The ‘Risky-Decoy’ hypothesis

by Bill Bateman & Trish Fleming.  If an ecologist asks you if you are good at modelling you might think that they are referring to something mathematical, and start running in the opposite direction. But a simpler kind of modelling is often used by behavioural ecologists who are interested in predation.

If you wanted to know what organisms get preyed upon, and by what, you could go out and spend countless hours hoping to see your study species getting attacked, or you could be smart and make ‘stand ins’ for your study species – stunt doubles – who will get attacked by the predators instead.

This is where the modelling comes in – modelling clay, or plasticine, is ideal for making stand in lizards and snakes which can be placed out in the wilds to be found by natural predators. Non-toxic and soft, modelling clay does no harm to the predators – apart from disappointing them – and the marks of the attack are left in the clay, often in enough detail to allow you to identify the predator.

We have used models in a recent study on lizards in the USA. We made models of Southeastern five-lined skinks (Plestiodon inexpectatus), a species that, although a dull brown as an adult, sports a fetching electric blue tail when young. A blue (or green, or red) tailed ontogenetic stage is surprisingly common in lizards, and seems counter intuitive as it makes the individual more conspicuous to predators, increasing their risk of being attacked. But a bright-coloured tail also seems to have a survival benefit (and this is where the ‘decoy’ bit comes in).

Bateman, Fleming, Rolek 2014 Bite me: Blue tails as a ‘risky-decoy’ defense tactic for lizards Current Zoology 60, 333-337
One of our highly-accurate models – it has been in the hot Florida sun, which can do bad things to plasticine.

By putting out models of lizards with blue tails or brown tails we found that, although a blue tail makes a lizard more obvious such that it is found sooner and attacked sooner (by birds) than are brown models, the attacks (as evidenced by the peck marks left in the modelling clay) are more likely to aimed at the tail (which can be autotomised) than the head, which would be fatal. All-brown models were attacked less, but attacked proportionally more on the head. Ouch!

Bateman, Fleming, Rolek 2014 Bite me: Blue tails as a ‘risky-decoy’ defense tactic for lizards Current Zoology 60, 333-337
Bateman et al 2014 Fig. 1: a) Time (days) until the first damage by predators on blue-tailed and all-brown models of skinks was noted, and b) distribution of attacks according to position on the model. Values are expressed in terms of ‘sample days’ (i.e. 24 models of each color, each checked over 7 days).

Young Plestiodon lizards often forage in different, more open, habitat than adults, and are smaller and less likely to survive an attack. So the ontogenetic stage of having a bright-coloured tail is likely to help these animals survive if they are exposed when foraging.  Flashing it about when foraging is also likely to ensure that it is your tail, and not your body, that is attacked.  [some neat footage of Ctenotus calurus tail flailing].

We called this the ‘risky-decoy’ hypothesis – if you risk attack, it is better if those attacks are aimed at parts of your body you can sacrifice or can survive being attacked.  So a bright-coloured tail means you risk more attacks, but decoy them away from your head.

A recent paper (Fresnillo et al. 2014) has supported our risky-decoy hypothesis, but with a lacertid lizard (Acanthodactylus erythrurus) that has a red-tailed ontogenetic stage.  [We also have to admit that they made some lovely, detailed plasticine models that put ours to shame.]  It will be good to have further studies test this theory in the future…

Fresnillo et al 2014.  they made some lovely, detailed plasticine models that put ours to shame.
Fresnillo et al 2014 made some lovely, detailed plasticine models that put ours to shame.
Fresnillo et al 2014 Fig 2.  The attacked models show clear marks indicative of particular predators: birds, rodents and other lizards.
Fresnillo et al 2014 Fig 2. The attacked models show clear marks indicative of particular predators: birds, rodents and other lizards.

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