Glowing squid and sly acacias

Bobtail squid seeks radiant bacteria. (Image: Wikipedia)

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.

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Acacia ants in the genus Pseudomyrmex. R. Somma (Flickr; creative commons licensed).

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