Biology

Today, two extant species of the Hippopotamidae coexist within two genera: (1) the small and rare pygmy hippopotamus (Choeropsis liberiensis, previously Hexaprotodon liberiensis) native to the forests of West Africa, and (2) the far larger, more widespread, and common hippopotamus (Hippopotamus amphibius). The geographic distribution of the common hippopotamus today is restricted to sub-Saharan Africa, where it inhabits rivers, lakes, and wetlands.

In 2006, this species was included for the first time in the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species and was listed as Vulnerable to Extinction.³¹ The common hippopotamus is listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora.⁹ The wild population is rapidly dwindling, with a 30% decrease in the past decade. In some countries such as the Democratic Republic of Congo, the wild population has diminished by as much as 95%.³⁶ Unfortunately, there are very few recent surveys of hippopotamus (“hippo”) populations and, therefore, a significant uncertainty concerning the momentary situation exist.

As its name indicates, the common hippopotamus has an amphibious lifestyle, gregariously resting in shallow water during the day and emerging at night to feed solitarily. Common hippos live in loose, social, polygynous groups of females with offspring and territorial males, based on mating territoriality. Spreading its dung by tail wagging while defecating, a behavior more commonly observed in males, is believed to have a signaling function, rather than a territorial one. Although conflicts are highly ritualized and usually restricted to threat displays, fierce territorial fights may occur when a bachelor challenges a territorial male. These territorial fights, which usually occur in the water, may result in considerable trauma and even death caused by the huge canines.

Similar to its larger relative, the pygmy hippo is facing a considerable population decline in the wild and is currently listed as Endangered on the IUCN Red List of Threatened Species.³¹ The pygmy hippo is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora.⁹ The species occurs in four African countries, namely, Sierra Leone, Guinea, Côte d’Ivoire, and Liberia, where the largest populations are found. Because of its secluded and nocturnal lifestyle, not much is known about the ecology of the species. The pygmy hippo is less gregarious compared with the common hippo and is usually found either solitary or in pairs. The few studies indicate that the pygmy hippo retreats into swamps, river banks, and heavily forested habitat during daytime and that it shows a strong affinity to water and is therefore found close to rivers or streams.^39,60

Both species are long lived, with reported ages of 35 to 50 years (61 years reported in a captive animal) in the common hippo, and up to 55 years in the pygmy hippo.^45,72 Weights range up to 2500 kg in the female and 3000 kg in the male common hippos, with a tendency to be higher in captivity; and around 160 to 350 kg in the pygmy hippo.

Unique Anatomy

Compared with its larger relative, the common hippo, which is designed to support its enormous body weight (the scapula is oriented vertically and forms a vertical column with the limb bones; the pelvis is oriented at a 45-degree angle), the skeleton of the pygmy hippo is built slightly lighter.⁴⁵ The shape of the skull and the position of the eyes differ among the species, according to their individual lifestyles. In the common hippo, the eyes face forward, whereas in the pygmy hippo, the skull is proportionally smaller and rounder, with the orbits positioned laterally. In the common hippo, the brain case is extremely small compared with the head, which should be taken into account when euthanasia performed with a rifle is considered. The jaw of the common hippo is capable of a 150-degree opening, which is less in the pygmy hippo.

Hippos are considered pseudoruminants, as they have a complex four-chambered stomach structure with a ruminant-type digestion, although they do not ruminate. As in ruminants, fermentation occurs mainly in the forestomachs, which contain a rich ciliate fauna. Cecum and gallbladder are missing. However, necropsy reports have described a structure that contains bile and have reported it as “gallbladder-like.”⁴⁵ In contrast to other species of artiodactyls, hippos have four functional toes with nail-like hooves on the end of the toes.

Both hippo species show marked physiologic adaptations for a (semi-)aquatic life style. Accordingly, the external sensory organs are positioned high on the head.⁵ The skin is thick, lacks sebaceous glands, and retains few hairs. Muscular valves close the ears and nostrils during diving. Recently, the hippo has been identified as the artiodactyl sister group of cetaceans.²

The unique morphology and the physiology of the hippo may constitute a challenge during medical procedures. The thick skin and the dense subcutaneous tissue may impede darting and the resorption and distribution of drugs. Difficulties caused by their large size, dive response, arousal during anesthesia, and limited vascular access have been previously reported.^45,56,68

Furthermore, knowledge gaps with regard to the anatomy and location of specific organs such as the testes have led to intrasurgical difficulties.^15,56 A decade ago, it was already realized that more research is required before surgical castration of the male hippo can be recommended.⁴⁵ The location of the testes in the hippo is still described as “internal” in analogy to cetaceans that have true intraabdominal testes. However, in the hippo, the testes are actually partially descended and remain in the inguinal canal.⁷¹ A scrotum, as such, is not developed in the hippo. The hippo evolving between land and water appears, in contrast to true terrestrial mammals, to have partially retracted testes.¹⁴

Special Physiology

The unique skin of the hippo, consisting of a thin epidermis and a thick dermis with dense subcutaneous and fat tissue, characterizes both species. Subepidermal capillary loops, considered to play an important role in thermoregulation, have a diameter 20 times larger and density three times higher than in other mammals and were recently described in the integument of the common hippo.⁴³ Furthermore, these capillaries are uniquely adapted for high blood pressure, allowing for efficient blood vessel–based heat transfer to the periphery.⁴³

No sebaceous or sweat glands exist, but large, subdermal mucous glands secrete a thick, oily red fluid—the so called “blood sweat” or “red sweat.”²¹ The secretions are believed to protect the thin epidermis against water loss, sunburn, and infections.

