Animals that change colour as they grow do so because their size influences how easy it is for predators to find them

Background

Many animals have a coloured body that help them to avoid being eaten by predators. There are two main types of colouration. They may try to avoid being found by being camouflaged, termed crypsis. Or they may contain poisons or other distasteful chemicals and advertise them with warning colours such as yellow, red and black. Using these defences is especially important for insects during their larval stage (such as caterpillars), because they can’t fly away to escape. There are some very elaborate colourations used by larvae. Many species change from one type of defence to the other as they grow, usually from crypsis to warningly coloured. We constructed a computer model of growth and defence in a generalised insect in order to explore what determines how animals defend themselves. We explored the effects of ecological factors such as how effective warning colouration is, how this effectiveness increases with body size, and how much being cryptic means having less opportunities or time to eat.

Findings

Our results show that under most circumstances it is best for animals to start off cryptically coloured when small, since it is difficult for predators to spot them anyway. The clearest exception are when warning colouration can be reasonably difficult to spot until close up when they should be warningly coloured all the time. However, most of the time animals should be warningly coloured when large because it’s harder for them to hide and a large signal is easier for potential predators to recognise and avoid. We also surveyed photographs at www.ukleps.org for 185 species of caterpillar and found that 62% of the species were cryptic at all ages, 16% were warningly coloured all the time, and 19% were cryptic when small and warningly coloured when older. We used the model to predict how large each of the possible colour types should be when they mature, and found the sizes matched quite well. This suggests that the model has captured most the important details in the things that determine colour types.

Implications

Our results show that under most circumstances it is best for animals to start off cryptically coloured when small, since it is difficult for predators to spot them anyway. The clearest exception are when warning colouration can be reasonably difficult to spot until close up when they should be warningly coloured all the time. However, most of the time animals should be warningly coloured when large because it’s harder for them to hide and a large signal is easier for potential predators to recognise and avoid. We also surveyed photographs at www.ukleps.org for 185 species of caterpillar and found that 62% of the species were cryptic at all ages, 16% were warningly coloured all the time, and 19% were cryptic when small and warningly coloured when older. We used the model to predict how large each of the possible colour types should be when they mature, and found the sizes matched quite well. This suggests that the model has captured most the important details in the things that determine colour types.

Subject

Behavioural ecology


Subject Group

Zoology and Ecology


Keywords

Herbivory

Predation

Sequestration


Posted by

Andrew Higginson

on Tue Aug 29 2017


Article ID

FK9JR8FBY


Details of original research article:

Higginson AD, Ruxton GD. Optimal defensive coloration strategies during the growth period of prey. Evolution. 2010;64:53-67.

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