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Corbiculate bees offer the opportunity to analyse the evolution of differences in body size between sexes in relationship to the social environment, because this group of phylogenetically related taxa have contrasting levels of social organization, ranging from solitary to highly eusocial. Sexual size dimorphism evolves because body size is usually related to reproductive success through different pathways in females and males. Female body size is strongly correlated with fecundity, whereas in males the body size is correlated with mating success. Within a clade, if sexual selection on males has been stronger than selection on female fecundity, a sexual size dimorphism bias to males would be expected (Rensch's rule), or an opposite trend when the fecundity of females has had a stronger effect. We tested these predictions by taking an allometric approach that considered differences in the capacity of a reproductive female to start a nest on her own or not. Phylogenetic analysis of the whole clade and another analysis including only species in which the females start nests on their own showed that the corbiculate bees do not seem to follow Rensch's rule. The males and reproductive females of the species in which the females have lost their capacity to start nests were smaller than the reproductive individuals of species in which the females start nests on their own, suggesting that selective pressures linked to the evolution of sociality might constrain the evolution of a large body size in both males and reproductive females. [ABSTRACT FROM AUTHOR]

In temperate regions of the world dominated by intensive agriculture, cities harbor a rich diversity and abundance of bee species, often exceeding those of the rural environment. In less industrialized tropical countries, in contrast, stressful conditions may exist in cities for bees with perennial colonies such as stingless bees because of the lack of resources such as amenity green spaces (flowers) and appropriate nesting sites. Yet, we currently lack studies on the ecology of bees in tropical urban areas to test this idea. Here, we compared the body size (as a possible indicator of floral resource supply) and the frequency of diploid males (as a measure of inbreeding) of a widespread stingless bee, Nannotrigona perilampoides, at mating congregations within the city of Merida, Yucatán, Mexico, and in surrounding rural areas. We found that males in Merida were significantly smaller compared with those in rural areas. While diploid males were found in both in the city and rural congregations, their frequency was significantly higher in urban sites. Our results suggest that the performance of colonies of N. perilampoides is poorer in tropical urban compared with rural sites, possibly as a consequence of limited resources and mating among a small number of colonies. We discuss the implications of these findings for the survival and fitness of stingless bees in the tropics. [ABSTRACT FROM AUTHOR]

It is generally accepted that temperate subspecies of honey bees can maintain stable temperatures inside their nests; however, little information is available on the cooling ability of tropical honey bees and the effect of high environmental temperatures on individuals. In this study, we registered temperatures in the brood area of strong- and medium-populated colonies of Africanized honey bees during heatwaves (maximum environmental temperature 44 °C) between April and May in the tropical Yucatán Peninsula of México. To evaluate the effect on colonies, we compared the body size of workers produced under high temperatures in the field and siblings produced at stable 34–35 °C laboratory conditions. We found that in the field, temperatures of the brood nest in both types of colony can increase above 36 °C during the hottest part of the day, especially in less strong colonies. Workers produced in the field were significantly smaller, compared to those reared in the lab at stable temperatures. This is a first study reporting body size reduction in honey bees as a consequence of possible thermoregulation disruption during heatwaves. We discuss our results considering potential effects of heat stress to Africanized honey bee colonies and tropical beekeeping. [ABSTRACT FROM AUTHOR]

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non‐Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed a male‐biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non‐Melipona species. Our results in the stingless bees question the classical explanation of female‐biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size. We performed an ancestral character reconstruction of the evolution of sexual size dimorphism (SSD) and controlling by phylogenetic effects, and we explored the allometric relationships among queens, males, and workers in stingless bees. Our results confirmed the existence of two opposite patterns of SSD, Melipona show a male‐biased SSD, and non‐Melipona species generally have a female‐biased SSD, which suggest that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. Proximal causes to explain the two observed patterns of SSD in stingless bees could be related to fecundity selection, sexual selection, and the relative length of development of queens in Melipona and non‐Melipona species, possibly reflecting an interplay between self‐interest and colonial interest which requires further analysis in this and other highly eusocial taxa. [ABSTRACT FROM AUTHOR]