Hippos have an exceedingly good predisposition for wound healing.⁴¹ Good wound healing is clearly necessary for the common hippo, as territorial fighting results in wounds of significant size and number.¹⁰ One possible explanation for the good wound healing properties is the particular “red sweat” observed in this species. The pigments responsible for the color have been described as hipposudoric acid (the red pigment) and norhipposudoric acid (the orange pigment).²⁷ The red pigment has been shown to have significant bacteriocidal activity by inhibiting the growth of pathogenic bacteria such as Pseudomonas aeruginosa and Klebsiella pneumonia.⁶¹ Similarly, the closest relatives of the common hippo, cetaceans, also have an epidermal defense mechanism against bacteria, fungi, algae, and ectoparasites.⁴²

Because of their size and strength, the common hippo requires robust housing and adequate barriers. Many existing barriers in zoos are very low and could easily be scaled by adult hippos. The animals are generally not considered to be frost resistant, although they are found in Africa at higher altitudes (1500–2000 m above sea level) with cool night temperatures and have been held in zoologic collections in Europe in subzero conditions. However, for animal welfare reasons, they should be provided with heated interior enclosures in temperate climates. Room temperature is suggested to range from 16° C to 20° C. The animals need interior and exterior pools, which should have a depth of at least 2.5 m so that the animals can remain totally submerged.^45,55 Additional shallower areas in the pools have been shown to be beneficial for the resting behavior.¹ Extensive filtering is needed to provide conditions for underwater viewing. Furthermore, the animals require adequate areas to rest and shaded areas in the outside enclosures. The interior enclosure design should allow for easy separation of individuals and removal of individuals from the pool for medical interventions. Naturalistic exhibits have been suggested to enhance animal welfare and provide the public with a clearer view of the lifestyle of the hippo.¹

Feeding

Both hippo species are easy to keep in captivity and usually pose few veterinary problems related to feeding. Disorders associated with the GI tract are mainly reported to be foreign body ingestion leading to obstruction of the small intestine.¹⁶ Hippos are considered strict herbivores, although cases of carnivory in the wild common hippo have been reported.¹² Common hippos are described as exclusive grazers with a capacious foregut fermentation system.⁶⁵ Interestingly, this species is reported to have a comparatively low food intake and also short feeding times.⁷ The diet of an adult common hippo should consist of some 30 to 45 kilograms (kg) of a mixture of good-quality grass, clover, lucerne, and green maize. Additionally, various vegetables may be provided. In winter, the grass may be replaced with good-quality hay and silo.⁵⁵ The few studies on pygmy hippos suggest that the diet in this species contains ferns, herbs, leaves, and fruits, with grass playing a minor role.²⁹ In captivity, a wide range of vegetables and fruits may be provided, along with high-quality hay. It is important to note that both hippo species are reported to have unusually low metabolic rates, and therefore the provision of energy-dense pelleted foods should be considered carefully in view of the species’ proneness to obesity.⁶⁴

Restraint and Handling

Physical restraint and handling are difficult in both hippo species, as they are extremely dangerous. Especially, the common hippo, because of its weight, size, sharp canine teeth, and the often-aggressive demeanor, may easily inflict fatal injuries. For any procedures that require closer contact with the animal, additional chemical restraint is recommended (Tables 59-1 and 59-2).

Fasting the hippos 24 to 48 hours and restricting their water intake for 12 to 24 hours before anesthesia are recommended by some authors to reduce the volume of gut content and, thus, the pressure on the diaphragm during recumbency.^40,44,45,56 However, other authors have successfully anesthetized hippos with fasting of the animals for only some 12 hours.⁶⁸

Anesthetic agents may be applied manually in trained animals, but usually pole syringes, anesthetic pistols, or guns are used. Especially in large adult animals, only the area caudal to the ear or the medial and caudal aspects of the hind leg are recommended sites for injection, as the dense subcutaneous tissue and fat of several centimeters thickness will impede darting and the resorption in other areas. In juvenile or smaller animals, other sites may be used for injection. To penetrate the skin and the fat layer, long, reinforced, nonbarbed needles of 60 to 100 millimeters (mm) length are recommended.

