Common Hippopotamus (Hippopotamus amphibius)
Status : Vulnerable
Pop. trend : Stable at the range level with evidence of regional declines
The Common hippopotamus is the only extant member of the genus Hippopotamus which during the Pleistoncene included a total cca 15 species. The members of the genus lived in Africa, Europe, and the Middle East, the extant species is distributed in Africa. Lydekker (1915) proposed 5 subspecies of common hippopotamus, however almost no revision has been done since that time and the validity of these subspecies was questioned (Eltringham 1999). A study based on mitochondrial DNA supported validity of three subspecies: amphibius, capensis, and kiboko (Okello et al 2005). However, more recent study using both mitochondrial as well as nuclear DNA failed to confirm subspecific differences but found differences at local levels instead (Stoffel et al. 2015).
Subspecies (unconfirmed, see above)
H. a. amphibius: Gambia to South Sudan, and Sudan, to DR Congo, Tanzania, and Mozambique; extinct in Egypt
H. a. tschadensis: Chad, Cameroon, and Nigeria
H. a. kiboko: Kenya and Somalia
H. a. constrictus: Angola, South of DR Congo, and Botswana
H. a. capensis: Zambia to South Africa
The species evolved in East Africa where it started to diversify in the Late Pleistocene 700 000 years ago (Stoffel et al 2015). Current distribution of common hippopotamus is limited to sub-Saharan Africa. The species ranges from Gambia and Senegal in the west to Ethiopia, South Sudan, and Somalia in the east, southwards to South Africa and Swaziland. In total, the species inhabits 38 political states (Angola; Benin; Botswana; Burkina Faso; Burundi; Cameroon; Central African Republic; Chad; Congo; Congo, The Democratic Republic of the; Côte d'Ivoire; Equatorial Guinea; Ethiopia; Gabon; Gambia; Ghana; Guinea; Guinea-Bissau; Kenya; Malawi; Mali; Mozambique; Namibia; Niger; Nigeria; Rwanda; Senegal; Sierra Leone; Somalia; South Africa; South Sudan; Sudan; Swaziland; Tanzania, United Republic of; Togo; Uganda; Zambia; Zimbabwe). In all countries, the distribution of common hippopotamus is restricted to suitable areas, i.e. grassland with rivers or lakes (Klingel 2013).
The species became extinct in Algeria, Mauritania, Egypt (early 1800s), Eritrea (70s of 20th century),and it is believed to be extinct in Liberia. The situation in Sudan remains unclear.
Head-body male common hippos measure 312 (260-350) cm, the females are a little smaller - 300 (260-337) cm. Their shoulder height is 150 (130-172) and 144 (110-158) cm for males and females respectively; the weight of adults averages 1546 (955-2000) kg and 1385 (995-1850) kg for both sexes. The maximum reported weight in the wild was 2065 kg in a male in Uganda (Klingel 2013). Captive specimens are larger (Blowers et al. 2012) mostly due to obesity caused by improper food (Schwarm et al. 2006). The record tusk length measured 1638 mm in length and came from animal from Congo (Klingel 2013).
Dominant features of common hippopotamus include a barrel-shaped body, disproportionally short limbs, and a large head with the eyes, ears, and nostrils high up on the skull so that the animal can see, hear, and breathe when completely submerged in the water (Eltringham et al. 2013). They possess four incisors in both upper and lower jaws with a strong development of the first lower incisors and canines which are called tusks. The enamel of lower canines exhibits strong convergent ridges (the characteristic of whole genus). The dental formula is 2/2, 1/1, 3-4/3-4, 3/3 = 36-40. One unique feature of Hippopotamus is a mandible that is heavier than the skull above it. The hippopotamus has therefore special arrangement of reinforcement neck and cranium which allows to the entire body to deliver and absorb jaw clashes and it also enables the cranial part of the skull to move upward vertically whereas mandible remains in horizontal position (Eltringham et al. 2013).
The ears are small relative to the size of the head, body dark reddish-brown, ventral surface pink with high individual variation in extent of pink (Klingel 2013). Skin is covered with short hairs (20-30 per 100 cm2), with thick bristles on tail and snout. The tail is short and flattened. Each foot has four toes. Females have a pair of inguinal nipples.
Chromosome number is 2n = 36.
