parasitism by larvae of the bombyliid Bombylius pulchellus and various forms of microbial infection of bee immatures or their pollen ball food. Bombyliid larvae consumed host prepupae or, less commonly, young pupae, restricted their attack mostly to the reproductive brood, and were contagiously distributed within the nest population. All immature stages were susceptible to fungal infection or disease but such pathogens did not seem to spread among cells within nests, indicating that bees may be able to prevent the spread of disease from one brood cell to another. Filling affected cells with earth may accomplish this brood hygiene. The survival rate of worker and reproductive brood immatures approximated 90%. Reproductive brood mortality affected females more than males. Reproductive brood mortality was probably underestimated: filled-in cells that may represent brood mortality were not included in these estimates.

The social organisation of Halictus ligatus (Hymenoptera; Halictidae) in southern Ontario

The social organisation of Halictus ligatus was studied at Victoria, southern Ontario. At this locality, the one worker brood has a protracted period of emergence; this results in small colony populations throughout the summer activity phase. Workers average 12.7% smaller than their queens, 60% of them have some ovarian development, and 42% of them mate. More males are produced towards the very end of the first brood than earlier in the spring provisioning phase. These late first brood males probably survive to mate with reproductive brood females. In orphaned nests, one worker dominates the others to become a replacement queen. Most replacement queens are mated and orphaned colonies produce reproductives of both sexes. Data from this population are compared with those of other studies of this, and other, halictine species.

Multiple-foundress associations in a temperate population of Halictus ligatus (Hymenoptera; Halictidae)

Thirteen pleometrotic (multiple-foundress) nests of the primitively social sweat bee Halictus ligatus were excavated in the summer of 1984 at Victoria, near Toronto, southern Ontario. Subordinate foundresses were significantly smaller than both dominant females in pleometrotic nests and females that nested solitarily. Most subordinates were smaller than the workers that they helped to raise. These small females could have been surviving workers from the previous summer or the offspring of workers. It seems unlikely that they were malnourished reproductive brood individuals produced as a result of parental manipulation. In successful nests, the number of workers produced was positively correlated with the number of founding females such that productivity per foundress remained fairly constant. Pleometrotic nests also produced more reproductives than haplometrotic (single-foundress) ones. Subordinates may occasionally lay reproductive brood eggs. The increased productivity of multiple-foundress nests was not quite sufficient, by itself, to select for subordinate behaviour. The small subordinates had lower potential productivities in comparison to the larger females. This decreased reproductive potential, when combined with the increased productivity of pleometrotic nests, was sufficient to make subordinate behaviour selectively advantageous. When both factors are taken together, subordinate behaviour is selected for as long as the coefficient of relatedness between dominant and subordinate individuals is greater than 1/4. This indicates that high coefficients of relatedness are not necessary for pleometrosis to be selectively advantageous under the conditions found in this study. Dominant females may suffer increased reproductive competition from their numerous workers. This, plus the difficulty of ensuring association with siblings in spring, may be the reason why multiple-foundress associations were uncommon at this locality. The data presented here are compared with those from other studies of this species. The factors promoting pleometrosis in halictines are compared with those that result in multiple-foundress associations in temperate polistine wasps.