African Hippopotamus

Hippo Sunset in the Okavango Delta

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The hippopotamus once inhabited virtually every river system in Africa from Egypt in the north to the wetlands in the very south of the continent. In the mid-17th century Jan van Riebeeck, the first Dutch commander at the Cape, recorded in his diary the presence of hippos in the swamp that is now Church Square in the centre of Cape Town. But not for long. The species was soon driven back ahead of the advancing colonial presence and less than two centuries later the last hippo had been shot in the Orange River far into the interior. The great, grey mammals fared little better in the north, and reputedly the last hippo in the Nile Delta was shot in the mid-1800s.

The hippo's persecution continued unabated, and for a number of reasons - with justification it is much feared as a mean-tempered killer, its voracious appetite can mean serious damage to crops, and its meat, fat, hide and tusks are prized. Today the hippo's only real sanctuaries are within Africa's network of national parks, reserves and the few remnants of true wilderness.

Despite its now much reduced distribution, however, Africa's 'river' or 'water' horse - a literal translation of its colloquial name - still occurs widely throughout the Afrotropical region, from The Gambia in the western bulge of the continent across to Ethiopia and the Sudan in the east. Southwards the range extends through the countries of West and Central Africa, East Africa and down into southern Africa to the reserves of KwaZulu-Natal.

The angle of the camera lens suggests that the jacana could be in for a nasty surprise. The bird is quite safe, however, as no meat, not even the flesh of fish, is part of the bulky mammal's diet. Instead the hippo is strictly vegetarian, coming ashore at night to graze.

As if participants in an opera on some vast aquatic scale, hippos open wide. The real purpose of such showmanship is not to vocalize, but to advertise their amazing gapes and jaws armed with huge teeth. Such yawning displays are only part of an extensive repertoire that also includes dung-showering, water-scooping, head shaking, rearing, lunging, roaring, grunting and chasing - all strategies and counter-strategies in the endless rituals expressing dominance and threat. Sometimes fighting does ensue, but more often than not the contest is settled in relatively harmless jousts employing the tactics described. Sooner or later one of the combatants will lose his nerve, showing submission by taking flight, slow tail-paddling while urinating, or lying prone.

With much head tossing and mud slinging, a dominance skirmish gets well under way. Males begin testing themselves by the time they become adolescent at about seven years old, engaging with their peers in jaw-to-jaw sparring and yawning contests. Any disturbance in a hippo pool will set off a wave of wheeze-honking, yawning, and sometimes chasing and fighting.

For the hippopotamus Hippopotamus amphibius to survive, it needs two essential things - water deep enough to submerge its considerable bulk, and nearby grassland for feeding. The opportunity to submerge is critical because of the hippo's unique skin - a thin epidermis and no sweat glands - which loses water at several times the rate of other mammals. A hippo out of water under the scorching heat of an African sun, therefore runs the serious risk of rapid dehydration and overheating. For its aquatic refuge, rivers and lakes with gently sloping, firm bottoms are preferred where herds can rest half-submerged and calves can nurse without swimming. Rapids and fast-moving water are avoided.

The hippopotamus is socially schizophrenic. In the water or resting ashore, it is highly social, tolerating closer contact with its fellows than is the case with most other ungulates. Nevertheless, or perhaps therefore, it is also highly aggressive towards its own kind and perceived intruders. It is hard to tell whether the tendency to cluster has evolved for the protection of calves against crocodiles, or simply to accommodate the maximum number of bodies. If it is primarily for the benefit of the calves, however, the strategy has its downside, as although the tightly packed herds do undoubtedly deter crocodiles, aggression and trampling by the larger members, especially the bulls, is an ever-present danger for any youngsters in the group.

