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.

 

Interview with a social insect scientist: Graham Birch

IMG_20181026_181932835

IS: Who are you and what do you do?

GB: My name’s Graham. I’m a master’s graduate from the University of Exeter, coming to the end of a year working as a volunteer research assistant in South Africa for the Kalahari Meerkat Project. I’ll be starting a Ph.D. back at Exeter in September on Banded Mongooses.

IS: How did you develop an interest in your research?

GB: I’ve always been interested in sociality and how it’s evolved, and ants specifically can form such huge and complex groups, with multiple distinct castes. Many species are intensely territorial, with group size and make-up largely determining success; therefore, conflict may have been a significant driver in the evolution of ant societies. But the threat of competition is not necessarily ever-present and being ready for battle all the time may be wasteful, so an intriguing question is whether coordination and behaviour of these complex groups can plastically respond to the level of threat in the local environment in response to cues, which is the subject of our paper.

IS: What is your favourite social insect and why?

GB: Termites moult multiple times before reaching their adult forms, but some primitive species can moult backward in time to a younger form! They’re therefore able to plastically change their development in response to the colony running out of resources. They can also switch development between different castes if they like to a certain extent, which I just find super cool.

IS: What is the best moment/discovery in your research so far? What made it so memorable?

GB: I haven’t done too much of my own research so far, but I (but mostly my dad) did start a meme where scientists left Amazon reviews for items they used in their research (but not based on the intended purpose). Started when we had the idea of using tea strainers to protect ants we introduced into another colony, and my dad left a review about how great a capsule for ants these tea strainers were (anything to get his ranking up!), which the tea drinking public found a bit odd/hilarious. It became the top review, started a twitter trend of other scientists leaving similar reviews, which got picked up by The Washington Post (easy to find on Google!). I even did a couple of phone interviews. It was all very surreal but definitely memorable!

IS: Do you teach or do outreach/science communication? How do you incorporate your research into these areas?

GB: Again it’s a bit early in my career to answer this question, but I have lead turtle conservation tours in North Cyprus, and Meerkat research tours in the Kalahari, which did involve communicating the science of what we were doing to the public. I hope to do some demonstrating and maybe teaching during my Ph.D.

IS: What do you think are some of the important current questions in social insect research and what’s essential for future research?

GB: Broadly the big issue, not just for social insects, is climate change. Does the level of sociality mean these species are more able to shift their ranges or, if they can’t, can they deal with new competitors or enemies that can (as well as changes in temperature itself), over other less social species?

IS: What research questions generate the biggest debate in social insect research at the moment?

GB: I fear I may be a bit naive at my career stage and may not have enough experience on social insect research generally (beyond group conflict) to comment on this. However, what I have found to be controversial is the definition of eusocial when looking at non-insect taxa. For example, there was a lot of buzz about naked mole rats being the first eusocial mammal due to their large groups and division of labour among reproductives and workers, but recently many have turned against this. You could even say the same for humans where, even though we don’t have fixed caste determination, we have orders of magnitude larger and more complex societies then eusocial insects in terms of numbers and the division of labour we see in the variety of jobs we can pursue. Maybe it’s just unhelpful to use the term for non-insects.

IS: What is the last book you read? Would you recommend it? Why or why not?

GB: Ultrasociety. Phenomenal book looking at how human societies have changed and evolved from egalitarian hunter-gathers to incredibly unequal archaic societies led by God kings, to how religion and war shape these into the relatively more equal modern societies we see today.

IS: Outside of science, what are your favourite activities, hobbies or sports?

GB: Badminton, swimming, diving, and travelling.

IS: How do you keep going when things get tough?

GB: Try to not have your research as the only focus taking up all of your time. Get involved in a sport, volunteer, or just something to focus on that isn’t science / your degree, so when you do get stuck you have something else to work on that refreshes you for when you come back to your science.

IS: If you were to go live on an uninhabited island and could only bring three things, what would you bring? Why?

GB: Anyone at the Kalahari Meerkat project, or who went with me on my expedition to Madagascar over a year ago, can vouch for me when I say I’ve almost already done this, but on the island of an isolated research base (days of travelling away from the nearest major town with no communication with the outside world in the case of Madagascar). Anyways, I’d bring a Kindle with a vast library of books so I can read to pass the time, a snorkel and fins (since I’m on an island I might as well enjoy the marine life), and a Camelback so that I don’t have to carry a water bottle around in my hands.

IS: Who do you think has had the most considerable influence on your science career?

GB: Again, I’m very early into my career, BUT I would have picked a Natural Science degree rather than Zoology at Exeter if I hadn’t read the Selfish Gene and Extended Phenotype during the summer before year 11. This made me incredibly passionate about evolution, especially of behavioural strategies. I am so thankful I made that decision, so for that alone, Richard Dawkins.

IS: What advice would you give to a young person hoping to be a social insect researcher in the future?

GB: Well, I’m not exactly old myself… but just read some books about behavioural ecology/evolution and see if you get hooked. If you do and find yourself on a zoology or similar degree, read up on the profiles/publications of all of the lecturers at your university and seek the ones that share your interests / get involved! Do this early, and you may be working with them on your thesis later on and perhaps beyond.