Musth as advertisement of male quality to females

Even more important than serving as a deterrent to rival males, musth may influence female choice of mates. As discussed, musth males advertise their condition through a specific infrasonic call that may be heard by females within a radius of 5-10 km. Temporal gland secretion seems to play a similar role in signaling the musth condition to estrous females, although the precise role of the signal is not yet clear (see chapter 4). Urine dribbling by a musth male, however, serves as a strong signal to potential mates in the area. Joyce Poole and Cynthia Moss were the first to point out that female elephants were attracted to musth males that had marked their paths with strong-smelling urine. Females inspected urine-marked trails with interest and were observed to follow such trails.

While high testosterone levels have been recorded in the urine of musth males, there are possibly other compounds as well that serve in communicating musth to females. Elephants may also be able to recognize information about individuals from such chemical signals.

The more fundamental questions are: What precisely are musth males conveying to estrous females? and Why do females evince such interest in mating with a musth male? Theory dictates that females should evolve a preference for mates with high genetic quality so that their offspring may inherit such traits. In this manner, the relative fitness of females discriminating among mates would be enhanced.

A signal used by a male to attract females, in principle, can be either an honest one of its inherent quality or a dishonest signal. According to existing theory, honest signals of genetic quality are costly to produce and maintain. For two individuals of equal quality, one honest and the other a cheat, the cheater is one that incurs less immediate costs for a given signal. In our case, cheating has nothing to do with the inherent quality (high, average, or low) of the signaler, but with incurring lower costs in signaling. If receivers (in this case, females) cannot discriminate between the two individuals, the cheater benefits in the short term. How can honest signals be maintained at all if cheaters can invade a population of honest signalers?

Alan Grafen has developed theory that demonstrates that, in fact, honest signaling of quality is evolutionarily stable, and that a cheater's average fitness would be reduced even when receivers (females) are nondiscriminat-ing. The theory is complex, but can be put in a nutshell. Any variation in genetic quality among the cheater's sons is not expressed in their signals. A cheater's son with higher-than-average genetic quality will not necessarily have a higher probability of being selected by a female. Daughters of non-discriminating females inherit this trait and end up with lower fitness as they select mates of lower quality on average (remember that some cheaters would have lower-than-average quality and would not be discriminated against). Thus, a costly but honest signal of male genetic quality is evolution-arily stable.

The evidence so far does suggest that musth is an honest indicator of male quality. It is well known among mahouts (elephant handlers) in Asia that captive male elephants do not come into musth if they are not in good body condition. In fact, they have used this knowledge to suppress musth deliberately in (otherwise troublesome) elephants by denying proper feed. Similarly, observations of wild elephants in both Africa and Asia indicate that undernourished or injured bulls do not come into normal musth.

Musth is an expensive proposition in the currency of energy. The reduced amount of time spent in feeding and greater time spent in searching for estrous females also means that a bull has to be in good condition in the first place to sustain the rigors of the musth period. High levels of androgens (e.g., testosterone) also increase metabolic rates and consume more energy. Thus, captive males in musth lose body condition even if they are chained and given normal quantities of forage. The constant dribbling of urine also exacts a physiological cost. A large male in musth may lose 350 liters of urine each day. When water is limiting, as in semiarid habitats or during the dry season, the physiological costs of urine dribbling for signaling can be substantial if the male is to leave uninterrupted trails.

Females choosing males in musth for mating thus seem to be choosing males of high quality. This can be demonstrated even more convincingly when the cost imposed by musth on the functioning of their immune systems is considered—but this is a subject that merits a separate discussion.

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