At night when hippos come ashore, their tightly knit daytime association alters dra-matically - they become completely unsociable, with each individual, except for females with young, moving off to forage independently. Sheer bulk and massive, formidably armed jaws are more than a match for any would-be aggressor and so, largely immune from predators, adult hippos have no need to herd together for safety on land.
Hippopotamus herds typically number 10-15 individuals, but range from 2 to 50 and, at very high density in large bodies of water, sometimes as many as 150. The average density in lakes is seven hippos for every 100 metres of shoreline and 33 over the same measure in rivers. Crowding is understandably greatest during the dry season when competition for available water is at a peak; but during the rains hippos, especially the males, disperse more widely and may occupy temporary pools and even wallows many kilometres from permanent water.

A mature bull (20 years and older) will control a stretch of river some 50-100 metres in extent (or as much as 250-500 metres of lakeshore) as an exclusive mating territory. Although some individual bulls have been known to keep the same territories for at least eight years in lakes and four years in rivers, under more variable climatic conditions which cause major changes in grouping patterns and density, territorial turnovers may occur as often as every few months. The attitude of territorial bulls towards other males is somewhat unpredictable. Territorial bulls are mutually intolerant, but they will usually accept bachelor males within their domain and even in cow herds, so long as they behave submissively and do not attempt to mate with females in oestrous. They can be, and often are, beastly towards younger males and at times will drive out potential rivals with great ferocity. And so bachelor herds living apart from females (usually in less favourable habitat) are also a characteristic of hippo society. Lone hippos may be either outcasts or territorial bulls without herds.

The infrastructure of female herds remains unclear. Although they may remain fairly stable for several months at a time, there appear to be no close ties between cows. Maternal bonds with daughters are known to persist at least to the subadult stage, however, and so cows may be followed by up to four successive offspring.

Hippos feed at night on land and shortly before dark they leave the water to trundle down the well-worn, branching paths that lead to their inland pastures. After grazing, they return to water in the early hours to rest and to digest the night's harvest. This nightly commuting to and from feeding grounds can mean an average round trip of between three and five kilometres, although journeys of as much as 10 kilometres and even more have been recorded.

On land hippos look clumsy - with their massive barrel-like bodies and short stubby legs it could hardly be otherwise. But a lack of grace should not be taken for a lack of speed as adult hippos can gallop as fast as 30 kilometres an hour in an emergency. More usually, though, they proceed at a jouncy trot. They can climb steep banks, yet cannot jump and are even reluctant to step over any obstacles in their path. They lie down and get up like pigs, sitting down on their haunches before reclining, and rising front legs first.

In the water it is a different matter altogether, for hippos are powerful swimmers, in effect they 'gallop' underwater, and can move disconcertingly fast. They also walk on the bottom while submerged, their feet leaving the river floor and landing again as lightly as those of an astronaut on the moon. When a hippo dives, the nostrils close and the ears fold into recesses. Resurfacing, the nostrils open wide to release pent-up breath, and the ears spring erect, throwing showers of droplets. Mature hippos can stay under for as long as five minutes, but on average they surface every 104 seconds, compared to 20-40 for a two-month-old calf. Hippos rise to breathe even in their sleep, the act of surfacing being as involuntary as breathing itself.

As hippos spend their days submerged and come ashore at night, visual contact and communication is limited, perhaps explaining why they display very little sexual dimorphism, have inconspicuous appendages and coloration, and have no facial expression apart from the ability to open their mouths very wide. By contrast, one would expect auditory, olfactory, and possibly tactile communication to be particularly important. And indeed this is so. The resonant honking call made by hippos in the water is one of the most familiar and impressive African wildlife sounds, and it is known that in exhaling their breath, hippos can express threat and alarm. On land, however, hippos are notably silent.

