Why are wee wasps big-headed?

A blog post highlighting the article by O’Donnell in Insectes Sociaux.

By Sean O’Donnell

Wasps in the family Vespidae are attractive to social insect researchers because they present nearly the full range of animal social structures, from solitary living species to species with some of the largest, most complex colonies known.  It has long been recognized that some social species of vespid wasps, such as Vespinae (hornets and yellow jackets), have strong caste-related allometry, or differences in body size and shape. Queens and workers are typically distinct to the point that they are readily recognized with the bare eye- there is no need to pull out the micrometer to distinguish a yellow jacket queen from her workers. Some other social vespids also have caste allometry (O’Donnell 1998).

Another potentially fascinating aspect of vespid wasp diversity is the wide range of body sizes exhibited by this group, yet species differences in body allometry in this family seem to have been largely overlooked by researchers.

In a recent pair of studies, my lab explored the evolution of brain versus body allometry among Vespidae species (O’Donnell & Bulova 2017; O’Donnell et al. 2018). We found that smaller species had larger brains relative to their body size. This work inspired me to examine whether vespid wasp species differ in body allometry.

When we analyzed brain-body allometry, we used head capsule volume as our measure of species mean body size. Some reviewers suggested this was not the best approach: what if wasps’ heads varied allometrically with overall body size, and head allometry drove the apparent brain-body patterns? This could happen, for example, if smaller-bodied species had relatively small heads, and their brain size was constant, relative to overall body size. These comments inspired me to test whether wasp head capsule size varied with overall body size. My new findings on wasp head-to-body allometry show that not only were our brain allometry findings supported, they were conservative.

To study head-body allometry, I started by measuring species mean dry weights of the main body trunk (thorax plus abdomen) for the subjects of the brain studies. Because we had photographed the head capsules of the subjects of our brain allometry studies, I had head volume measurements for some species. I added new data and increased the sample size by weighing the head capsules of some of the species for which I had volume data, and I measured both head and body weights for several additional species. I included solitary vespids (potter wasps; Eumeninae), species from the subfamily Vespinae, and species from all tribes of eusocial subfamily Polistinae. The species examined ranged from some of the largest Vespidae (Vespa hornets, and the giant Asian paper wasp Polistes gigas) to some of the smallest swarm-founding Vespidae (Protopolybia and Leipomeles).

Importantly, I found that the two measures of head size, head dry weight and head volume, were tightly isometrically correlated. I then asked how head size varied with body size. All analyses supported a strongly significant negative head-body allometry, in other words, smaller-bodied species had relatively larger heads. This pattern held for head weight, head volume, and when only social species were included in the analysis. I used a special analysis to account for the potential effects of species relatedness on the negative head-body allometry, and the pattern was still highly significant. The magnitude of relative head-size variation was striking: in one of the largest species, head capsule weight approached a mere 5% of body weight, while in a small species, the head comprised nearly 30% of body weight.

Adult female Mischocyttarus sp. (left, a medium-sized species) and Protopolybiaholoxantha (right, a smaller species). I scaled the photos so the wasps appear to be about the same body length, and the relatively large head capsules of the Protopolybia workers are evident. The red scale bars represent approximately 1 cm in each photo.

I believe the strong interspecific head-body allometry in Vespidae is surprising, given that wasps are flying animals. Large heads could affect the wasps’ ability to fly by altering aerodynamics or by shifting the center of gravity. What factors might drive the evolution of allometrically enlarged heads in smaller species?

Our previously published brain size data suggest an answer. Remember that brain size relative to head size increased as smaller body size evolved. Since we now know that smaller wasps also have relatively larger heads, this means that the negative allometry of vespid brain size outpaces the negative allometry of head size. In other words, smaller species’ brains make up an ever-increasing portion of their relatively larger heads. Again, variation in the magnitude of brain size to head size was dramatic: brain volumes ranged from about 2% of head volume in the largest species, up to 12% of head volume in the smallest species.

In other vertebrate and arthropod lineages, the relatively large brains of the smallest species are associated with modification of heads including thinning of skulls (vertebrates) or head capsule walls (arthropods), and with reductions and bodily displacements of tissues such as muscles. Have the relatively large brains of vespid wasps driven similar changes in head capsule structure or physiology? Perhaps the need to house large brains has affected the behavior and ecology of small vespid wasps: limits on head musculature or head cuticle thickness might limit the wasps’ abilities to bite and chew building materials or food. If so, the need to house relatively large brains could set biomechanical lower limits on body size evolution in the family.

 References

O’Donnell S (1998) Reproductive caste determination in eusocial wasps (Hymenoptera: Vespidae). Annual Review of Entomology 43:323-346.

O’Donnell S, Bulova SJ (2017) Development and evolution of brain allometry in wasps (Vespidae): Size, ecology and sociality. Current Opinion in Insect Science 22:54-61.

O’Donnell S, Bulova SJ, Barrett M, Fiocca K (2018) Size constraints and sensory adaptations affect mosaic brain evolution (paper wasps- Vespidae: Epiponini). Biological Journal of the Linnean Society 123:302-310.

 

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