Hamilton’s anomaly: drifting wasps deepen our understanding of altruism

Deep after hours in the Life Sciences Building, the elevator doors closed on the faded light outside. They reopened to the sight of Patrick Kennedy, seated on a chair, perched between the printer, glass panels and offices. He was reading an edition of “Narrow Roads of Gene Land” by W. D. Hamilton, with well-turned pages. I asked him what he was still doing there, but the question didn’t feel like the right one. Why not? Patrick is known by many of his research group to be easily found with books in hand. He reads much and many, remembering snippets and quotes which he weaves through his own work. This particular book, a collection of Hamilton’s papers and theories, formed some of the first written roots to theories that explain how social animals have evolved and they have been the basis of thousands of papers, discoveries, and Patrick’s award-winning PhD thesis.

Hamilton’s rule is well engrained in studies of how social animals navigate their interactions and choices to live within or without their group. Altruism is the effort donated by one individual to another at its own cost. Such selfless acts for others would be favoured by natural selection if the benefit for the related recipient outweighs the cost for the altruist (image 1). Hamilton’s rule explains why animals are altruistic, and his theories were woven with living examples, wasps in attendance from the very beginning. “Hamilton was really taken with wasps” Patrick says smiling. “His revolutionary ideas about social evolution needed to be tested” and wasps were brilliant social insects to do so. Hamilton’s work took stage in 1963 and has inspired generations of social insect biologists ever since.

Image 1: Hamilton’s rule. Image courtesy of Isla Keesje Davidson

Insects are the most diverse group of animals, with their species tallying up more than half of all we know to live on this planet. Sifting down the biological behaviours of this incredibly diverse and abundant groups of organisms, we come to the eusocial insects. Their Greek-given name translating as “good” and “social”, these social insects divide labour to support the collective, and this behaviour places them within a small proportion of insect diversity. Of these wasps, ants and bees, the Polistes wasps form just over 300 recognised species and make up a lineage that is almost entirely ‘primitively’ eusocial (meaning members of the group haven’t lost the capacity to reproduce). This small subsection of particular wasps have been the workhorses to test Hamilton’s ideas for at least four decades in terms of inclusive fitness theory and how eusocial insects could have evolved to behave in this way (Nowak et al 2010).


Definitions (from Oxford Languages):

Kin selection: natural selection in which an apparently disadvantageous characteristic (especially altruistic behaviour) increases in the population due to increased survival of individuals genetically related to those possessing the characteristic.

Inclusive Fitness: an individual’s success at spreading copies of its genes in the population, whether by helping close relatives or by reproducing itself,

Altruism: behaviour of an animal that benefits another at its own expense.

Yet even back in 1964, when Hamilton’s theories described how relatedness (the proportion of shared genes) was what held societies together, it was those very societies of social wasps that did not fit the rules. “These are the big themes that we use now to explain social behaviour…” Patrick says. “At the end [of Hamilton’s book] there was a section called “anomalies” and among these anomalies were his observations of the paper wasps of Brazil moving between nests”. The irregular behaviour of these “drifting” wasps did not quite fit expectations of social animal behaviour. Like all animals, Polistes wasps should strive to increase the success of their own genes in the population. They have two options: attempt reproduction themselves or altruistically help their closest relatives who share the same genes.

Patrick mentions that biologists have observed what would be 10% of 10,000 wasps drifting between their nests. He leans forward: “We all like to think to be a wasp is quite simple, you just have to help at home, help your own family and be mean to everyone else”. But these wasps fly out of their own nest and go babysit for the neighbours, constantly moving between the nests next-door and helping each other. Patrick opens his palms “what on earth are these wasps doing? It’s like they haven’t read the textbook”. Some wasps may be moving for selfish reasons, such as attempting to lay eggs in other nests or attempt to inherit the role of queen. However, most of the wandering wasps show little interest in personal reproduction, and instead appear to lend a hand. Why waste effort on more distant relatives next-door when your closest relatives are in your home nest? This was part of what Patrick investigated.