Olfactory responses, especially dung- and urine-showering in the water and on land, are also clearly of central importance in hippo social life. In addition to normal olfactory reception, urine-testing with the vomeronasal organ presumably functions to communicate the reproductive status of females and possibly of males as well. The vomeronasal, or Jacobson's, organ is the pair of narrow blind sacs in the nose which are present in most mammals. The sacs are lined with sensory cells which seem to serve the primary function of enabling individuals of the same species to monitor the reproductive condition of their kind by assaying hormonal products excreted in the urine. The grimace, or flehmen, that signifies urine-testing in most other ungulates is not displayed by hippos. A probable explanation for this is that the hippo's vomeronasal organ appears to be designed to function under water, operating rather like a syringe bulb to draw in a sample of voided urine. Supporting this deduction is the fact that a thick layer of muscle surrounds the large, incisive ducts leading to the vomeronasal organ (and the organ itself) and that the ducts do not penetrate the nasal passageway as is usual in ungulates. The possibility of the hippo's urine-testing equipment being able to function under water gives added significance to the habit of females and subordinate males of urinating when approached by bulls.

As stated earlier, visual communication in hippos is limited by their daytime aquatic habitat and their nocturnal feeding habits on land. Nevertheless, visual signals obviously do play a role in the daytime interactions of submerged hippos, most notably in threat displays such as yawning and charging. During these impressive encounters, the greater development of male teeth, the hippo's most obviously dimorphic character, is doubtless observed by individuals for whom the display is intended. Dung-showering is also conspicuous, leading to the oft-heard quip that all social interactions between hippos revolve around the two opposite poles of the body!

Encounters between territorial bulls are frequent and highly ritualized. After approaching the common boundary, they stop and stare at each other, then turn tail, elevate their rumps, and shower dung and urine over each other with rapidly paddling tails, following which they withdraw. Should this show not be enough fighting may ensue, but these are also highly ritualized frontal combats with much water splashing, advancing and retreating, and hardly any physical contact. Where serious fights do occur it usually stems from a non-territorial bull challenging for possession of an already held stretch of river or lakeshore. Now the struggle is in earnest, the combatants lining in reverse parallel and slashing at each other's flanks with their lower tusks. Great gouging wounds inflicted in this manner can lead to disability and even death.
Conspicuous dung middens are found along hippo paths, but whether or not they have territorial significance is open to question as various passing hippos add their excrement to the pile. It could be that the middens may well assist hippos generally in orientation and communication at night. A number of observers believe that the middens are only used by bulls; certainly the paths start at the water's edge and therefore lead out of and into territories, whose owners take a proprietary interest in them. Also, middens are most frequent near shore and these are renewed nightly by bulls on their way to pasture. Territorial bulls also come ashore during the day to defecate on their shoreline dung heaps, and on rocks or islands in the water. Before adding a new contribution, a bull will first smell the existing deposit, only then backing up to urinate and defecate to the rear, all the while broadcasting his excrement by vigorous tail-paddling. Bull excrement is particularly smelly and interesting to other hippos; for example, a juvenile will often follow a bull to a midden, intently smelling or licking his superior's backside on the way. After the event the subordinate hippo will spend minutes nosing and even eating the newly deposited dung.

Although the showering of excrement in territorial males is apparently an assertion of dominance, if not also a form of territorial advertising, its full significance, as well as differences in the behaviour between males and females, has yet to be clearly explained. When females and subordinate males are approached by a bull they respond by turning their rears and tail-paddling, but slowly, splashing the water and probably urinating but not necessarily defecating. This behaviour appears very similar to the urination-on-demand/urine-testing sequence so common among antelopes. And like them, perhaps hippos also employ the tactic to appease or divert aggression. Likewise, lying prone in submission may gain added appeasement value through resemblance to the posture of oestrous females during copulation.

Males begin testing themselves by the time they become adolescent at about seven years old, engaging with their peers in jaw-to-jaw sparring and yawning contests. Any disturbance in a hippo pool will set off a wave of wheeze-honking, yawning, and sometimes chasing and fighting. Usually such fights are undamaging, because the combatants can fend each other off with their jaws. But disengaging is a problem for the loser who thereby exposes his body to the winner's tusks. Deep gashes are cut during hot pursuits but often look worse than they are, for skin up to six centimetres thick protects the hindquarters and sides. The most serious injuries result from crushing bites on the legs, head and neck.

