Insectivores (Insectivora, Macroscelidea, Scandentia)

Insectivores are considered the most primitive of all placental mammals and are believed to be the ones from which present day mammals have evolved. The order consists of six extant families: Erinaceidae (hedgehogs and gymnures), Chrysochloridae (golden moles), Tenrecidae (tenrecs), Solenodontidae (solenodons), Soricidae (shrews), and Talpidae (moles, shrew moles, and desmans). A seventh family, Nesophontidae (West Indian shrews), is considered extinct. The order Insectivora comprises the third largest group of mammals, with over 400 species identified. Some controversy exists in classification within this order, and some have proposed the formation of a new order, Afrosoricida, based on molecular studies. This new order would include the families Chrysochloridae and Tenrecidae. However, most current literature maintains all six families in Insectivora.^18,32

Insectivores are terrestrial, fossorial, or semiaquatic and are almost completely nocturnal. They are found worldwide except in Australia, Antarctica, and most of South America. Over one third of all species in the order Insectivora are listed on the International Union for the Conservation of Nature (IUCN) Red List, with 21% of those listed as critically endangered and 26.5% as endangered. The major causes of population decline are human-induced habitat loss, fragmentation, and degradation. Overall insectivores are poorly understood, and further study is needed to determine practices that will aid in their conservation. As a group, insectivores make important contributions to the environment by controlling insects that damage crops and controlling vermin. Fossorial species also effectively aerate the soil.¹⁸

Insectivores are small mammals that typically have long, narrow snouts and five clawed, nonopposable digits on each limb. The pelage consists of short, dense fur; short nonbarbed spines; or a combination of both. External ears are very small or non-existent (Talpidae). Insectivores have very small eyes and poor eyesight, with some species (desmans and moles) being completely blind and lacking a palpebral opening. They have a very keen sense of smell, and the nasal chamber consists of scrolls of coiled bone covered in olfactory epithelium for enhanced olfaction.^18,32 Most talpids have an elongated, mobile snout featuring arrays of small bumps called Eimer's organs. Eimer's organs contain a dense array of mechanoreceptors that allow detection of very small surface textures and features.²⁷ The star-nosed mole (Condylura cristata) has a specialized snout that contains 22 fleshy appendages with 25,000 mechanosensory receptors to help it locate food. Solenodons have a small bone called the os proboscis at the tip of the nose to support snout cartilage. Vibrissae, located on the snout, ears, tails, and sometimes feet, are large in diameter and relatively rigid, and they aid in the location of prey items.^18,32

The dental formula in many insectivores is 3/3, 1/1, 4/4, 3–4/3–4, with a total of 44 to 48 teeth, although significant variation does exist within the order. All teeth are rooted but are primitive. Deciduous teeth serve no purpose and are shed very early in life. The upper molar pattern in shrews and moles is dilambdodont (W shaped), whereas tenrecs, solenodons, and golden moles have a zalambdodont pattern (V shaped). The upper molars of hedgehogs and gymnures have four main cusps. Solenodons (Solenodon sp.) and some shrews (Neomys fodiens and Blarina sp.) produce toxic saliva from the submaxillary gland, and the saliva is delivered through deep grooves in the lower incisors.^18,32

The majority of insectivores have a plantigrade stance, and fossorial species have short, powerful forearms specialized for digging. Talpids have a falciform bone, sometimes called a “sixth digit,” which expands the palm and supports the digits. In talpids, the radius articulates with the humerus in an S-shaped cavity, causing the forearm to be permanently rotated outward from the body. This allows the forelimb to act as a spade for digging. Aquatic species have webbed feet with hairy fringes, which allows them to run on water surfaces for several seconds. The tails of moles are able to detect ground vibrations, and Microgale has a modified tail that is prehensile.^18,32

Insectivores have a low, flat skull, with reduced or absent zygomatic arches. The brain is small, and the cerebral hemispheres lack fissures and do not extend over the cerebellum. All species except Potamogale spp. have a clavicle. All insectivores lack a cecum, and many species have a cloaca.³²

In males, the testes are abdominal, inguinal, or in a sac in front of the penis. Some species have a baculum. Females of some species in the Talpidae family have ovotestes with a functional ovarian segment and a larger testicular segment that lacks germ cells. These animals are considered true fertile hermaphrodites.⁴

