Musth is a period among adult male elephants, (over 15-20 years old) sometimes referred to as similar to male deer's "Rot". There is a three weeks pre-musth-condition, about one month high-musth, and one post-musth condition. Their temporal glands become swollen, from where a strong smelling fluid, rich of testosterone, runs down on their cheeks. During musth the males are very aggressive, and sexual active. According to Cynthia Moss, author of the book elephant memories, the older females in a heard gives passage to musth males when a female is in heat, while they block the way for younger males, not in musth. But males are always able to cover a female, also outside the musth period.
The reason for the musth seems to be that it prevents inbreeding. If the bulls would not be at their aggressive peak only for a month a year, only the largest, most dominant bull would cover the females all the time. In captivity, the limited space makes it dangerous to let a bull in must condition to a female, since she may be attacked and wounded if she is not willing to mate.
Most cases of elephants killing their handlers, or reports of a"rogue" wild elephant running wild through villages and attacking people, is during musth, when they get out of control, and their level of agression is high.
Younger bulls usually also go into a similar condition, from about their tenth year, but not so strong, and not so long time usually, a couple of days or a week, developing stronger musth for every year, with a peak between 30 and 40, then gradually showing less symptoms.Bulls aging over 55 does seldom show musth at all. Many Indian mahouts claim that Makhnas show lesser musth than tusker´s. In captivity, bulls are claimed to have been in musth for over a year, (Hellabrunn Zoo, Munich) or longer. (CopenhagenZoo)
Asian Wild bulls often go into musth during rain season, and when they have had unnormal access of food, so the reason for the longer musth periods in captivity may have a connection with the short days during winters, (hormones often depend on daylight)over feeding, and under stimulation.
Recently it was discovered that long-term administration of leuprolide acetate (LA) depot reduces the musth. Early musth signs ceased within 3 days of drug administration after 10 of 12 injections.
Literature: de Oliveira, C. A., West, G. D., Houck, R., and Leblanc, M. Control of musth in an Asian elephant bull (Elephas maximus) using leuprolide acetate. Journal of Zoo and Wildlife Medicine 35, 70-76. 2004.