(see Graphic) Each dry season, Conrad Brain is plagued by the same melancholy thought – that he may be watching the dying days of the troop of chacma baboons he has been studying since 1986. The baboons’ home is the Namib desert, the most arid environment known to be inhabited by a nonhuman primate. And their struggle is not against poaching or habitat destruction, the usual foes of wildlife, but against something more fundamental: a scarcity of drinking water.FIG-mg18795001.jpg
But each year, in spite of his fears, Brain witnesses yet another extraordinary feat of survival as the waterless days stretch out, the summer heat rises and the baboons go without drinking for seemingly impossible lengths of time. In all other field studies, baboons have been recorded as drinking at least every other day. Indeed the availability of drinking water seems to be the key factor influencing where most of Africa’s hundreds of thousands of baboons choose to live. But not so for the Namib troop, which regularly goes without drinking water for up to 11 days at a time – and sometimes much longer. In 1990, for instance, Brain’s records show that the troop went without drinking for 26 days.
At the beginning of his field studies, Brain believed that the unusual drinking habits of the baboons were just part of the picture of life in the desert and probably an infrequent phenomenon. But he soon discovered otherwise. Going thirsty for prolonged periods seemed to influence every aspect of the baboons’ social and reproductive lives: their activity patterns, how aggressive the males are (to each other and to members of other troops), the length of female reproductive cycles, rates of infant mortality, how many male immigrants are allowed to join the troop, and so on.
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Kidnap and murder
‘Drinking water as a determinant of behaviour’ became the central theme of his research. ‘It was like watching two different troops when they had water and when they didn’t,’ Brain says. In the course of his research, he also uncovered a dark side of the baboons’ behaviour. Infant mortality is so high in the troop, because of the harsh conditions, that some infantless females regularly kidnap – and ultimately kill – babies belonging to other females. The females make up the troop’s core (male baboons tend to migrate from troop to troop), so this behaviour is particularly self-destructive; in the years to come, it could threaten the troop’s very existence.
Brain’s findings raise several important questions. One is how the Namib baboons survive without drinking water for such long periods. Have they evolved special biological and behavioural mechanisms to cope with the problem? If so, are these mechanisms unique to baboons living in arid conditions, or common to all members of the species (Papio ursinus)? It may be some time before Brain has definitive answers, but interesting clues are already beginning to emerge.
The troop’s home range is a 30-kilometre stretch of the deep, rocky canyon that is cut, east to west through the heart of the Namib desert, by the Kuiseb river. To the south lies a sea of sand dunes, and to the north a flat gravel plain almost devoid of vegetation. The river flows for only a few weeks each year, normally in January and February, after rain falls in its catchment area. But it provides the canyon with enough water to support a river bank forest.
Any pools left by the passing flood soon dry under the fierce sun, leaving no surface water in the canyon for around eight months of the year. During this period the only sources of drinking water for the baboons are a tiny seep from a crack in a rock high up the wall of the canyon and excavations in the sand where gemsbok (Oryx gazella) and mountain zebra (Equus zebra) have dug down to water. So meagre are these sources that the baboons must drink from them one at a time, and even these supplies often fail towards the end of the dry season.
In the last months of 1992, the baboons went without water for 116 days. Just when Brain believed they could survive no longer because the succulent wild figs – one of the main alternative sources of moisture – were also coming to an end, the flood arrived, snaking its way through the canyon, its frothy brown waters carrying a crown of rattling acacia pods. Brain has seen the flood arrive every year since he began his study. ‘It’s the most spectacular thing imaginable,’ he says. The baboons drink almost continuously and sometimes they swim.’
Shrinking core
The first scientific papers to mention the Namib baboons were published in the early 1970s. But the troop has probably been eking out its perilous existence in Kuiseb Canyon for much longer. The troop’s size fluctuates depending on how many immigrant males join it in any one year, but it usually comprises between 10 and 20 animals. Brain believes that the troop’s core has been gradually shrinking over the past few years and soon may be wiped out altogether if infant mortality continues to exact its toll. Yet travel up the canyon, towards the more fertile highlands, and the number of baboons increases sharply. It seems that Brain’s troop is living at the extreme end of the population’s desert range.
Brain first made contact with the troop in the Kuiseb Canyon during school holidays, when he used to accompany his father, a palaeontologist, on field trips to the Namib. As a small boy he spent hours following baboons in the foothills of the Magaliesberg near Pretoria, where his father had an excavation site. But the hyper-arid Namib desert, which receives a paltry 27 or so millimetres of rain a year, was a dramatically different environment from the Transvaal. And when he saw that baboons lived there, too, Brain became curious and began visiting the canyon regularly. Then in 1989, having just qualified as a vet, and disillusioned with the prospect of looking after small, pampered pets in Pretoria, he devised a research project, got backing from the Witwatersrand University and moved to the Desert Ecological Research Unit at Gobabeb, some 20 kilometres west of the baboons’ home range.
