When predators are present in the environment, prey species must change an aspect of their biology to give themselves a better chance at surviving. There are several ways that this can be acheived; changes in behaviour, in life history or in morphology. Of course these changes are not either or choices, and in fact many prey species combine several traits into a strategy.
Sometimes I find that non-biologists grasp this easier when I use the analogy of personal security. Imagine for a moment that you have a large sum of cash and that you need to get across town with your money intact, but on the way home there are some seedy areas where it is likely that you will get mugged. You have three choices:
- Go via a bank and deposit the money before going home.
- Avoid the seedy areas and take a longer route home
- Go to the gym, get big and muscley and learn to defend yourself, or at least look like you could.
Banking is the equivalent of making a life-history change, putting the money in the bank offsets the cost of getting mugged. If its in the bank already the mugger doesnt get it. Taking the longer route is a behavioural change – by avoiding risky areas the chances of being mugged are reduced. Finally, going to the gym and becoming big and scary is a morphological change – if you are (or look) big and mean, muggers will think twice and the chance of being mugged is decreased. Naturally the best strategy is to combine all three and change behaviour, life history and morphology, but there are costs involved with each change: banking is inconvenient, and your cash is not as easily accessible, taking the long way takes more time and energy and might mean you dont pass the supermarket on your way, and getting big and scary takes time, resources and dedication. These costs must be balanced against the benefits that they provide, and thats where a strategy comes in – balancing costs and benefits delicately to maximise the benefit whilst paying the lowest cost.
The model evolutionary ecology system I use to examine this is a nifty example of co-evolution between Daphnia pulex and Chaoborus sp. in a classic predator-prey interaction:
Daphnia are freshwater crustaceans that reproduce asexually (for the most part anyway). They feed on algae suspended in the water and once they reach maturity, like many animals they produce several offspring at once – in general, the bigger the daphnia the more babies she will produce. Chaoborus (a midge) larvae are voracious predators and will eat anything that fits in their mouth. When they are very young they mainly eat rotifers but as they get older they go through a phase where the preferred food are Daphnia. When the predator (Chaoborus) is present, the prey (Daphnia) are able to change their behaviour, avoiding predation throughout the day by inhabiting different areas of the pond than where the predators are (known as diel vertical migration), they change their life-history by growing quickly so they become too big to be eaten – changing their growth rate independent of maturation rate- and perhaps most importantly, they change their morphology: they grow spikes on their head which make them harder to eat. The interesting thing is that different clones use different strategies, employing a mix of behaviour, life history and morphology changes to outwit (in an evolutionary sense) their predators. Whats really really exceptionally interesting is that these changes occur in a clone. They are plastic changes – there is no change in genotype, when the predators disappear (seasonally) Daphnia revert to their “no predator” strategy and just carry on!!!
Now thats a cool story, but the really interesting question from my perspective is HOW DO THEY DO IT? What is the mechanism that allows these changes to occur. Clearly the Daphnia sense the predators and respond to them. As it turns out, the predators give off a chemical signature – technically its called a kairomone – that the Daphnia sense and through changes in endocrine signalling they generate alternative phenotypes.
My research uncovered the principal pathways involved – its principally through changes in juvenile-hormone-like signalling cascade, but the precise details are still elusive. We do know that the pathway is is conserved between responding to predators and also to parasites
This research continues in collaboration with Dr Andrew Beckerman (University of Sheffield) and the Daphnia Genome Consortium (all over the world),
Publications from this work:
Beckerman, A.P. Rodgers, G.M., Dennis, S.R. (2010) The reaction norm of size and age at maturity under multiple predator risk. Journal of Animal Ecology. 79:1069-1076
Dennis, S.R., Carter, M.J., Hentley, W.T., Beckerman, A.P. (2011) Phenotypic convergence under predation risk. Proceedings of The Royal Society: B 278 1687;16
Beckerman, A.P., de Roij, R., Dennis, S.R., Little, T. (2013) A shared mechanism of defense against predators and parasites: chitin regulation and its implications for life-history theory. Ecology and Evolution 3 (15) 5119-5126
Dennis, S.R., LeBlanc, G.A., Beckerman, A.P. (2014) The physiological basis of predator-induced plasticity. Oecologia 176 (3), 625-635
Carter, M.J., Dennis, S.R., Hentley, W. and Beckerman, A.P. (submitted) Adaptation and natural selection of the anti-predator reaction norm.