Aggression in any hippo community is most frequent and intense during the dry season when living conditions are most crowded. At such stressful times serious injuries and deaths from fighting are not uncommon and it is also when most attacks on calves occur. Tensions are not just a consequence of cheek-by-jowl living - they are exacerbated by mating rituals, for although breeding is not strictly seasonal, most conceptions probably occur during dry-season concentrations, with the rainy season being the time of peak births.

In the quest for mating opportunities bulls may wander through basking nursery herds sniffing at cows' rear ends to test their 'readiness'. The procedure is not without its attendant danger, however, as the adventuring male runs the risk of being mobbed should the cows become disturbed. No bull, no matter how dominant and powerful, would be a match for a bunch of angry matrons, so he moves very carefully and at the first sign of trouble, lies down - a display of submission that is unusual in a dominant male animal.

Having located an oestrous female, a bull hippo wastes no time on the niceties of elaborate courtship. He simply pursues her into the water until she turns and clashes jaws with him. He then forces her into prostrate submission, whereupon he mounts. The female's head is often forced underwater during the encounter, and when she raises it to breathe, the bull may snap at her. The affair is often punctuated by much wheeze-honking.

Females conceive for the first time when they are about nine years old, calving thereafter at two-year intervals. Prior to calving (gestation is eight months) a pregnant cow will isolate herself from the herd, on land or in shallow water, to give birth, only rejoining the herd some 10-14 days after the event. Baby hippos weigh 25-55 kilograms at birth and are adapted for nursing underwater. Even out of water, their ears fold and their nostrils close while sucking. Every few seconds a submerged sucking calf pops to the surface, breathes, and goes back to the nipple, gripping it between the tongue and the roof of the mouth.

Expectant and new mothers tend to be savagely protective, keeping other hippos at a distance. The bond between mother and calf is close. The mother licks, nuzzles, and scrapes the calf with lower incisors, and calves reciprocate. The only time that a very young calf may be left alone is when the mother is away at pasture, but even then they are not exposed to any danger as they are left to playfight and chase one another in crèches, while carefully watched over by one or a few hippos. It is not long, however, before a calf is able to join its mother on the nightly feeding expeditions as, at about a month old, calves are already grazing a little, and in earnest by five months. They are weaned at about eight months. When accompanied by young on land, a mother punishes any tendency to stray by nudging, sideswiping, or even biting the calf, which responds by prostrating itself.

Baby hippos would be easy prey for lions, hyaenas, or crocodiles if not protected by their mothers and other adults. Lions are capable of killing a full-grown hippo if they can get it down on its back, with its throat and chest exposed to their jaws, but in reality and with jaws wide and powerful enough to bite a three-metre crocodile in two, adults are all but invincible. Best illustrating this point is the account of a bull hippo set upon by three famished lions. He simply dragged and carried them along until he reached the river, when he entered and submerged, little the worse for wear, leaving his tormentors, unsuccessful and soaked into the bargain, to swim back to shore.

HIPPOS PAST AND PRESENT
Pigs and hippos had common ancestors which diverged in the late Eocene, but the early history of hippos is unknown. They appear in the fossil record only in the lower Miocene in a form already more advanced than in the living pygmy hippo Choeropsis liberiensis. There is no record of the pygmy hippo's ancestry nor of links between it and the more widely found common hippopotamus Hippopotamus amphibius. The pygmy hippo is living evidence, however, that hippos started out as solitary, forest-dwelling herbivores with semi-aquatic habits. Whether the line arose in Africa or Eurasia is unknown, but during the Pleistocene a number of different species lived on both continents.In Kenya's Lake Turkana, four different hippos, at least one of which was more specialized than H. amphibius, formerly shared the ecological niche now exclusive to this single species.

The pygmy hippo, standing 70-80 centimetres and weighing only 250-270 kilograms, occurs in the lowland forests of West Africa from Guinea to the Ivory Coast. Found on the edges of swamps and rivers it spends the day on land, in a hole, wallow or dense undergrowth. At night it enters the water to forage on aquatic plants. It also forages in the forest, but stays near to water which serves as a refuge from predators.

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