‘Same same but different’ Exploring evolutionary and ecological causes of eye diversification in bumblebees

Abstract

Bees rely heavily on vision during most of their interaction with the environment, but so far, visual abilities have not been included into functional investigations of these crucial pollinators. This is probably due to the lack of comprehensive and phylogenetically-controlled quantification of visual traits across species. In the present study, we used high-throughput micro-CT tools to quantify, compare and understand the diversity of visual traits of compound eyes in bumblebees. Visual systems of bumblebees were far from identical, with variations across sizes, castes and species. While phylogenetic proximity poorly supported interspecific variations, these were better explained by two ecological factors: social parasitism and habitat. The eye parameter – a metric that measures the relative investment of a compound eye into resolution or sensitivity – was lower in queens of social parasitic species than of non-parasitic species. Workers of species associated with forested habitat had distinct visual traits, including a higher eye parameter, than those of species living in open landscapes. These diverse visual traits are likely to provide selective advantages to bumblebees given their specific ecological requirements. We thus propose that social parasitism and forest habitat are drivers of the diversification of compound eyes in bumblebees. Finally, we discuss how the present study can inspire trait-based approaches in ecology and conservation biology.

Shining a light on species coexistence: visual traits drive bumblebee communities

“…This choice was supported by a fourth-corner analysis, where the eye parameter was significantly correlated to light intensity, while the inter-ommatidial angle and the facet area were not. To obtain the eye parameter for different species [ 2 ], we used measurements taken on bumblebee specimens from the Biological Museum at Lund University collected in Scania and Abisko (Sweden), as well as from specimens previously analysed in Taylor et al [ 30 ]. For each specimen, the left eye was prepared as described in Taylor et al [ 30 ].…”

“…We estimated eye parameter for all castes except queens of B. soroeensis , B. hypnorum and B. lapidarius for which measurements were not available. Since differences in eye parameter between castes within a species are smaller than those between species [ 2 ], we used the values of workers for these missing measurements. For the BTC complex, we averaged the data available for the two dominating species B. terrestris and B. lucorum (electronic supplementary material, table S2).…”

“…For the BTC complex, we averaged the data available for the two dominating species B. terrestris and B. lucorum (electronic supplementary material, table S2). As no data were available for the cuckoo species we observed ( B. sylvestris ), we used the average eye parameter of three other cuckoo species available in the dataset (electronic supplementary material, table S2), as the eye parameters of parasitic bumblebee species are significantly different from those of non-parasitic species [ 2 ]. Trait values and associated sample sizes are provided in the electronic supplementary material table S2.…”

“…Coexistence has traditionally been explained by niche partitioning, i.e. that species partition resources needed for survival and reproduction in space or time [ 1 , 2 ]. Local coexistence of bees is an iconic example of niche partitioning that has been explained by foraging specialization on flowers [ 3 , 4 ] based on trait matching [ 5 , 6 ].…”

“…A low eye parameter indicates an investment in resolution—which would aid in the detection of flowers in bright light—and a high value an investment in light sensitivity—which would improve vision at low light intensities [ 33 ]. The eye parameter trait has previously been associated with bumblebees' habitat preferences [ 2 ].…”