sexual ornaments, such as increased eye span. David demonstrated that some male genotypes develop large eye spans under all conditions, whereas other genotypes progressively reduce eye spans as environmental conditions deteriorate. Several non-sexual traits, including female eye span and male and female wing length, also show condition-dependent expression, but their genetic response is entirely explained by scaling with body size. Unlike these characteristics, male eye span still reveals genetic variation in response to environmental stress after accounting for differences in body size. Thus, David inferred that these results strongly support the conclusion that female mate choice yields genetic benefits for offspring as eye span acts as a truthful indicator of male fitness. Eye span is therefore not only selected on the basis of attractiveness, but also because it demonstrates good genes in mates. Directed at other species The signal receiver need not be a conspecific of the sender, however. Signals may also be directed at predators, with the function of showing that pursuit will probably be unprofitable. Stotting, for instance, is a sort of hopping that certain gazelles do when they sight a predator. As this behavior gives no evident benefit and would seem to waste resources (diminishing the gazelle's head start if chased by the predator), it was a puzzle until handicap theory offered an explanation. According to this analysis, if the gazelle simply invests a little energy to show a lion that it has the fitness necessary to avoid capture, it may not have to evade the lion in an actual pursuit. The lion, faced with the demonstration of fitness, might decide that it will not catch this gazelle, and thus, avoid a wasted pursuit. The benefit to the gazelle is twofold. First, for the small amount of energy invested in the stotting, the gazelle might not have to expend the tremendous energy required to evade the lion. Second, if the lion is in fact capable of catching this gazelle, the gazelle's bluff leads to its survival that day. Another example is provided by larks, some of which discourage merlins by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture. See also Aposematism Game theory Multiple sexual ornaments Parasite-stress theory Sacrifice Signaling game Signaling theory References External links Honest Signalling Theory: A Basic Introduction. By Carl T. Bergstrom, University of Washington, 2006. Animal communication Ethology Selection Sexual selection