Cost, expenditure and vulnerability

The handicap principle (HP) stipulates that signal reliability can be maintained if signals are costly to produce. Yet empirical biologists are typically unable to directly measure evolutionary costs, and instead appeal to expenditure (the time, energy and resources associated with signaling behavior) as a sensible proxy. However the link between expenditure and cost is not always as straightforward as proponents of HP assume. We consider signaling interactions where whether the expenditure associated with signaling is converted into an evolutionary cost is in some sense dependent on the behavior of the intended recipient of the signal. We illustrate this with a few empirical examples and demonstrate that on this alternative expenditure to cost mapping the traditional predictions of HP no longer hold. Instead of full information transfer, a partially informative communication system like those uncovered by Wagner (Games 4(2):163–181, 2013) and Zollman et al. (Proc R Soc B 20121878, 2012) is possible.