Brain wasn’t the first to notice the unusual drinking habits of the baboons. Bill Hamilton, a zoologist from the University of California at Davis, had been studying the Kuiseb baboons from time to time on day visits to the field from Gobabeb. But having never been exposed to human beings before, the animals were extremely shy and hard to study in the long, deep stretches of the canyon. Brain decided that if his research was going to produce anything worthwhile the troop must become accustomed to his presence. So he moved into the canyon for weeks at a time, where he was joined eventually by Ginger Mauney, a wildlife film-maker.
‘It was extremely frustrating,’ says Brain. ‘At first we would climb out of the Land Rover and see them disappear off up the canyon, and that would be my day’s observation.’ Reducing the observation distance, he notes, ‘took about ten months, lots of sitting round drinking beer, and lots of patience’. Today, Brain and Mauney are accepted as members of the troop. ‘It’s a really spectacular achievement,’ says Duncan Mitchell, head of physiology at Witwatersrand University and supervisor of the research project. ‘Nobody has got baboons as well habituated as this before.’ The baboons regularly pick thorns from their sandals, and even make sexual overtures at times.
Until January this year, when Brain completed his thesis and took the job of veterinary officer at Namibia’s Etosha National Park, he and Mauney spent an average of 18 days a month living in the canyon, observing the baboons from the moment they came down from their sleeping cliffs at first light until they returned there at night.
The two researchers’ early attempts to set up a base camp were frustrated by spotted hyaenas that roam the canyon in small packs and made short work of their tents, equipment and ingenious shower arrangements. So they lived out of the Land Rover and slept under the stars with nothing but a sjambok (rhino-hide whip) to crack at the hyaenas when they came sniffing around. ‘Mostly,’ says Brain, ‘they were after the vehicle tyres which they like to shred.’ Living was pared to the bone as they ate out of tins and rarely lit a comforting fire for fear of alarming the baboons. But the desert landscape was magical, and they shared the canyon ‘oasis’ with many species of bird and small animal.
It soon became clear that, when the waters fail, the baboons make for an area downstream where the canyon opens out and clumps of mustard tree (Salvadora persica) grow among abundant acacia trees. There they concentrate on eating food with a high moisture content: the berries of Salvadora persica (about 70 per cent moisture), wild figs (Ficus sycamorus, 80 per cent), the pith of wild tobacco (Nicotiana glauca), an exotic plant growing in the canyon that is toxic to domestic animals, and the bark of Acacia albida. This last appears to be the baboons’ favourite. Having pulled off the tough outer bark, they tear strips off the succulent bark underneath, which they chew before spitting out the dry wads.
This concentrated feeding activity takes place in the cool of the morning and evening. For the rest of the day, as the heat builds up, a profound lethargy grips the troop: all grooming ceases, youngsters stop playing, incidents of male aggression fall drastically, and the baboons adopt strange postures as they rest for six to eight hours at a time in the shade of cliffs or trees. They lie on their backs with their limbs stretched out to expose hairless parts to the breeze, or scrape away hot sand before lying belly down on the cooler sand beneath the surface. Brain frequently observed male baboons showering handfuls of cooler sand over their chests – ‘sand bathing’ behaviour which has never before been reported for primates. Trying it himself, he found his skin temperature dropping by 5 to 7 degreeC within minutes. ‘It’s just like having a cold shower if you select the right sand,’ notes Brain.
Tragic death
Daytime temperatures in the canyon frequently reach 45 degreeC, and in 1991 Brain decided to do experiments to find out how heat coupled with lack of drinking water affected the baboons physiologically. The plan was to implant telemeters in a few animals, which would give readings of their core body temperatures, and to inject an isotope into the bloodstream to monitor how much water was passing through their bodies during drinking and non-drinking periods.
First the animals had to be darted, so Brain learnt to use a silent blowpipe. The whole idea would have failed without the trust built up between the humans and the troop. As it was, however, Brain and Mauney’s first attempt ended in tragedy. Their plan was to anaesthetise a baboon at the back of the troop on the move, quickly covering the animal with a dark sheet as the drug took effect. They chose an old male and successfully anaesthetised him with a dart, but he scampered off towards the cliffs some 150 metres away and sat down on a ledge. Just as the anaesthetic was taking effect, another baboon approached and pushed him over the cliff, killing him. ‘We weren’t expecting that to happen because we chose the lowest ranking male, thinking he’d be the least vulerable to any sort of rank assertion at such a time,’ says Mauney.
It took another eight days of continuously following the troop before ideal conditions, well away from cliffs, presented themselves. In all, they darted three animals and Brain implanted the telemeters into the animals’ abdomens, in the peritoneal cavity. ‘It’s minor surgery,’ he says, ‘but carrying all our equipment into the field and making sure it was sterile took a lot of preparation.’