Image 2: Closeup of a Polistes wasp. Image courtesy of Patrick Kennedy

Patrick initially learnt of this from a 2007 paper by Seirian Sumner, documenting the extraordinary extent of nest drifting observed in tagged wasps moving between the walls of a leprosy hospital near Panama City (Sumner et al 2007). As the study demonstrated, 56% of the observed wasps became drifters, but not due to mistaken nest identity, social parasitism, or bids for a succession of queens. So why this altruistic self-sacrifice of your own survival and reproduction to help less related neighbours? Patrick had read Sumner’s paper whilst working as a research assistant on a beetle project in the Panamanian rainforest and contacted Seirian to learn more. It was an enquiry borne out of curiosity, but then became the very questions nested into his PhD. Patrick explains how almost all other social insects are very hostile to other colonies, “bristling with rivalry”, earning colonies the title of ‘social fortresses’. The mystery of neighbourliness, generosity and babysitting in the wasps of South America earnt the drifting workers the title of ‘anomaly’ in Hamilton’s work in the 1960s.

The first question of Patrick’s PhD was to explore why the paper wasps of South America drifted. At first, it seemed like drifting might be a clever response to risk in a world of rapid changes and harsh consequences. “These wasps live in a really risky world. One day their nest is fine and the next it’s collapsing because all the babies are being eaten alive by flesh-fly maggots”. These wasps might therefore also invest in the nests of more distant relatives as a form of ‘bet-hedging’ (Sumner et al 2007). From the perspective of the drifter, your home brood and closest relatives might not make it from one day to the next, so spreading helping effort to your (albeit less-related) nesting neighbours could become more beneficial overall. When the stakes are high, “investment bankers would hedge their bets”. No one puts all their eggs in one basket, and no Polistes wasp drifter all their energy into one nest. Yet Patrick found that though this seemed so inherently logical, it did not work so neatly  when modelled mathematically. So why drift, Polistes? Together with biologists at UCL and Exeter, he investigated another possibility. Using long-term observations of broods, the team showed that as the number of workers in a colony rose, their relative contribution to the home brood diminished. With nest sizes varying from 10s to 100s of individuals, a drifter might simply be more useful to its distant kin in neighbouring nests than those in a busy home, and therefore might replace its bet-hedging ideas with calculations of diminishing returns (Kennedy et al 2021).

This volatile reality of a wasp’s world, however, did snag in Patrick’s thinking. How might a variable environment tilt the equation of Hamilton’s rule and thereby impact the choice to be altruistic? This formed Patrick’s second question for his PhD thesis. Reaching out to various gurus of evolutionary biology, Patrick dove into theoretical modelling and resurfaced with the idea that altruism in a volatile world may not directly impact the expected quantity of related offspring produced, but increase the certainty that those offspring will survive (Kennedy et al 2018). Patrick concluded this section of his thesis saying “sheilding relatives from a volatile world can drive the evolution of sociality”. This showed that these ‘rule-defying’ wasps may not be defending their fortresses as viciously as other insects, but they fly as an example of how variable conditions can, and do, affect the evolution of altruism.

Image 3: Polistes wasps tending their nest. Image courtesy of Patrick Kennedy

What Patrick seems reluctant to accept is praise for the unique quality and creativity of his work. A sheepish smile admits to the awards he won for best PhD thesis. He enjoyed the unravelling of function as a form of appreciation of the natural beauty of the biological world. “It’s great you can explain animal behaviour in this way”.

“The breadth of approaches adopted by Patrick for his PhD alone is unusual” says Professor Andy Radford, one of Patrick’s PhD supervisors. “Through a combination of wide reading, deep thinking and detailed discussions with academics in several different research fields, he drove forward a theoretical reworking of Hamilton’s rule”. This formed part of first winning the University of Bristol prize for the best thesis in the Faculty of Life Sciences 2018/2019, followed by being awarded the Zoological Society of London Thomas Henry Huxley Award and Marsh Prize for the best PhD thesis submitted by a university in the UK. “By focusing on important, profound questions (rather than getting distracted by low-hanging peripherals), drawing together disparate research fields, and combining theoretical and empirical work, Patrick produced a thesis of rare quality” Andy says, and this goes along more generally with him being “interested, and interesting”.