The literature and reports on anesthesia of hippos are rare. Very early reports on capture and chemical restraint provide descriptions of field captures in which various muscle relaxants such as succinylcholine chloride,^53,70 varying combinations of phencyclidine hydrochloride with α₂-agonists, acepromazine, or ethorphine,^59,69,76 or fentanyl-acepromazine were used.²⁵ These historical immobilizations in the field often resulted in drowning of animals, as hippos tend to retreat into water when threatened. This fact should be kept in mind when considering anesthesia, and access to pools should be blocked to prevent accidental drowning.

The use of detomidine–butorphanol and medetomidine–butorphanol–azaperone (BAM) combinations have been described.^8,13,19,47 Spontaneous arousal of animals following stimulation makes these protocols only suitable for sedation and very minor or nonsurgical procedures.

The most commonly used anesthetic is the potent opioid etorphine, sometimes in combination with xylazine or acepromazine.^40,45,56,69 Reported complications with these opioid-based anesthetic combinations include apnea, cyanosis, bradycardia, and fatal respiratory arrest.⁴⁴ In a retrospective study, 6 of 16 immobilizations were shown to have resulted in complications from bradypnea and apnea. In 3 of 4 cases, these respiratory complications were successfully resolved with the administration of doxapram, a respiratory stimulant. In the same study, only 2 of 16 procedures provided an anesthetic depth sufficient for surgical interventions.

The successful use of a medetomidine–ketamine combination in 10 adult male hippos undergoing castration has been reported.⁶⁸ In this study, self-limiting apnea was observed in 5 of 10 animals. The authors considered this a physiologic process related to the dive response in this semi-aquatic species, as no intervention was necessary, and vital parameters remained unchanged throughout.

The literature on anesthesia in the pygmy hippo is scarce. Various case reports provide brief descriptions of anesthetic protocols.^{4,17,20,35,46} Anesthesia in this species has been described in more detail elsewhere.^{3,33,34,44,45,52,73}

The protocols used are similar to the ones used in the common hippo. Etorphine, alone or in combination with acepromazine; azaperone; propionylpromazine; or xylazine were used most commonly. The use of ketamine in combination with α₂-agonists has been reported.³³ Ketamine and medetomidine (with isoflurane maintenance) has been successfully used in the pygmy hippos.^3,4,35 Combinations of detomidine or medetomidine with butorphanol have been described, but as in common hippos, pygmy hippos also showed arousal because of stimulation.^44,45 Ketamine–butorphanol combination is reported to be used for anesthesia induction.^44,45 Midazolam–zolazepam–tiletamine combination has been used for mild sedation in this species.

Although difficult, anesthesia monitoring is crucial for successful anesthesia in hippos. The most serious adverse effects of anesthesia in hippos include hypopnea or apnea, bradycardia, and hyperthermia. This is especially true with regard to long procedures with poor oxygen saturation. Respiratory rate may be assessed directly by observing thoracic excursions or indirectly with a respirometer, capnography (side or mainstream), or observation of the rebreathing bag when the animal is intubated.

Endotracheal tubes of 24 to 30 mm are recommended for an adult hippo.^40,45,47 To keep the hippo’s mouth open and to safely intubate, a mouth gag (e.g., wood block) may be used. Because of the pygmy hippo’s smaller, more caudal pharynx and smaller trachea, intubation is reported to be more difficult than in the common hippo and is usually performed by manual palpation with a tube size of 14 mm for an adult pygmy hippo.^17,44,73 Heart rate may be auscultated in smaller animals or by pulse oximetry (with the probe attached to the tongue, inside lip, eyelid, ear [after removing the corneal layer of the skin] vulva, prepuce).^45,68 Doppler ultrasonography (with probes placed on the cornea),⁴⁵ or palpation of an artery (sublingual artery) may also be used (Stalder, personal communication). Electrocardiography (ECG) is generally difficult to perform under routine anesthesia. Saturation of peripheral oxygen (SpO₂) may be measured by pulse oximetry at the same locations as stated for the heart rate or by blood gas analysis (saturation of arterial oxygen [SaO₂]). Oxygen supplied by nasal insufflation or via a tube with a flow rate of up to 15 liters per minute (L/min) is recommended to improve oxygen saturation.^40,68 The use of a Hudson demand valve may be useful in view of the very low respiratory rate in anesthetized hippos.

Arterial access is generally difficult, but blind puncture of the ventromedial tail artery is possible.⁶⁸ Several venipuncture sites have been used in the hippo (ventral tail vein, sublingual vein, cephalic vein, medial vein at the antebrachium, or the palmar digital vein, caudal on the lower limb, medial saphenous and plantar digital vein, auricular vein).^45,47,68 Intravenous catheterization is usually difficult. The thickness of the skin may necessitate a cut-down procedure or the use of ultrasonography to locate the veins. On the legs, the veins are extremely thin walled and collapse easily, so these veins are less suitable for catheterization.

Especially during longer procedures, body temperature should be closely monitored, as hippos could become hyperthermic. Water should be available to keep the skin moist and support thermoregulation. A cold-water rectal enema may also be used to decrease body temperature.