As its name suggests, the common hippo is an amphibious creature, which spends the day in water and emerges at night to feed. The hippo uses the water as day time habitat and typically does not eat aquatic vegetation, rather they forage on grass at varying distances from a water source most typically at night. Hippos rely on a range of water sources - rivers, lakes, and wetlands - although seasonally the animal can survive in muddy wallows. Hippos require some form of permanent water particularly in the dry season as their skin must remain moist and will crack if exposed to the air for long periods. The skin physiology is complex and not fully understood but is clearly adapted for an amphibious existence. Hippos secrete a red liquid from subdermal glands which is thought to function as both a sunscreen and antibiotic (Saikawa et al. 2004).
Food and Feeding
Common hippos are predominantly grazers, although isotope studies shows that many individuals have a significant browsing component in their diet (Eltringham 1999). The list of the plants they consume is as following: Botriochloa spp, Brachiaria decumbens, Cynodon dactylon, Chloris gayana, Heteropogon contortus, Hyparrhenia filipendula, Panicum repens, Sporobolus homblei, Sporobolus pyramidalis, Themeda triandra (Eltringham 1999). They also eat fruits of Kigelia pinnata (Klingel 2013) or rarely aquatic vegetation such as Pistia stratioides, Nymphaea cerulean, Ruppia cirrhosa and Potamogeton pectinatus (Klingel 2013). Except grasses, hippopotamus forage considerable mass of dicotyledonous plants (sedges, forbs) and utilise lower nutrient aquatic plants during seasons when palatable higher quality terrestrial grasses are unavailable (Michez et al. 2013). Based on its foraging strategy common hippopotamus is considered as one of model species of so called central place foraging theory, i.e. they increase the intake when they travel farther from the river (Lewison and Carter 2004).
Common hippopotamuses are not very selective in food choice, which is reflected by higher plant consumption closer to the water where they are resting. Grazing areas range on average to 3.5 km far from the water, but they could be as far as 7 km (Harrison et al. 2008).
Although common hippopotamus was long time regarded as obligate herbivores and short/grass specialist, field studies have demonstrated that hippopotamuses are facultative carnivores that can consume flesh and intestinal tissues from the carcases of other animals like other hippopotamuses (Dorwrad 2015), eland, impalas, wildebeest, plains zebra, or even juvenile elephants (Dudley et al. 2016). Hippopotamus are even able to chase away the predator from its prey (e.g. crocodiles) or more often feed on carcasses left by predators (Dudley 1996, 1998). Carnivory of hippopotamus is an infrequent but widespread behavioural pattern that occurs in populations in eastern and southern Africa and thus not restricted to any particular locations. It is frequently associated with communal feeding involving multiple individuals or even entire groups (Dudley et al. 2016).
In severe drought, the consumption of woody plants and elephant dungs were observed as well (Dudley 1998).
By defecating in the water, common hippos fertilize rivers and lakes. Nutrients from hippo faeces has been identified as an important factor to maintaining fishery harvests in natural lakes as well as in artificial dams, i.e., areas where hippo populations have declined have anecdotally reported drops in fishery harvests.
Movements, home range and social organisation
Common hippos are gregarious, social, polygynous animals when resting in water by day, with herd sizes ranging from the tens to a hundred. The social system is based on mating territoriality Males (the strongest ones representing 10% of them) defend territories only in the water, including shoreline and a narrow strip of a bank. A territory is defended against other adult males. Non-territorial males form bachelor groups, often within territory keeping submissive posture towards the dominant male. However, these males also settle outside territorial areas, especially seasonal wallows. In a defended herd, females with offspring and sub-adults group together. In general, the sexes remain segregated although a few members of the opposite sex are often found within a male or female gathering. Social behaviour or social relationships within the group is challenging to observe because it occurs in typically murky water (Beckwitt et al. 2016; Eltringham 1999). During the feeding on the ground, no association between animals, except between females and their dependent young were observed. In addition, the males do not then behave in a territorial fashion on the ground.
Common hippos are well known to spread their dung by wagging the tail vigorously while defecating, both in the water and on land (Klingel 2013). This behaviour is observed primarily in males. The significance of this behaviour remains unknown, although several hypotheses were suggested: it used to be thought that it is territorial behaviour, or it serves to orientation or may have social significance when submissive individual is spreading faeces and spraying urine towards the dominant one. The dung piles on land are often on a river bank or shrub on the way to a foraging area and may also serve to orient hippos or may mark territories.
