Nest guarding by male bees

In their recent PNAS article on biparental care in Ceratina nigrolabiata Friese, Mikát et al. (1) demonstrate biparental care and nest guarding by males and present these strategies as previously unknown in bees. However, nest guarding by males has been documented in multiple other bee species, including another species of Ceratina. I am aware of 13 studies that document or suggest nest-guarding behavior in male bees, though I am limited here to the most well-documented examples.

The most in-depth descriptions of nest guarding are on macrocephalic males of Lasioglossum erythrurum (Cockerell) by Kukuk and Schwarz (2). Like the present study by Mikát et al. (1), males of L. erythrurum primarily defended the nest against conspecific males; they also fought ants that were artificially introduced to the nest and even engaged in a small amount of nest construction and maintenance (2). Similarly, Houston and Maynard (3) observed macrocephalic male Leioproctus muelleri Houston and Maynard guarding nests against conspecific males and attacking a nylon thread inserted into the burrow.

Multiple observations have been made of Lasioglossum zephyrum (Smith) guarding nest burrows. In a 1903 paper, Melander and Brues (4) provided detailed descriptions of guarding of nest entrances by males who defended against mutillids by blocking the nest entrance with their abdomen [they reported the species as Lasioglossum pruinosum (Robertson) but were most likely observing L. zephyrum (5)]. Additional observations were made on males of L. zephyrum by Rau (6, 7), as well as by Barrows (8), who observed L. zephyrum and Augochlora pura (Say) males biting conspecific males who attempted to enter nests.

Finally, males of another Ceratina species, Ceratina smaragdula (Fabricius), have been reported to guard nests. Males were observed guarding burrows and blocking the entrance with their abdomen (9), and dissected nests revealed older males (rather than young, newly emerged males) inhabiting nests with females (10).

Mikát et al. (1) do an excellent job examining the foraging, reproductive success, and paternity of C. nigrolabiata and present a convincing case that males are largely motivated by increased mating opportunities. However, while Mikát et al. (1) do a more thorough job than previous studies on nest guarding in male bees, those previous studies provide valuable biological context and have implications for the interpretation of the results of the present study. In particular, Mikát et al. (1) present biparental care as an evolutionary alternative to eusociality, but this claim is undermined by the fact that male nest guarding occurs in a range of different social structures, including solitary (Leioproctus), communal (L. erythrurum), and eusocial colonies (L. zephyrum). Instead, this suggests that the evolution of male nest guarding is driven by processes separate from the evolution of sociality.