Over hundreds of millions of years, pollinators and many plant species have coevolved intricate mutualisms through which both partners benefit – plants see their genetic material dispersed to new mates, and pollinators receive rewards in the form of nectar. Different species have evolved floral morphologies that restrict floral reward access to particular pollinator species; for example, flowers with long corollas have rewards that are accessible only to long-tongued pollinators. The function of different floral morphologies is, in part, to ensure that pollinators that gain access to a plant’s nectar are also contributing to the plant’s reproductive success. However, the phrase “cheaters never prosper” doesn’t really apply to natural systems, and many pollinators “rob” nectar by drilling holes in a plant’s corolla. As is often the case, some of the first descriptions of nectar robbing can be traced back to Charles Darwin, who, in 1872, was the first to suggest the nectar robing was a socially-learned behavior.

Social learning, simply described, occurs when social interactions bias the behavior that individuals learn. Most studies of social learning have examined vertebrates, but some recent work has shown that social learning also occurs in invertebrates, specifically eusocial bees. In a recent paper in Behavioral Ecology and Sociobiology, Goulson et al. studied nectar robbing of the plant Rhinanthus minor in different alpine meadows, and found that, within a meadow, nectar robbing holes were predominantly found on one side (either left or right, from the researcher’s perspective) of the corolla. This result suggests that bees not only observe which flowers other individuals are visiting, but also notice specific details of nectar robbing behavior and copy that behavior. While it is possible that “handedness” (left or right) of nectar robbing could be an evolved trait, this explanation seems unlikely for a few reasons: 1) meadows were close enough for bees to move between them, so if handedness was coded into bee DNA we’d expect to see a mixture of left- and right-side holes in the same field; and 2) within a field, patterns of handedness were not consistent from year to year.

Now let’s bring this back to a common topic of this blog: indirect genetic effects (IGEs). Since the late 1990s, indirect genetic effects have been recognized as integral parts of social learning and evolution (e.g., Moore et al. 1997, Wolf et al. 1998). (This post has focused on social learning in non-human populations, but IGEs may be important to social learning in humans as well). Despite the importance of IGEs to social learning, I haven’t been able to track down studies that have explicitly discussed the implications of IGE theory for nectar robbing. If “nectar robbing” is in fact reciprocally-displayed trait, then the rate of phenotypic change in this trait could be magnified by IGEs (Moore et al. 1997) and it’s possible that IGEs were integral to evolution of nectar robbing in the first place. But that’s getting fairly speculative. What’s clear is that Goulson et al.’s work contributes to a novel but expanding body of literature showing that social learning (and therefore IGEs) is important to invertebrate species.  

Goulson, D., Park, K. J., Tinsley, M. C., Bussière, L. F., & Vallejo-Marin, M. (2013). Social learning drives handedness in nectar-robbing bumblebees. Behavioral Ecology and Sociobiology, 1-10.

Moore, A. J., Brodie III, E. D., & Wolf, J. B. (1997). Interacting phenotypes and the evolutionary process: I. Direct and indirect genetic effects of social interactions. Evolution, 1352-1362.

Wolf, J. B., Brodie III, E. D., Cheverud, J. M., Moore, A. J., & Wade, M. J. (1998). Evolutionary consequences of indirect genetic effects. Trends in Ecology & Evolution, 13(2), 64-69.