By Sophie Berdugo
North-south divides are found worldwide, with stereotypes and tropes being associated with people living above or below a designated national boundary. Surprisingly, this phenomenon is not uniquely human. Paper wasps (Polistes fuscatus), whose range spans North America, also show distinct regional differences, with northern wasps being highly cooperative compared to their southern counterparts. And the secret to this difference? Northern paper wasps can recognise their nesting partners, but southern wasps cannot.
Paper wasps—named after the papery texture of their nests—are eusocial and cooperatively breeding insects, meaning only certain individuals reproduce within the colony and unrelated individuals help to rear the offspring. The process of nest founding by the reproducing females, called ‘gynes’, is either a cooperative or solo act. Cooperative foundresses team up with other gynes to share the workload, and a dominance hierarchy determines how many eggs each gyne can lay in the resulting nest. But it is essential to be able to tell individuals apart to have a cooperative nesting system. Dr Michael Sheehan, along with his lab at Cornell University, focuses on this exact link: the association between the evolution of individual recognition and cooperation in paper wasps.
Individuals within any animal population need to vary in appearance (known as ‘phenotypic diversity’) to recognise one another. Phenotypic diversity is found in paper wasps too; crucially, however, only in northern populations. Recent research from Sheehan’s lab found that latitude explains 74% of the variation in facial pattern diversity, with increasing latitude corresponding strongly with increasingly distinct features between individuals. Although this was studied using quantitative measures, a quick Google search of northern and southern paper wasps will show you just how much this rings true. Southern wasps in Louisiana and Georgia have uniformly red faces across the board. On the other hand, northern wasps in New York and Michigan have a mixture of red, yellow, brown, and black faces, all in distinctive patterns.
The researchers tested the assumption that northern foundresses should be able to recognise each other more than southern foundresses using a common garden experiment. Sets of four foundresses, three previous co-nesters and one outsider, were grouped in enclosures together, reflecting natural founding patterns. The enclosures contained either northern (New York) or southern (Louisiana) wasps, and each had nest building supplies provided. By looking at the which other gynes each foundress associated with before the nest building began, the researchers were able to establish a social network for the wasps. From this, it was clear that the southern wasps were not differentiating between individuals, suggesting a lack of knowledge about who was who. By comparison, northern wasps were selecting who to associated with, consistently forming huddles with certain foundresses more than others. This shows that individual recognition only appears to exist in northern wasps, and that this impacts the social organisation of the populations.
Evolutionarily speaking, the interest lies in whether there are corresponding fitness benefits—in terms of survival and reproduction—incurred from the northern ability to recognise one another. Incredibly, no multi-foundress nests built by southern wasps were successful, whereas all northern multi-foundress nests were. Although southern gynes did cooperate to build nests, no eggs ever became larvae, and foundresses either ate or simply removed the eggs of their supposed teammates. Northern gynes did not have this issue, with large nests being built and eggs developing into larvae. In fact, two of the nests made by cooperating northern gynes had worker wasps after a couple of months. Solo southerners were successful though, building nests which produced larvae. These findings mirror what is seen in natural populations, where there is a positive relationship between latitude and foundress number per nest. This north-south difference in the fitness benefits incurred by cooperative nesting is compelling, showing that being able to recognise fellow foundresses is critical for nest success.
This ability to recognise individuals in northern paper wasps stems from a recent evolutionary phenomenon. Population genetic research on P. fuscatus found that within the past few thousand years (which is recent in evolutionary terms) there has been a rapid spread of genetic information associated with the learning, visual processing, and long-term memory capacities needed for individual recognition. This rapid spread is known as a ‘selective sweep’, where a beneficial chance mutation takes over the existing DNA base pairs in that location, and then becomes stable within a population. In northern paper wasps, the selective sweeps are highly localised to cognition-related genetic regions, but these sweeps have not occurred in closely related species. This seems to suggest that the selection of genetic loci related to these particular cognitive capabilities resulted from the selection pressure of individual recognition. Although there were undoubtedly other pressures throughout paper wasps’ evolutionary history, individual recognition appears to have been an especially strong one.
This all begs the question: why do southern paper wasps not have individual recognition if there are advantages to the cooperative relationships it would allow them to form? There is currently no concrete answer to this. There is very little genetic variation between northern and southern wasps as there is gene flow across the range. This suggests that there are external factors favouring individual recognition and foundress cooperation in northern regions which do not exist in the south of the paper wasp range. Given that cognitive complexity is very energetically expensive, if cooperation is not necessary—for whatever reason—in the south, there would not be strong selection on the neural evolution required for individual recognition. Another possibility is that there is selection for individuals to look similar or have all red faces, which would also hinder the development of individual recognition. A next step would be to test these different hypotheses to better understand the evolutionary forces at play.
The recent and rapid evolution of facial recognition in northern paper wasps is a fascinating means to better understand the evolution of cooperation and social cognition more generally. The research by Michael Sheehan and his colleagues has established that social interactions can drive cognitive evolution and provides an avenue to identify specific genes of interest for a multitude of cognitive processes. Thus, there is definitely a north-south divide in paper wasps—the next step is trying to figure out why it exists.
Bluher, S. E., Miller, S. E., & Sheehan, M. J. (2020). Fine-scale population structure but limited genetic differentiation in a cooperatively breeding paper wasp. Genome biology and evolution, 12(5), 701-714.
Miller, S. E., Legan, A. W., Henshaw, M. T., Ostevik, K. L., Samuk, K., Uy, F. M., & Sheehan, M. J. (2020). Evolutionary dynamics of recent selection on cognitive abilities. Proceedings of the National Academy of Sciences, 117(6), 3045-3052.
Tumulty, J. P., Miller, S. E., Van Belleghem, S. M., Weller, H. I., Jernigan, C. M., Vincent, S., … & Sheehan, M. J. (2021). Evidence for a selective link between cooperation and individual recognition. bioRxiv.