I’m stuck in a rut

All around, the bush is reverberating with deep snarling roars and growls.

Imagine the rumbling growl of an attacking Rottweiler, only crank up the volume until it's audible 2 km away. Oh yeah, and there's the occasional explosive snort too.

For the uninitiated these bursts of apparent violence are quite intimidating. I've become accustomed to my new volunteer assistants returning prematurely from the field, ashen faced and muttering, "Er... um... I heard this growling...".

Lions?  Wild dogs?  Fighting hyenas?   Impala.

Impala (Aepyceros melampus) are the most numerous antelope in southern Africa, with more than one million living in the region. And right now it sounds like they're all just outside!

Each May (synchronised between the two full moons) the local impalas mate. Now, before you start thinking that impalas have wildly exotic sex lives, the mating itself isn't noisy. It's the males arguing over breeding territories that create the racket. A study of rutting males at Sengwa, in Zimbabwe, noted 180 roaring displays in just one hour, on a romantic moonlit night.

Displacement grooming is frequent in territorial conflicts, as males waver between attacking their rivals and playing it safe (just as humans scratch their heads when making up their minds).

In East Africa, male impalas hold territories all year round, but here in southern Africa, they relax in mixed-sexed herds for most of the year. However, as the short, intense breeding season approaches, their testosterone levels soar and they go into a frenzy of aggressive competition. In a bid to stake out breeding territories (of around 8 – 13 ha), they use macho displays to intimidate rivals, standing very erect while flexing their neck muscles, tossing their heads, roaring, horning vegetation and even flicking their tongues in and out like professional punk-rockers.

Actual horn-clashing fights are rare (occurring in around 7% of conflicts) and males have a 'dermal shield' of thickened skin that protects their head and neck from stab wounds. Photo by Dougie Cunningham and borrowed from here.

Males that fail to gain a territory retreat to bachelor herds, where they spar with one another to establish a pecking order. Only those at the top of this hierarchy go forth to challenge the territory holders.

Territorial males spend up to a quarter of their time tending the females that pass through, chasing them into the territory's core, tasting their urine for signs of oestrus and, of course, mating. They also patrol their boundaries, face down trespassing neighbours and fight off challengers. As a consequence, they rarely get a chance to feed or sleep, and quickly exhaust themselves. Their average tenure, during the peak rut, is only 8 days. Once they're ousted by a rival, males limp back to the bachelor herd (rejoining at the bottom of the hierarchy) to regain weight and recuperate.

Horning vegetation is not only an impressive visual display, it leaves behind a smear of strong-smelling secretion, from the skin on the male's forehead, that indicates his status.

Female impalas show little interest in the males' squabbles, wandering from territory to territory. Although the population is female-biased, with 1.5 to 2 females for every male, most of the mating is monopolised by just a handful of prime territory holders.