Castanets at 50 paces

When you're out in the bush on your own, miles from anywhere, bumping into people is seldom a good thing.

So when I heard the sounds of someone hammering in stakes I was worried.
Clonk, clonk, clonk... pause.
Clonk, clonk, clonk, clonk....
The sound was rhythmic, and quite close by, but the bush was just too thick for me to see what was happening.

Who could it be? I knew the park staff weren't working in the area. And it wasn't a bird (although admittedly some of them make some pretty weird clicks and thunks).
Didn't poachers knock in stakes when setting snares?
I stood listening anxiously to the intermittent hammering until I realised that the source of the sound was moving. It shifted slowly, in fits and starts, in a wide arc around me, as if the perpetrator knew exactly where I was. That freaked me out. I hurriedly packed up my gear and headed home. I didn't know who was knocking in pegs, and I didn't want to find out!

This incident occurred during my first months working with the mongooses, and the mystery of my phantom stake-basher remained unsolved. Once or twice a year, I'd catch a momentary hint of the same noise again, but it was always fleeting and very far off.

It wasn't until last February, when my old Toyota bakkie/ute/pick-up/van (depending on which continent you call home) uttered its terminal splutter, that I finally learnt the truth.
While my car enjoyed a rejuvenating overhaul, the university lent me a rental car: a tiny, three-door hatchback that was incapable of passing over an apple without creating pulp. Trying to coax this thing along the rough, rocky tracks at my study site was a nightmare. It took me hours to reach the groups, inching around boulders and over culverts, sliding across sand-filled creek-beds and braving the paint-scratching embrace of overhanging thorn bushes. Oh, and did I mention it was black? In February. Who would purchase a black car in a region where midday temperatures play cat and mouse with 40C for nine months of the year?
Anyway, it was while I was edging along in my sweaty little black box - fervently cursing all car manufacturers and university administrators - that I met a herd of eland.

Eland (Tragelaphus oryx) are incredibly shy of people, avoiding all signs of human activity. They've the longest flight distance of all African game (300-500m), presumably thanks to thousands of years of human carnivory. Photo borrowed from here.

Although eland are common here, I very rarely see them so I stopped my travesty of a car to watch as they fled into the bush. Only they didn't. They just stood staring at me. Then they started walking toward me. As they inched closer, I suddenly realised I was an utter novelty; no one had ever been insane enough to drive a tiny hatchback out here.
Eland are massive, hefty animals, air-brushed in salmon-pink and equipped with tall, erect horns embellished with a candy-cane twist. Being the world's largest antelope, they towered over my car, crowding around cautiously to peer and snuffle at this strange new animal. I gazed back in awe, stunned by the amazing privilege (it's an ill wind...).

Then it happened.

I heard the clonk, clonk, clonk of my phantom stake-basher.

It took a while, but I eventually figured out that it was emanating from a large eland bull. What was wrong with him? Was he suffering some ghastly form of arthritis?

Once I (eventually) made it home, I trawled the literature. Sure enough, there it was: mature eland bulls make a castanet-like clicking when they walk. It's audible for several hundred metres. But why? Surely it's a 'come and get me' invitation to predators?

This is where it gets interesting. Eland are nomadic creatures who congregate in loose herds, often hundreds of animals strong. Although the herds are constantly changing composition, the bulls maintain a strict hierarchy, with only those at the top wooing the females. But how does a randy eland bull, joining a herd of strangers, figure out where he fits in? He clearly can't test out everyone, yet he needs to know how strong each male is (i.e. how big), how experienced he is (how old) and how aggro he's feeling (because eland bulls aren't all equal in the testosterone stakes; like elephants in musth, they go through periods of heightened aggression, called 'ukali'). The answer? He looks and listens.

A macho eland bull parading his signals. Photo by Blake Matheson.

Research by Jakob Bro-Jørgensen, in the Masai Mara Game Reserve in Kenya, found that a bull's dark face, his woolly forehead tufts (normally smeared with urine for added effect) and his grey colour (due to balding) are all features induced by testosterone, the hormone that also promotes aggression. By checking out these traits, a strange bull will get a pretty good idea of how antsy a rival is likely to be. (Note to self: steer clear of dark grey eland bulls). In contrast, the bull's goitre-like dewlap grows bigger over time, regardless of his size or hormone levels, so accurately reveals his age (and hence experience).

But the most ingenious of an eland bull's signals, is their weird knee-clicking. This clonking sound is generated by the leg tendon slipping over the carpal bone. And it works just like a plucked guitar string, with the pitch (or frequency) of the click varying precisely depending on the length and thickness of the tendon. Hence, the bigger the bull, the deeper his knee-click. Bro-Jorgensen found that the pitch of the knee-click didn't just reflect skeletal size but muscle mass too. When one of his study bulls lost body condition, his knee-click got higher, as the thickness of his leg tendon shrank in response to the dwindling muscles.
So canny eland bulls can figure out the size of a potential rival from a distance and - if the 'stake-basher' is threateningly big - skedaddle before he arrives.

 Fighting in eland bulls is extremely rare, testament to the efficiency of their 'look and listen' signalling system. Photo posted on Flickr by kibuyu.

Bro-Jørgensen J. and Dabelsteen T. 2008. Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals. BMC Biology, 6:47.