Vocalizations take the form of complex bellows and grunts, which serve as signals as well as in social context (dominance-submissive). Sounds may be made either on land or in the water and may be transmitted simultaneously through air and water (Barklow 2004; Maust-Mohl et al. 2015). Most calls are emitted under the water (Maust-Mohl et al. 2015).This is the only known case of amphibious calls in a mammal.
It is likely that thermoregulation has determined the nocturnal feeding habits of the animal. Hippos typically leave wallow or pool soon after sunset and spend night hours grazing on short grass swards for up to several kilometres from water. The distance animals travel to grazing areas likely varies seasonally and among different areas. Grazing areas, which are kept short by the activities of the Hippopotamus, are often referred to as Hippo lawns. Although Hippos can graze every night, there are usually animals present in the water all night, as some return after a few hours and others leave later. The animal 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 and food may be retained in the gut longer than most large herbivores. The ecological requirements for common hippopotamus, therefore, include a supply of permanent or seasonal water, and adequate grazing on open grassland within a few kilometres of the aquatic habitat.
Information on breeding involves big discrepancy based on data source whether the data were collected in the wild or in captivity. In the wild, the reproduction is highly seasonal with birthing correlates highly with rainfalls (Smuts and Whyte 1981; Eltringham 1999). This seasonal pattern was reported mostly from the Southern parts of the range (Eltringham 1999) and partly also from East Africa (Laws and Clough 1966). However, in the latter the births might occur during the whole year. In captivity, reproduction is aseasonal (Pluháček 2017). Mating occurs under the water where the female remains submerged most of the time (Laws and Clough 1966). Copulation lasts around 15 minutes. The oestrous cycle length is not known well and it is estimated between 37-50 days. The gravidity lasts for 240 days. It is assumed that many foetuses will survive to term (Eltringham 1999). Although no deviation of the sex ratio (from 1:1) at birth was recorded in the wild, the study of captive animals using large sample size revealed the deviation in sex ratio is favour of males (54%; Pluháček and Steck 2015). The most important factors affecting the sex ratio in common hippopotamus were the origin of the mother (wild/captive) and the number of adult females in the group (sex ratio was deviated only in cases where one adult female was present; Pluháček and Steck 2015).
The female gives the birth mostly in shallow water separated for couple of days from the herd. The newborn common hippopotamus weight ranges between 35 and 52 kg (Laws and Clough 1966). Twins are extremely rare but they were recorded as in the wild (Laws and Clough 1966) as well as in captivity (Pluháček 2017). The suckling occurs in both environments - in the water as well as on the land (Eltringham 1999; Pluháček and Bartošová 2011). Allosuckling (suckling of non-filial offspring) was observed in captivity (Pluháček and Bartošová 2011), nevertheless it is presumed to occur in the wild (Smuts and Whyte 1981) as well. Most studies reported that the lactation in common hippopotamus stopped when the offspring is of the age of 8-15 months (Laws and Clough 1966), however suckling of offspring older than 2 years was recorded in captivity (Pluháček and Bartošová 2011). The mean interbirth interval lasts for 2 years in wild and in captivity (Smuts and Whyte 1981; Laws and Clough 1966; Pluháček 2017).
Infanticide by males in terms of sexual selection was reported from several wild populations (Lewison 1998).
The studies on sexual maturity in common hippopotamus resulted in very inconsistent conclusions (Dittrich 1976). Whereas captive females may successfully conceive when 2 years and 3 months old (and most of them do this from 4 years on) and males are fertile at 2 years and 5 months (Dittrich 1976; Pluháček 2017), the studies from the wild reported a much longer time for maturity: not earlier than 6 years for males (Laws and Clough 1966; Smuts and Whyte 1981) and 7 for females (Laws and Clough 1966), with averages as of 7.5 years for males and 9.5 years for females (Laws and Clough 1966; Smuts and Whyte 1981). One of the factors which might be responsible for this huge difference is density-dependence which might cause delayed reproduction in the wild. Both sexes can breed till at least the age of 45 years (captive data: Pluháček 2017; 43 years in wild: Laws and Clough 1966).
