Musth, not tusks, confers advantage to duelling males

Tuskers fighting.

Two tusked bulls fighting. Photo by Karpagam Chelliah.

Bull Asian elephants come in two forms: tusk, and tuskless (this is termed dimorphism).  It’s long been thought that tusks must confer an advantage in competitions between males for dominance and mating rights.  However a recent study by Karpagam Chelliah and Raman Sukumar of elephants at Kaziranga National Park, India, puts a wrinkle on this common story.

With some dedicated field research, Chelliah investigated how three factors influence a male’s competitive edge: tusks, body size, and musth.  It is common among mammals as among many other species that body size generally determines the winner in physical contests. In the case of elephants though, the possession of weapons such as tusks could upset this outcome.  In addition, elephants come into a physiological state termed ‘musth’ which is characterized by higher levels of testosterone and aggression toward other bulls, which can last from weeks to months.  In African elephants, where most bulls tend to carry tusks, it’s body size (and age, which determines body size) that settles the outcome. Musth, however, can trump size, conferring the ability on smaller animals to dominate larger ones [2].  Because bull Asian elephants can appear with or without tusks, this system sets up the chance to test the interplay of all three factors.

After 400+ days of observations on foot over a period of three years meticulously documenting 116 interactions among animals of varying sizes and physical characteristics, the authors found that musth was still the most important predictor of success in winning, followed by body size, and lastly tusks.  Those with tusks were more likely to win when evenly matched in the other two respects, but being in musth overwhelmingly decided the winner. (*Edit: confrontations between matched tuskers can be fatal!  See news report here)

This leads one to wonder naturally why males should have tusks in the first place.  Chelliah speculates that tusks possibly appeared earlier in the evolutionary history of elephants, whereas musth appeared later.  They may also be maintained through female choice (though it’s difficult to see how females could actively maintain such a preference if multiple males challenge the suitor of choice).  Another possibility is that tusks confer some advantage earlier in life – perhaps during adolescence, as males attempt to sort out their social positions.  There is some indication of this sometimes in Uda Walawe, as juvenile and subadult males engage in sparring – young bulls such as Little Fangs can sometimes turn the table on bigger, older, males once they learn to use their tusks as weapons (video below).  But it may take experience to develop this skill adequately.

Here is another hypothesis, based on our observations in Uda Walawe.  Unlike in India and much of south/southeast Asia, where there tuskers constitute a non-negligible proportion of the population, in Sri Lanka tuskers are in the minority (possibly just 2-5% of the population).  In Uda Walawe, the few adult tuskers who appeared in the park always did so during musth – therefore it was impossible to observe their behavior out of musth.  However, there were plenty of other males of about comparable size, who were also in musth.  The tuskers however always succeeded in dominating the other bulls and monopolizing access to reproductive females.  This leads me to wonder if there might be some sort of frequency-dependent selection acting on top of learning experience.  In populations where there are lots of tuskers as well as tuskless males, the tuskless males may eventually learn tricks to overcome tusked opponents through their repeated encounters (such as grabbing hold of the tusks).  But in populations where tuskers are rare, like in Uda Walawe, tuskless bulls may not have enough experience with them to have developed successful tactics.  This is similar to the case of left-handed athletes such as tennis players, cricket bowlers, or baseball pitchers. In a right-handed world, players who are lefties may have an edge against their opponents, who lack the experience to counter their plays effectively.

This hypothesis is yet to be tested, but perhaps future studies will be able to do so. Till then, this study is a great example of how careful observation can upend our common intuitions about nature.

Read more details of this fascinating study here.[1] Chelliah, K., & Sukumar, R. (2013). The role of tusks, musth and body size in male–male competition among Asian elephants, Elephas maximus Animal Behaviour, 86 (6), 1207-1214 DOI: 10.1016/j.anbehav.2013.09.022

[2] Poole, J. (1999). Signals and assessment in African elephants: evidence from playback experiments Animal Behaviour, 58 (1), 185-193 DOI: 10.1006/anbe.1999.1117

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