I have complained before (who? me? complain?) about the common journalistic fallacy that proof of X is not proof against Y. All proving X does is show that X is true, not that Y is false. However, it’s easy to see how this comes about – many ‘obvious’ anatomical features certain seem to have, or are famous for having, just a single function. Deer fight with their antlers, peacocks display with their tails and so on. However, even some supposedly monofunctional features can have more functions, if more minor ones – the antlers of deer and horns over various bovids are used for fighting off predators as well as rivals, but also act as rather minor heat-loss features. Sure, it’s pretty minor, but it is a function that they perform, and may even be selected for.
Some features however can have a great many functions and all of them may be important. Both the tusks and trunk of an elephant can be used for all manner of things – fighting, signalling, collecting food, manipulating the environment and others. It’s not only hard to say which of these is the most important now, but which (if any) may have been the original driving force behind it’s selection is very hard to tease out. Thus making explicit statements about other functions and the selection pressures behind them is a very risky thing to be do without good evidence. Sorting this out properly requires, as far as possible, testing individual hypotheses about function in isolation and rejecting or accepting them based on the available data.
This should be quite obvious, and yet inevitably in the media (and sadly, on quite a few occasions in the literature) this simple error is repeated. Single functions are tested and found to be a use of a feature but then this is then incorrectly extended to be assumed to be the sole or primary function. Watch out for these, they can be very misleading and it is an exceptionally easy trap to fall into.
Thanks to Rachel Kilby for the photo.