Armored shrews (Scutisorex somereni) have unique interlocking lateral, dorsal, and ventral vertebral spines, which create an exceptionally sturdy vertebral column. Despite this feature, these shrews still have considerable flexibility and may bend dorsoventrally and laterally.³²

Special Physiology

Most insectivores have a very high metabolic rate. Compared with most mammals, insectivore body temperatures are usually lower (33° C to 35° C), the exception being Sorex spp. (37° C to 38°C). Some species such as the hedgehog exhibit true hibernation for part of the year. They spend the months preceding hibernation building up fat stores. Some species in the families Macroscelididae, Chrysochloridae, and Tenrecidae exhibit torpor (heterothermy) on a daily or seasonal basis when ambient temperatures decrease or food sources become scarce. During hibernation and torpor, heart rate, body temperature, and metabolic rate drop significantly. The body temperature of the Southern African hedgehog (Atelerix frontalis) may decrease to 1° C during hibernation.²⁹ Solenodons and some species of moles and shrews emit high-frequency vocalizations that may have an echolocation function for navigation.

Hedgehogs exhibit a unique self-anointing behavior, also called anting. When a new or irritating substance is encountered, the hedgehog will lick the substance until saliva is produced and then vigorously groom its quills. The reason for this behavior is unknown, but theories suggest the method may serve as protection from predators or to apply a unique scent to itself or its home range. This normal behavior and the resultant saliva production are often confused for a disease such as rabies or a dental condition.^18,20,32

Special Housing Requirements

Insectivores are rarely used as exhibit animals because of their secretive nature. Most are nocturnal, and the fossorial species spend most or all of their time underground. For the most part, insectivores are solitary and socialize only during mating and during rearing offspring. In a captive situation, some species may be exhibited in groups without problems; however, significant aggression may be seen. The most successfully exhibited species include elephant shrews, tree shrews, African hedgehogs, and tenrecs.

Insectivore species have a well-developed sense of hearing; therefore, the area around the exhibit should be kept quiet. Double-glass barriers have been used for this purpose. Walls should be smooth, nonclimbable, and easily cleaned. Absorbent bedding may be used to decrease the exposure of the animals to urine and feces, which may lead to dermatitis. Shrews may become entangled in shredded wood bedding and may be averse to the noise of shredded paper. Litter should be deep enough for the animal to plough through.

A nest box is essential. Insectivores are sensitive to disturbances, so cleaning should be done only as needed. The use of a second enclosure to shift animals may minimize disturbance. Shrews and some other insectivores establish latrines and tend to defecate near water, even if provided in a bottle.

Environmental temperatures depend on the natural history of the species but usually are between 23° C and 28° C. All insectivores should be kept free from drafts and abrupt (greater than 5° C) temperature changes. Cages should be well ventilated. Exhibits should be kept dry to prevent the animals from losing body heat through damp pelage.^2,32

Feeding

As would be expected, insects are the bases of the natural diet of most insectivore species. However, most species have varied diets, including small vertebrates, carrion, berries, nuts, and vegetation. Aquatic species prey on worms, crustaceans, fishes, and frogs. Many insectivores have a voracious appetite and consume large quantities of food.^{18,20,32–34}Hedgehogs possess chitinase in the gastric mucosa and pancreas, which presumably aids in digestion of the exoskeletons of insects.⁶

In captivity, diets should contain high protein (30%–50%) and moderate fat (10%–20%) content on a dry-matter basis.⁶ Commonly exhibited insectivores may be maintained on a commercial cat diet or insectivore diet and on insects. Small amounts of fruit, vegetables, and leafy greens may also be added. Raw meat and eggs should be avoided, as they may harbor Salmonella sp. Milk should not be fed, as it may cause diarrhea.²⁰ Periodontal disease has been reported frequently; therefore, dry food may be beneficial to dental health.⁶

Feed consumption varies widely by species. Some shrew species may eat up to several times their body weight daily. Some species may undergo wide fluctuations in appetite and body weight because of seasonal changes and so may be a challenge to manage. Transient weight gain may be physiologically normal in species that exhibit heterothermy; however, in captive situations, diets may need to be rationed to prevent obesity. Nutritional problems may be the cause of the high rates of early mortality and poor rates of survival frequently seen in insectivores in captivity.^2,18,32

Restraint and Handling

Many insectivore species are sensitive and prone to stress when handled. Plastic tubes and clear containers may be useful for visual examinations. Hedgehogs and tenrecs tend to roll up when touched, making physical examination difficult without sedation or anesthesia. Species that are habituated to handling may be more easily examined under manual restraint. Light gloves may facilitate handling of species that have spines or those that are venomous.⁹

