1 month to go till I slip away back to tropical storms and toucans. Preparations from afar continue... A taxi driver conversation today took a weird turn when he asked (in deep Bristolian) what I'm doing with my life. Turns out "trying to attach miniature radio-tags to hundreds of wasps around abandoned buildings on the edge of South American rainforests" is a hilarious answer. Cue a long drive to Temple Meads station with the taxi driver doubled over in hysterics.
I won't give away exactly what I'm doing, in case other South-American-abandoned-building-wasp-radio-taggers (?) go in for a bit of industrial espionage. (Though I suspect that isn't too much of a risk.) In short, there's a fascinating evolutionary mystery concerning animal cooperation that we're interested in at Bristol, part of a rich tradition in evolutionary biology stretching back to the greats like W. D. Hamilton and John Maynard Smith. It's fantastic to have the chance to explore this enigma in high resolution using techniques like radio-tagging on wild populations.
I'm finding it an interesting exercise setting up a field project from abroad! Logistics are just about sorted now. And I'm very much looking forward to meeting Professor Alain Dejean (of the University of Toulouse) and Dr Bruno Corbara (Global Canopy Programme), who are both kindly lending their advice in French Guiana.
That's all for now then. Ciao!
Imagine you and some friends really really want ice cream. But to get it, one of you is going to have queue for hours. Someone has to take the costly plunge. Who will do it?
This kind of scenario - the 'volunteer's dilemma' - is the sort of thing that arises when only one individual (or, at least, a set number) is needed to generate a public good. In this case, it is ice cream for everyone. If nobody volunteers, nobody gets ice cream. If somebody volunteers, they can do the queuing. It pays everyone else to remain silent. Nobody wants to queue for hours.
The volunteer's dilemma has recently been under a bit of scrutiny in the context of punishing cheats. When you've got a group with more than two actors (an n-player group), and a cheat arises, that cheat can be deterred from cheating in the future if they are punished. The positive pay-off from preventing cheating is a public good, the benefits of which are shared across the group. But who will step up and pay the personal cost of punishing the cheat? Exotically, we end up with what we call a 'second-order' cheating problem: it pays to free-ride on the goodwill of someone else who'll punish the first free-rider (1). To keep with the ice cream example, it's like somebody has stolen everyone else's delicious chocolate 99-flakes, but nobody particularly wants to step up to punish them because it's a costly behaviour that will be injurious to them (let's say it involves a bit of a punch-up), even if it benefits everyone by deterring 99-flake thieving in the future.
There are two main ways the pay-offs in this second-order problem can work. It could be that more punishers means a proportionally better result. Or it could be that only one punisher (or, in more complicated set-ups of the volunteer's dilemma, a set number of punishers) are needed. The first is an n-player prisoner's dilemma scenario; the second is an n-player volunteer's dilemma. Another way of describing this is to say that the former is based on a linear pay-off function, whilst the latter relies upon a non-linear, binary pay-off function, which makes a sudden change at a certain level of punishment. There are, of course, a number of intermediate non-linear functions.
Let's imagine the n-player volunteer's dilemma in its simplest form, in which only one volunteer need step up in order to achieve the step change in pay-off to the group by getting on with the punishment. The evolutionary outcome of such a game is that both second-order free-riders and second-order volunteers persist in the population at equilibrium, and perturbations from this equilibrium result in a return to it (i.e. it is evolutionarily stable). Why? Because fitness in this game changes in a negatively frequency-dependent fashion. To see this point, have a think about this: the fitness pay-off of being a volunteer in a population where everyone is free-riding is higher than being a free-rider, but the fitness pay-off of being a free-rider in a population where everyone is volunteering is higher than volunteering.
There's a school of thought that says the volunteer's dilemma is a much more realistic vision of punishment than the prisoner's dilemma. Nichola Raihani and Redouan Bshary, for instance, think that we should expect the real world to involve step-changes: a set number of individuals should step forward, and any more would be silly, increasing the cost to the group for no gain (1). In the prisoner's dilemma (i.e. when the pay-off function is linear), we end up with free-riding winning unless individuals are able to assort in some way (such as by grouping with relatives, or remembering reputations). Non-linearity creates a stable mixed equilibrium through negative frequency-dependence (unless the cost of punishing is simply too big), so does away with the need for these additional assumptions (2).
The next step is demonstrating that such non-linearity of pay-offs is common in real punishment situations. Recognising this non-linearity may help transform our understanding of public goods problems, so watch this space!