The longevity in the wild is not known exactly and most animals die by 40 - 45 years old. In captivity the oldest male died when more than 59 years old (in Cleveland, USA) and the oldest female was as of 61 years old (in Munich, Germany).
Status and Conservation
The IUCN Red List classifies common hippopotamus as Vulnerable (VU A4acd), and it is listed under Appendix II of CITES. The species still occupies much of its former range from half of the 20th century, although population sizes have declined over the past 30 years. The most recent population assessment suggests that total wild population is likely between 115,000 and 130,000 (Lewison and Pluháček 2017). Although the continent-wide population trend is stable, in some regions, remaining hippo populations are experiencing greater pressures and threats. Eastern and Southern African countries represent the conservation stronghold for this species and are the regions where the largest numbers of common hippos occur. Although common hippos are found in many West and Central African countries, overall population sizes tend to be much smaller and conservation actions are needed to protect remaining populations there (Lewison and Pluháček 2017).
The primary threats to common hippos are habitat loss or degradation and illegal and unregulated hunting for meat and ivory (found in the canine teeth; Lewison and Pluháček 2017).
Habitat loss and conflict with agricultural development and farming are a major problem for hippo conservation in many countries (Brugière et al. 2006, Kanga 2013, Kendall 2011, Brugière and Scholte 2013; Stommel et al. 2016). In many West and Central African countries, habitat loss has contributed to a growing regional threat of population fragmentation, as isolated and small populations of hippos are confined to protected areas, with poor or even no management and increasing pressure from local communities (Brugière and Scholte 2013).
The other increasing threats include conflicts with fishermen and gold miners (Mackie et al. 2013).
Reports of human mortalities from common hippo interactions have also increased in recent years, which is another indication of habitat loss. In some agricultural areas, hippo population increases in restricted habitat has also fuelled conflict (Kanga 2013). Ten countries reported growing numbers of hippo-human conflicts, in several cases exacerbated by drought conditions. This problem is more pronounced in countries where common hippos reside outside of protected areas (Lewison and Pluháček 2017).
Illegal or unregulated hippo hunting has been found to be particularly high in areas of civil unrest (Lewison and Pluháček 2017). The most significant examples of this were reported at Democratic Republic of Congo (DRC) and Mozambique. In DRC at the beginning of the 21st century, the common hippo populations declined more than 95% as a result of intense hunting pressure, during more than eight years of civil unrest and fighting (Hillman Smith et al. 2003). In Mozambique hippo hunting by military forces during the civil war during 1980-1992 resulted in decline of more than 70% of the hippo population in the country (Mackie et al. 2013). Widespread poaching for meat has also been reported from Burkina Faso, Burundi, Ivory Coast, and South Sudan (Associated Press 2003, Dibloni 2010, H. Rainey pers. comm.) as well as from politically stable countries like Zambia (Wilbroad and Milanzi 2010).
Estimates of the amount of hippo ivory illegally exported were found to be increasing in the 2008 RLA. A 1994 assessment by TRAFFIC, the monitoring agency of international trade for the IUCN, reported that illegal trade in Hippo ivory increased sharply following the international elephant ivory ban in 1989. Between 1991-1992, approximately 27,000 kg of hippo canine teeth were exported, an increase of an estimate 15,000 kg from 1989-1990 (Weiler et al. 1994). In 1997, more than 1,700 Hippo teeth en route from Uganda to Hong Kong were seized by customs officials in France (TRAFFIC 1997). Five thousand kilograms of Hippo teeth (from an estimated 2,000 Hippos) of unknown origins were exported from Uganda in 2002. Seizure records of illegal ivory in 2007 and 2008 corroborate that this trend continues as Hippo teeth and products from Hippo teeth have been consistently found with seized ivory and ivory products from elephant tusks.
Lewison (2007) evaluates the relative impacts of the known threats to the species'persistence-habitat loss (from agricultural or larger-scale development) and hunting pressure-on a model population. While accounting for rainfall variability and demographic stochasticity, the model results suggest that combinations of habitat loss and even moderate levels of adult mortality from hunting (1% of adults) can lead to relatively high probabilities of population declines over the next 30-40 years. In particular, hippopotamus populations are very sensitive to regular rainfalls (Lewison 2007; Stommel et al. 2016) which start to vary due to global climate changes.