Anesthesia and Surgery

Parenteral anesthesia has been described in the literature but is reserved mainly for use in the field or for research purposes (Table 34-1). Parenteral anesthetics may be given by subcutaneous (SQ), intramuscular (IM), or intraperitoneal (IP) injection. IV injection is very difficult in most insectivores, but injections may be given via the cephalic vein or by the intraosseous (IO) route into the femur, humerus, or tibia. Inhalant anesthesia is more commonly used and is the preferred method in insectivores. Anesthesia may be induced using an induction chamber or a face mask. Because of the small body size of insectivores, intubation may be considerably difficult. Fasting is generally not recommended because of the high metabolic rate and small glucose reserves. High metabolic rate and oxygen requirement make insectivores prone to hypothermia, hypoglycemia, and hypoxemia, so the animals should be monitored closely. Anesthetic events should be kept as short as possible to minimize complications. Isoflurane may increase blood flow to the skin in the smaller species, so a heating pad is recommended to avoid rapid temperature decrease. Insectivores may be monitored by using pulse oximetry, electrocardiography (ECG), or Doppler ultrasonography.^9,14,20

Some surgical procedures reported in insectivores include enucleation, dental extractions, limb amputations, spay or neuter, biopsy and resection of various neoplasms, and treatment for traumatic injuries and abscesses.

Diagnostics

Collecting samples from insectivores may be challenging because of their small size, unique anatomy, and tendency for some species to roll into a ball when handled. Blood may be collected from the jugular vein, the cranial vena cava, and the cephalic, femoral, or saphenous veins. Reference ranges for complete blood cell count and serum biochemistry for select species are presented in Tables 34-2 and 34-3. SQ injections may be given in the spiny or furred areas. The dermal layer under the spines is poorly vascularized in hedgehogs, and absorption of drugs or fluids may be delayed in this area. IM injections may be given in the muscles of the hindlimb. The spines of hedgehogs and tenrecs may diminish radiographic details, and often anesthesia is required for proper positioning. With the use of plastic clips or tape, the dorsal skin may gently be pulled away from the chest and abdomen to achieve improved detail.²⁰

Infectious disease has been reported relatively uncommonly in insectivores. Bacterial diseases reported include enteritis caused by Salmonella sp., pneumonia caused by Corynebacterium sp., and respiratory disease caused by Pasteurella sp. and Bordetella bronchisepticum.²⁰ Insectivores may be reservoirs for several bloodborne diseases, including Rickettsia spp., Borrelia burgdorferi, Babesia microti, and Anaplasma phagocytophilum, which are transmitted by parasite vectors; however, clinical disease associated with these organisms has not been noted in insectivores.^17,23,28 Several fungal diseases, including adiaspiromycosis, cryptococcosis, paecilomycosis, histoplasmosis, and dermatophytosis, have been reported.^{2,12,20,37,39} Viral disease is rare in insectivore species; however, foot and mouth disease and paramyxovirus have been reported.²¹ Several species of shrews have been shown to harbor novel hantaviruses.²² Herpesviral infection has been reported in European hedgehogs (Erinaceus europaeus), and one case of an African hedgehog (Atelerix albiventris) that succumbed to infection with herpes simplex virus 1 (HSV-1) has been published.¹ The bicolored white-toothed shrew (Crocidura leucodon) has been shown to carry bornavirus, suggesting that it may be a reservoir species.³⁵ Table 34-4 lists commonly used antibiotics and dosages.

There is little information on the reproductive biology of many insectivores. Most appear to breed seasonally, but some of the smallest, short-lived species breed year round. Insectivores have a chorioallantoic placenta, which allows the young to fully develop in the uterus.¹⁸ Gestation length and litter size vary considerably across the order. A commonly kept insectivore, the lesser hedgehog tenrec (Echinops telfairi), has a mean gestation length of 67.5 days (range 57–79 days) with an average litter size of three to five.²⁵ The tenrec (Tenrec ecaudatus) appears to be one of the most prolific mammals, with litter sizes up to 32 reported.¹⁸ Selected reproductive parameters are listed in Table 34-6.

Some species of shrews exhibit caravanning behavior: The young follow each other in a single file while gripping the rump of the one in front. The first of the offspring in the line grips the dam's rump.³²