habitat_narrative

Terrestrial|Freshwater (=Inland waters)|Marine

The Hippo is an amphibious creature, spending the majority of its day in water, and emerging at night to feed on dry land (Eltringham 1999). Subtropical floodplain forest, grassland and coastal grassland are especially important habitat types for this species. Thus, the ecological requirements for Hippos include a supply of permanent water, large enough for the territorial males to spread out at a depth of about 1.4 m (Taylor 2013), and adequate grazing on open grassland within a few kilometers of the daytime resting sites. Freshwater for drinking is essential when they live in a saline environment – such as at St Lucia (Taylor 2013). Although, they are restricted to regions in the proximity of water, they are able to disperse efficiently from one water source to another. Open water is not always essential as Hippos can survive in muddy wallows but must have access to permanent water to which they can return in the dry season. The essential factor is that the skin must remain moist as it will crack if exposed to the air for long periods. A curious feature is the red secretion from modified sweat glands, which is thought to have an antibiotic function.

Wright (1964) argues very convincingly that the greatest benefit of an amphibious lifestyle is thermoregulation as a large body produces a considerable amount of metabolic heat. The water environment acts as a heat sink. Hippos leave their wallows soon after sunset and graze nocturnally on short grass swards up to several kilometers from water. These swards, which are kept short by the grazing activities of the Hippo, are known as Hippo lawns. Although the Hippo grazes every night, except for mothers with very young calves, there are usually individuals present in the water all night, as some return after a few hours and others leave later. Hippos consume approximately 40 kg of grass each night (Klingel 1983), and may walk up to 35 km during these nocturnal foraging activities. Their movements are not hindered by general fences. Natal Buffalo Grass (Panicum maximum), Bushveld Signal Grass (Urochloa mosambicensis) and Couch Grass (Cynodon dactylon) were commonly selected for in northern KwaZulu-Natal (KZN) (Scotcher et al. 1978), whereas Ischaemum fasciculatum is known to be a preferred species in the St Lucia region. During droughts, Hippos need to range further, as food resources near to water become depleted. In severe droughts when food resources are too far for a daily return to their pools, some Hippo may not return to water every night, and will lie up in the shade of a tree during daylight hours. This species feeds by plucking the grass with its wide, muscular lips and passing it to the back of the mouth to be ground up by the molars. The front teeth (incisors and canines) play no part in feeding. The amount of food ingested is small relative to the size of the animal but its resting habits by day reduce its energetic demands. The stomach is a complex four-chambered structure in which fermentation digestion occurs, i.e. this species does not ruminate.

The Hippo social system is based on mating territoriality (Klingel 1991). Hippos are gregarious, social and polygamous animals. Territorial males monopolise a length of the shoreline of the river or lake but tolerate bachelors within the territory provided they behave submissively. Non-breeding males may also settle outside of territorial areas. Fights for the possession of a territory can be fierce and the animals may inflict considerable damage on each other with their huge canines. Minor conflicts are usually settled by threat displays, of which the "yawn" is the most conspicuous. There is little association between animals when they are feeding at night, except between females and their dependent young, and during these periods males do not behave in a territorial fashion. Both males and females spread their dung by wagging their tails vigorously while defecating, both in the water and on land, where it is thought to have a signalling rather than a territorial function. These dung piles may serve for orientation. Vocalisations take the form of complex bellows and grunts, which presumably also have a signalling function. Sounds may be made either on land or in the water and may be transmitted simultaneously through air and water, which is the only known case of amphibious calls in a mammal.

A study in the Kruger National Park (KNP) (Smuts & Whyte 1981) examined carcasses of 225 Hippos randomly culled between 1974 and 1975, and 238 between 1976 and 1977 during population reduction programs. This study showed that Hippos are born at a sex ratio of 1♂:1♀. Male mortality rates appear to be much higher than females as this ratio changed to 1♂:2.97♀♀ in the adult age classes. Calves weigh 50 kg at birth and may be born at any time of the year, but the majority are born during the mid-summer (rainy) months. The gestation period is just 8 months – an extraordinarily short time for such a large animal. Growth of the Hippo foetus in the womb is therefore very rapid. Black Rhinos (Diceros bicornis), which are very similar in size to Hippos, produce a calf weighing about 40 kg, but their gestation time is almost twice as long as Hippos. Growth for the first ten years of life is also very rapid, the average increase in weight is in the order of 100 kg per year, so that at 10 years old they can weigh in excess of 1,200 kg (Whyte, unpubl. data). Males and females are very similar in size, males being only slightly larger than the females. The average weight for 86 adult males culled in KNP was 1,546 kg while 192 females averaged 1,385 kg (Whyte unpubl. data). Females may conceive as early as eight years old, but the average age of sexual maturity is 9–10 years. The mean calving interval is 21.8 months (Smuts & Whyte 1981), and lactation lasts for 10–12 months (Laws & Clough 1966). The comparison of data collected in the droughts of 1964 (Pienaar et al. 1966) and data of Smuts & Whyte (1981), which were acquired during a period of above average rainfall (1974/75), showed that Hippo reproduction is very sensitive to deteriorating environmental conditions. Both conception and the percentage of females lactating were significantly higher in the pluvial 1970s.

Ecosystem and cultural services: Hippos form a vital component and are a flagship species of natural water ecosystems. They are important ecosystem engineers, acting as carbon and nutrient vectors between savannah grassland and aquatic habitats (Subalusky et al. 2015). They also physically alter the environment they live in, creating feeding lawns, paths, and channels in swampland. Hippos may be responsible for increasing the fertility of coastal waters, for example, Taylor (2013) estimated the quantities of nutrients brought into the St Lucia estuarine system by Hippos.