All of the above conditions can be arguably linked to changes in androgen levels. Yawns, thereby, facilitating transitions between distinct physiological states differing in terms of respective androgen and neurotransmitter levels, by lowering CSF pressure and aiding in CSF circulation. Costly signals are defined by Zahavi and Zahavi, among others as phenotypic signals that offer a measurement of genetic and physical fitness since they infer a handicap.
Costly signals are signals that are costly to maintain or display.
Only healthy animals have the necessary and excess energy and time available to maintain costly signals. The peacocks showy tail is the analogical epitome of runaway selection on a costly signal. The need for reliability links the signal directly to the message it conveys and guarantees that the cost of the signal is reasonable for an honest signaler but prohibitive for a cheater. In the case of a threat, the cost that the honest signaler is willing to undertake, and that a cheater cannot afford, is the risk that the signal entails. Rather than being a mere convention, the signal itself provides the genuine information that is needed to resolve the conflict.