Musth as a signal of intent to males

There has been much discussion and theory developed around the issue of how rival males should signal their fighting abilities or aggressive intentions. The approach of the more traditional ethologists was that the expression of excitement, aggression, fear, or pain conveys the signaler's motivational state. Some of these views have been challenged by game theory as applied to animal behavior by J. Maynard Smith, G. R. Price, and G. A. Parker. Maynard Smith developed the concept of the evolutionarily stable strategy (ESS), a strategy that, if adopted by most individuals in a population, cannot be invaded by any other mutant strategy. Game theory models have shown that, although signals conveying information about fighting ability (or resource-holding potential as the theorists often put it) can be maintained by natural selection, those that signal motivation (i.e., intentions during conflict situations) would quickly lose their value. Announcing to an opponent what you would do next is like plac ing all your cards on the table. Cheaters would be able to invade a population of honest signalers.

During conflicts between male elephants, a large male in good condition but not in musth almost always defers to a smaller musth male in poorer condition. It is hard to believe that, in actual combat, the small male would have a decisive advantage even if it is in musth. The musth condition of the small male thus cannot be an honest signal of its fighting ability alone. Why then does the large, nonmusth male give in to the small musth male?

Geoffrey Parker and Daniel Rubenstein developed a model in which they considered two types of asymmetries: (1) asymmetry in fighting ability or resource-holding potential; thus individuals may differ in size, strength, or weaponry, which would translate into probable costs incurred in contests between rivals; (2) asymmetry in resource value or rewards, by which the contested resource (say, estrous female) may be worth more to one opponent than to the other. Parker and Rubenstein considered how animals should behave in a contest when both these asymmetries interact. The mathematical formulation of the model is complex, but the solutions can be expressed in simple terms. When roles A and B refer to the asymmetry between opponents, which sustain costs at rates KA and KB, and resource values are VA and VB, a unique ESS obeys the following rule: Fight on estimating role A, where VA/KA > VB/KB, retreat in role B. In other words, if you are able to assess your opponent's fighting ability with few mistakes, the rule approximates a commonsense ESS: Retreat if you would be the first to spend more in a contest than the resource is worth to you, otherwise persist.

Joyce Poole applied the Parker-Rubenstein model, for which an interaction exists between fighting ability and resource value, to the observed patterns of musth in the Amboseli elephants. The data from Amboseli showed that, on average, twice as many female elephants come into estrus during the wet season (February-July) compared to the dry season (August-January). Thus, resource value is higher during the wet season. All male elephants have equal chances (assumed to be 75% in this case) of locating an estrous female. However, high-ranking males are able to guard and monopolize all estrous females they find, a medium-ranking male mates with any female not already located by a high-ranking male, while a low-ranking male obtains any female not mated by the others. Musth has certain costs that may reduce future fitness; these are loss of condition and injury or death in escalated contests. Dry season costs from urine dribbling (when water is scarce) are higher (by 50% in this model) compared to wet season costs. Further, fitness costs are higher in low-ranking males as they are more likely to meet higher ranking individuals.

The payoffs (value of winning versus sustaining costs) are considered for three possible strategies by males of different ranks:

1. Come into musth for a full year, but only in alternate years.

2. Come into musth each year during the wet season only.

3. Come into musth each year during the dry season only.

The relative payoffs are summarized for the different strategies and bulls in table 3.3. From this, it is seen that for high-ranking bulls, it is best to come into musth during the wet season each year. The medium-ranking bulls are now able to assume the resource value (estrous females) and costs of high-ranking males during the dry season. Thus, they maximize their payoffs by expressing musth during the dry season. Given the above scenario, the best strategy for low-ranking males seems to be to come into musth during the wet season, overlapping with the high-ranking bulls. When challenged by a high-ranking bull, a low-ranking bull would also do better by retracting the announcement of musth (stop secreting from the temporal gland or urine dribbling, if any). This would also lower their costs of musth and further increase their payoffs.

Some direct evidence for this game theoretic explanation came from sound playback experiments that Poole conducted at Amboseli during 1988-1990 (see chapter 4 for details of sound communication). Two types of infrasonic calls, the musth rumble of males and the estrous call of females, were used in the playbacks. When the musth rumble of a high-ranking bull was played, the other musth bulls in the vicinity approached the speaker with aggressive intent, while nonmusth bulls walked away. Similarly, the estrous call playback attracted the attention of musth bulls, which quickly approached the speaker, whereas the nonmusth bulls listened for some time before moving away from the source of the call.

The observed patterns of musth at Amboseli fit the predictions derived from the application of this model. By and large, the older and bigger males do come into musth during the wet season, the medium-ranking ones come into musth during the dry season, while the younger males come into musth sporadically during the wet season. Even when a large, high-ranking nonmusth male can potentially dominate a smaller musth male, it does not attempt to contest because any risk of injury may reduce its capacity to come into musth

Table 3.3

The 2-year payoffs (V/K) for male elephants of high, medium, and low ranks in expressing musth at various times of the year.

Table 3.3

The 2-year payoffs (V/K) for male elephants of high, medium, and low ranks in expressing musth at various times of the year.

Season of Occurrence of Musth

Male Rank and Payoffs High Medium

Low

Full year in alternate years

0.9

0.2

0.05

Wet season in both years

1.5

0.3

0.08

Dry season in both years

0.6

0.1

0.03

Source: From Poole (1989b). Reprinted from Animal Behaviour with the permission of Elsevier Science. K, costs incurred; V, value of a resource.

Source: From Poole (1989b). Reprinted from Animal Behaviour with the permission of Elsevier Science. K, costs incurred; V, value of a resource.

during a later (wet) season when its fitness returns would be much greater. Obviously, the spacing of musth in an elephant population would be site specific and depend on a variety of other factors, such as climatic seasonality, density and spacing of other males, forage availability, or even susceptibility to parasitic infections.

Observations by Rob Slotow and associates in South Africa even point to a control of musth in younger bulls by the older, higher-ranking bulls. During 1992-1997, the 10 young orphaned male elephants introduced into Pilanes-burg from Kruger (after culls in the 1980s) showed deviant, aggressive behavior such as killing several white rhinos in the park when they came into musth at a relatively young age of 18 years. The introduction of 6 older bulls from Kruger into Pilanesburg rectified much of this behavior by suppressing the onset and duration of musth in the younger bulls.

The recent findings of Bets Rasmussen and associates that the chemical nature of temporal gland secretion is different in younger and suggests a more complex nature of male-male signaling that needs to be unraveled. A combination of sophisticated laboratory chemistry, field testing of the chemical signals of musth, and behavioral observations fitted into the game theoretic framework are perhaps needed to provide a better understanding of the role of musth in male-male competition.

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