Despite being awarded particular recognition for his PhD thesis, Patrick has had to learn comebacks to justify the worth of his work. Friends and strangers alike, awaiting their pint at the pub, have challenged the “usefulness” of understanding wasp behaviour. “I once was cornered in a bar in Clapham by a plumber who told me I should be ashamed of wasting taxpayer’s money on something as completely pointless as wasps”. He smiles and says that it is important to “eye each other with a questioning suspicion”, as this challenge pushes you to justify the question of ‘why’, especially “in the context of a 6th mass extinction where I’m fiddling about with wasps”. People often carry the assumption that research has to be immediately practically useful to be valuable, but Patrick argues that it is important to keep asking these evolutionary and ecological questions. If we cannot answer them in wasps, then where is our basis for further and perhaps deeper understanding?

34 years ago, the ecologist E. O Wilson started his address to an audience at an invertebrate exhibit: “Let me say a word on behalf of these little things that run the world”. Enter the jungles of Brazil (where Patrick’s study species also roam) and gather up 200kg of dry weight of animal tissue per hectare – 93% of that will be invertebrate (Wilson 1987). We live in an invertebrate world. They were the masters for some 100 million years before we vertebrates evolved, their small size enabling them to divide up niches into domains of countless little specialists. This 500-million-year-long co-existence should remind us that we humans are not the only movers and shakers of our planet and “the truth is that we need invertebrates but they don’t need us. If human beings were to disappear tomorrow, the world would go on with little change… but if invertebrates were to disappear, I doubt that the human species could last more than a few months” (Wilson 1987). Patrick laughs: understanding the many unanswered questions about these creatures, their behaviour and how it evolved to be so is at least as useful as fixing the burst pipes of London.

An important part of Patrick’s work is his openness with fascination. In his thesis he quotes monks and their biological theories from the 1800’s as readily as new publications on theoretical models of animal behaviour. When pestered for his advice to PhD researchers, Patrick hesitantly responded “I suppose my one bit of advice would be to never feel guilty for reading something that doesn’t obviously link to your PhD title”. He shares ideas, he writes, both academic and not, on the story in the research and he is a favourite amongst his research group to delve into any idea for the fun of exploring it. Surely this passion to question plays a major part in his successes? It is definitely an aspect of Patrick’s way of thinking and doing that encourages those around him.

Richard Dawkins describes in his book “Unweaving the Rainbow” that to better understand nature is to better place ourselves within life’s wonders. Many people feel explaining the world’s mysteries removes the very marvel and magic they possess, while many scientists would argue the very opposite (Dawkins 2006). Patrick says, “we live in such a mind-bogglingly weird universe and it is boggling to think we even evolved at all”. Evolution links us all to common ancestors, and so studying how and why we are, as we are, allows us to reweave the world, which is not only meaningful, but it is beautiful. Patrick not only sees that but recognises the need to convey that in his work. Perhaps it is in the telling of these unravelled mysteries wherein lies the opportunity to inspire, ignite or re-kindle the fascination with what we perceive on this planet.

Image 4: painting of Polistes paper wasp and its nest. Image courtesy of Isla Keesje Davidson

Written by Isla Keesje Davidson (PhD)

To learn more about papers that have been published as part of Patrick’s thesis see:


Dawkins, R. (2006). Unweaving the Rainbow. Penguin Books

Kennedy, P., Higginson, A., Radford, A. et al. (2018) Altruism in a volatile world. Nature 555, 359–362 (2018). https://doi.org/10.1038/nature25965

Kennedy, P., Sumner, S., Botha, P., Welton, N.J., Higginson, A.D. & Radford, A.N. (2021) Diminishing returns drive altruists to help extended family. Nature Ecology and Evolution, https://www.nature.com/articles/s41559-020-01382-z

Nowak, M., Tarnita, C. & Wilson, E. (2010) The evolution of eusociality. Nature 466, 1057–1062. https://doi.org/10.1038/nature09205

Sumner, Seirian., Lucas, Eric., Barker, Jessie., & Isaac, Nick. (2007). Radio-Tagging Technology Reveals Extreme Nest-Drifting Behavior in a Eusocial Insect. Current Biology, 17(2), 140-145. https://doi.org/10.1016/j.cub.2006.11.064

Wilson, E. (1987). The Little Things That Run the World (The Importance and Conservation of Invertebrates). Conservation Biology, 1(4), 344-346. Retrieved February 22, 2021, from http://www.jstor.org/stable/2386020

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