To find out more!
(1) Raihani, N. & Bshary, R. 2011. The evolution of punishment in n-player public goods games: a volunteer's dilemma. Evolution, 65: 2725-2728
(2) Archetti, M. & Scheuring, I. 2010. Coexistence of cooperation and defection in public goods games. Evolution, 65: 1140-1148
You are a bacterium, and you are strenuously trying to persuade a squid to go into business together. The way you see it, you're a match made in heaven. You've got something bobtail squids want - the funky skill to produce bioluminescent light - and a squid could offer you a home. Only problem is: how does a humble bacterium convince a squid to take it on?
Evolution really likes signals of quality (just ask a peacock to show you his tail). To be believable ('credible'), these signals need to be costly. But the quirky, counter-intuitive world of handicap signals - famously proposed by Amotz Zahavi in Israel and mathematically elucidated by Alan Grafen at Oxford - is a story for another time. It involves individuals demonstrating their quality to a discerning watcher through a costly exhibition of quality that can only be borne by high-quality individuals. Since you are a bacterium trying to enter a bobtail squid, you're soon going to run up against a close cousin of costly signalling, a form of costly honesty known as 'biological screening'.
Screening occurs when the watcher actually imposes a costly task upon these individuals, like King Eurystheus imposing intense tasks upon Hercules. Again, we end up with an honest exhibition of quality: low-quality individuals will simply fail the test, so screen themselves out.
Your squid has set you a biological gauntlet: only high-quality individuals like yourself can run the gauntlet. Cleverly, this is because life within the internal niche set aside by the squid for bacterial applicants carries a risk of death from a bunch of chemical nasties called reactive oxygen species (ROS). Bacteria that carry out the task the squid wants - bioluminescence - do so with the enzyme luciferase, and actually using your luciferase ends up reducing ROS generation back down to safe levels. Thus, the squid screens its bacteria: those who fail to work simply cop it. This brutal job-hunting game was highlighted as a case of screening in the real world four years ago by Marco Archetti (1).
Archetti and colleagues also highlighted a peculiar form of screening they christen 'competition-based screening' (2). In this case, the applicants have to struggle against one another in an arena created for the purpose by a host. As an example, they offer the famous mutualism between acacias and ants. A little acacia colonised by a bunch of different ant colonies in different regions of the plant might find that colonies differ in their usefulness as defence against herbivores. The idea is that those regions of the acacia with useful colonies will grow better than those without, and eventually only the good colonies will be left.
My own PhD is on wasps (which, incidentally, means I have to spend a huge amount of time arguing with people who advocate mass waspocide). Archetti's ideas on competitive arenas remind me of a strange theory concerning the swarm-founding wasps of South America, who rush the building of their nests ('explosive nest construction'; 3). There are a few hypotheses for this behaviour kicking around, including that it is simply a good idea to protect your brood from the slings and arrows of the world by building your nest quickly. Another, however, is that building the nest hyper-quickly is a means through which workers (seemingly unable to nepotistically favour queens based on relatedness) deliberately create manic competition amongst hopeful queens in order to select them based on fecundity: those who can't exhibit their fecundity (by laying loads of eggs in the shiny new array of empty nest cells) end up excluded as candidates for the eminent role of reproducing (3,4).
So, next time you're taking on a new job, be thankful: if you were a bacterium, a moments slacking might spell death, if you were an ant colony you might die for not keeping up with the Joneses, and if you were a wasp queen you might be pumping out eggs like there's no tomorrow. Perhaps interviews don't seem that scary after all.
To find out more!
(1) Archetti, M. 2011. Contract theory for the evolution of cooperation: the right incentives attract the right partners. Journal of Theoretical Biology, 269: 201-207
(2) Archetti, M., Scheuring, I., Hoffman, M., Frederickson, M., Pierce, N., & Yu, D. 2011. Economic game theory for mutualism and cooperation. Ecology Letters, 14: 1300-1312
(3) Jeanne, R. & Bouwma, A. 2004. Divergent patterns of nest construction in eusocial wasps. Journal of the Kansas Entomolgical Society, 77: 429-447
(4) Loope, K. & Jeanne, R. 2008. A test of adaptive hypotheses for rapid nest construction in a swarm-founding wasp. Insectes Sociaux, 55: 274-282
Adventures of a
Dr Patrick Kennedy, Radford Lab, University of Bristol | Zoology