Elephants have a reputation for being gentle, and it is mostly earned. They cooperate, they look after each other’s calves and they even show clear signs of grief . Then all of a sudden, a mature bull will kill a rhinoceros that was doing nothing to him, or charge a vehicle on a road he has walked past a hundred times before. It looks like a break from everything else we know about the species, but it isn’t one.

The state responsible is called musth. The word comes from Persian and Urdu and means something close to intoxicated. Mahouts on the Indian subcontinent were using it long before anyone studied elephant endocrinology, which tells you how obvious the condition is from the outside.

A bull in musth is easy to pick out, and the outward signs track the hormones closely. A 2005 study published in Hormones and Behavior found that elevated androgens (male sex hormones) showed up only in bulls displaying the visible signs, meaning the swollen temporal glands leaking down the sides of the head and the near-continuous dribbling of urine are a reliable readout of what is happening internally.

The state lasts anywhere from a few days in a young bull to several months in a prime one. Musth bulls also make a particular low-frequency call, the musth rumble, that carries for miles.

The Problem Musth Solves For Bull Elephants

To understand why any of this exists, it helps to look at the females rather than the males. Elephant reproduction is extraordinarily slow. Gestation runs about 22 months. A cow (mature female elephant) typically raises one calf (young or baby elephant) every four or five years, and within that long cycle she is fertile for only a few days. So at any given moment, a population containing dozens of adult bulls may have one or two receptive cows in it.

Biologists call that a skewed operational sex ratio, and it produces intense competition among males in every species where it occurs. The genetic evidence shows how much rides on it. A 2007 study published in Animal Behaviour , which matched paternity across a long-studied population in Kenya, found that most calves were fathered by bulls who were in musth at the time of conception.

Musth is how bulls compete for that narrow opening. What makes the condition interesting is that it works as a temporary promotion. A 1989 study published in Animal Behaviour showed that a smaller musth bull in poor condition can dominate a larger, normally higher-ranking bull who isn’t in musth, and that the larger animal generally backs down rather than fight. That sounds strange until you consider what backing down avoids. A musth bull will escalate further than is sensible, so even a rival who would probably win has good reason to walk away.

The signal holds up because it can’t be faked. Musth is metabolically expensive, and bulls lose condition over the course of it. What a bull is advertising is not how big he is but how far he is willing to go, and only an animal in genuinely good shape can afford to make that claim for long. This is what biologists mean by a costly, condition-dependent signal. The expense is the whole point, since it guarantees the information is honest.

Why Is A Bull Elephant In Musth Driven To Kill?

Two consequences follow. The first is that when bulls do fight, it is usually a musth bull driving the escalation, and the outcome can be fatal. Elephants that commit to a contest have no easy way out of it, and tusks driven by that much body weight cause injuries that kill.

The second is that the aggression spills onto animals that aren’t competitors at all. A bull in musth will go after rhinos, buffalo, vehicles and people, none of which have any bearing on his chances of mating. In parts of India, Nepal and Sri Lanka, where elephants and dense human populations share the same ground, encounters kill hundreds of people every year.

A 2021 study published in Ecology and Evolution , which compiled two decades of attacks across lowland Nepal, found that solitary bulls or small groups of subadult males were involved in most of them.

There is normally a brake on all this, and it is social rather than physiological. The presence of older bulls appears to suppress musth in younger ones, which staggers the timing so that a population isn’t full of musth bulls simultaneously. What made this clear was an unhappy natural experiment at Pilanesberg National Park in South Africa. A 2000 study published in Nature reported that young bulls orphaned by culls, growing up with no adult males around, entered musth years earlier than they should have and killed more than 40 white rhinos between 1992 and 1997. Introducing six older bulls from Kruger National Park stopped it.

Plenty about musth remains unresolved. Whether the testosterone surge causes the aggression directly, or whether social circumstances drive both together, is still an open question. Musth also differs enough between Asian and African elephants that findings from one don’t always carry over to the other.

What is clear is that musth isn’t a malfunction in an otherwise placid animal . It is a functioning part of how elephants reproduce, and it depends on old bulls being present to regulate it. Poaching and hunting take the biggest tuskers first, which means they remove precisely the animals that keep the whole arrangement stable.

Musth bull elephantsare a reminder that behavior which looks senseless usually has an evolutionary logic underneath it. Find out how sharp your grasp of that logic really is with this science-backed test: Evolution IQ Test