Saturday, 23 May 2015

More Poo For Creationists.

Bent posture improves the protective value of bird dropping masquerading by caterpillars

The thing about evolution is that it's opportunistic. If something can be done by an organism and it gives it an advantage in their particular environment at that particular time, there is every chance that it will be done.

Some time ago I wrote a series of blogs on mimicry showing how a perfectly harmless species can come to resemble a harmful one if that deters predators. For this to work, the predator also needs to be a potential predator on the harmful species or to have already evolved a set of avoidance strategies otherwise it won't 'see' the mimic as representing a threat. The predator needs to be repulsed or deterred from eating the mimic.

Of course, there are other forms of mimicry such as cryptic shape and colouration to resemble the background. This can be amazingly effective making some animals almost invisible to a casual observer and even to a trained observer of a hungry predator. A very good way to avoid being eaten is to masquerade as something inedible like a stone or a twig.

Falling somewhere between cryptic mimicry and deterrent or repulsive mimicry is this one which has been known about for some time but never formally proven to be effective until now. It's a caterpillar that resembles a bird dropping, not just in its colouring but in the way it curls itself.

  • We tested whether a bent posture enhances bird dropping masquerade in caterpillars.
  • Bent posture improved the survival of bird dropping-coloured caterpillar models.
  • Bent posture did not affect the survival rate of green caterpillar models.
  • We show for the first time the protective value of posture in masquerading prey.

Masquerade describes a defence by animals that have evolved to closely resemble inedible objects such as twigs, stones or bird droppings. Animals that masquerade often match their models in size or shape, and may even adopt specific postures in order to enhance their resemblance, causing predators to misclassify them as their model objects. The caterpillars of some moth species resemble bird droppings, and bend their bodies while resting on branches or leaves. We hypothesized that such bending might enhance the caterpillars' resemblance to real bird droppings. In this study, we tested this hypothesis by pinning artificial caterpillars with green or bird dropping coloration onto tree branches in both straight and bent postures, after which we exposed them to bird predation in the wild. We found that the adoption of a bent posture resulted in a lower attack rate by birds on the artificial caterpillars with the bird dropping coloration, while green caterpillars experienced no benefit from the same treatment. This study is the first experimental demonstration of the protective value of a specific posture in masquerading prey, and highlights the importance of considering an organism's shape and posture in the study of masquerade.

Copyright © 2015 Elsevier B.V. or its licensors or contributors

Obviously, few birds are going to eat their own droppings or those of other birds. Indeed, there are probably very good reasons why they would be repulsed by them as they would probably have very little nutritional value but might well carry diseases such as the eggs of parasitic worms.

So, we have a species which is a member of an ancient order - moths probably split off from other insects about 190 million years ago - masquerading as the droppings of an order which only evolved much more recently. It doesn't take a genius to recognise that there would have been no benefit in resembling a bird dropping before there were birds. Not only would it not deter birds from eating the caterpillars but it wouldn't deter anything else either. In fact, it wouldn't have been a case of cryptic mimicry or any other sort of mimicry. It would have been a complete waste of time.

So, any creationists or 'Intelligent Design' hoax dupes willing to have a go at explaining why an intelligent designer would have designed a moth caterpillar to resemble a bird dropping several tens of millions of years before there were birds? Or did it change it's mind later when it realised it had just designed birds and so changed the caterpillar?

Or is it that this is just a case of opportunistic evolution in a species in which it became an advantage to avoid getting eaten by birds once birds had evolved?

'via Blog this'

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