They followed the baboons for six weeks, taking regular readings of their core temperatures. ‘We found that there were two very clear pictures in body temperature changes depending on whether the baboons were drinking or not,’ says Brain. During non-drinking periods the baboons’ core temperatures climbed throughout the day from an early morning reading of around 36 degrees, dropping only as the cool desert night set in. However, when they were drinking, their core temperatures appeared to fluctuate across the day. Brain recorded temperature changes of up to 5.7 degreeC between dawn and dusk – a fluctuation much wider than any measured in captive primates or human beings, whose body temperature seldom fluctuates by more than 0.5 degreeC. Physiologists at Witwatersrand University are now doing comparative studies on baboons to find out if this is a general characteristic of the species or something peculiar to the Namib troop.
Because of the stress under which the Namib baboons live, Brain adopted an unusual procedure for measuring water flux. Darting the same animal twice is extremely difficult in a free-ranging primate. So instead Brain injected each drugged animal with water labelled with a radioactive isotope at the same time as implanting a telemeter. The researchers then collected faecal samples on each of the following 10 days, distilled the water from these samples and counted the activity of the isotope. The results showed that, far from dehydrating when there was no drinking water, the baboons could maintain their body water levels by eating succulent foods and reducing faecal water content from about 78 per cent to 53 per cent.
Teetering on the brink
Despite such measures, however, the troop continues to teeter on the brink of extinction. Infant mortality is exceptionally high. Of the 36 infants born during Brain’s six-year study, 30 died in the early weeks of life. A troop living higher up in the canyon, by contrast, suffered only one infant death over the same period. Why the difference? The answer, says Brain, lies partly in the physical inertia that water deprivation encourages.
Resting in the shade of trees, the lower baboons are prey to ticks (Ricicephalus gertrudae) that lurk in the undergrowth. Baboons in the higher troop are not affected in the same way because they do not lie for long periods under the trees.
Brain has counted more than 400 ticks on a dead adult male baboon, and more than 70 on a week-old infant. All baboons have scarred and bleeding ears as a result of the parasites. ‘Ticks cause fantastic damage to the muzzle and infants simply can’t suck,’ says Brain. ‘Sometimes a mother’s teats are damaged, too.’
Next to tick damage as a cause of infant deaths is kidnapping. This phenomenon – sometimes called ‘aunting to death’ – is usually extremely rare in baboons, but not in the Kuiseb troop. According to Brain, it resulted in the deaths of nine infants over the six-year study period. The offenders were high-ranking females, who would often snatch newborn infants from lower-ranking females – especially if they had themselves lost offspring to tick infestation. The kidnapped infants died because they were too young to survive without being breast-fed (a need their kidnappers could not fulfil because they were not lactating).
One female in the hierarchy, who had lost three offspring to tick infestation, was alone responsible for five of the kidnapping deaths. ‘Without fail,’ says Brain, ‘if a lower ranking female had an infant and the kidnapper didn’t have one at the same time, the infant was snatched.’ At present, it is unclear why certain females in the Kuiseb troop behave in this way. ‘In humans, this behaviour would be called psychopathic,’ says Mitchell.
From an evolutionary perspective, investing time and energy in trying to raise an infant carrying someone else’s genes seems to make little sense. One idea is that the females who snatch infants are trying to protect their status in the hierarchy – a status that is vital to attracting mates. But if this were the whole story, one would expect all the highest-ranking females to kidnap infants, and in reality only some of them seem show this behaviour.
The dearth of infants in the troop affects the behaviour of males, too. Brain observed that an alpha male in his troop never conceded rank to a challenger without a fight in which he was badly wounded or killed. In one memorable fight he watched as the challenged baboon had half its jaw torn off and crept off to die. By contrast, in reports of such challenges in other troops, the show of aggression is described as ritualistic, with neither baboon being wounded in the battle for dominance. Brain thinks the difference has much to do with the fact that in his troop, the alpha males being challenged had no living offspring of their own, and were therefore reluctant to give up a rank that carries special mating rights.
Female baboons that lose their infants return to fertility more quickly than usual. So the average interval between births in the Namib troop is eight months, instead of the more usual two years. Brain believes that the rapid reproductive cycle of the females is responsible for the unusually high rate of immigration to the troop – the opposite of what one would expect, given the inferiority of the habitat.
Despite the baboons’ phenomenal ability to adapt their behaviour to the the heat and dryness of their desert home, the troop will surely die out soon if it cannot improve the survival of its offspring. As Mitchell grimly states: ‘The combination of ticks and kidnapping to death is proving lethal.
Sue Armstrong is a freelance science writer based in Johannesburg, South Africa.
