Physical characteristics

The size of living insectivores ranges from a small mouse to a large house cat. Savi's pygmy shrew (Suncus etruscus), is believed by many scientists to be the world's smallest living mammal weighing 0.04-0.10 oz (1.2-2.7 g) and measuring 1.4-2.1 in (36-53 mm) without the tail. The 1998 discovery of a 65 million year old fossil jaw measuring 0.3 in (8 mm) and belonging to the extinct Batonodoides, suggests that some prehistoric insectivore mammals were even tinier than some Savi's pygmy shrew.

The moonrat, (Echinosorex gymnura) is the largest living insectivore, weighing as much as 4.4 lb (2 kg) with a body measuring up to 16 in (40.6 cm) and an 8 in (20.3 cm) long tail. Found on the Malay Peninsula, Sumatra, and Borneo this invertebrate and fruit-eating mammal keeps predators at bay by releasing a foul odor. The largest extinct member of Insectivora is thought to be Deinogalerix which means "terrible hedgehog." This 2-ft-long (61-cm-long) hairy mammal with elongated snout, sharp teeth, and short legs lived in Europe during the middle Miocene period about 15 mya.

Many members of Insectivora share the following characteristics: five-clawed digits on each limb (pentadactyly); short legs; annular (ring-shaped) tympanic bone; long, flat, small skull; flat cranium (brain case); a small, smooth brain with hemispheres that do not extend backward over the cerebellum; an incomplete zygomatic arch (cheek bone); largely interorbital, well developed and sharply demarcated olfactory bulbs; small ears (exterior ones nonexistent in Talpidae) often lacking ossified auditory bulla (the bony covering of the middle ear cavity); no intestinal caecum; pollex (thumbs) and hallux (big toes) are not opposable; tibia and fibula are often fused near the the ankle whereas the radius and ulna are separate. The otter shrew (Potamogale) is the only Insectivora genus that does not have clavicles. A cloaca, the common posterior chamber into which the digestive, urinary, and reproductive tracts all discharge, which is uncommon in most placental mammals, is frequently found in Insectivora species. Male testes are abdominal, inguinal, or borne in a sac in front of the penis; a baculum (os penis) is present in some species. All members of this order have a chorioallantoic placenta that allows the young to develop fully in the womb.

Shrew Penis
A short-tailed shrew (Blarina brevicauda) eats a worm. (Photo by Dwight Kuhn. Bruce Coleman, Inc. Reproduced by permission.)

Tiny eyes and poor eyesight are typical of insectivores. Moles and desmans (talpids) are almost blind and in many mature species the eyes are entirely covered with fur. Most insectivores have small external ears, some concealed under fur. Moles have no external ears. Asiatic water shrews (Chi-marrogale) have tiny ears featuring a valvular flap that seals the opening when they submerge themselves in water. The cone-shaped snouts characteristic of insectivores come in many different sizes and perform a wide variety of tasks. The ethmoturbinal bones (bony plates that support nasal membranes) are large. Coiled scrolls of bone make up the nasal chamber that is covered with olfactory epithelium, the receptors of which are stimulated by airborne chemical molecules. The insectivore olfactory sense is very keen. Some snouts are long and thin with flexible tips for prodding in muddy areas, others are short and stubby with leathery pads on the tip. Solenodons have a small round bone—os proboscis— located at the tip of their hairless nose that supports the snout cartilage.

Vibrissae—sensory hairs located on many insectivoran tails, snouts, behind ears, and even on feet—are large in diameter. They are relatively rigid so that they do not bend much, but instead act as levers, transmitting the applied force to their base. They are embedded in a fluid filled sack, which allows them to move about under the skin. Water shrews, like many other insectivores, have vibrissae on their snouts believed to help them locate prey. It is thought that unless these shrews actually touch their prey with their vibrissae, no matter how close they are, they cannot capture it. Long-tailed shrews (Sorex dispar) rotate their long, slender, vibrissae-adorned snouts constantly. Moles and desmans (Talpidae) depend primarily on their olfactory (smell) and tactile (touch) senses. The talpid snout is usually long, narrow, very mobile and extends beyond the end of the upper jaw. The tip is hairless, apart from a few vibrillae, and features Eimer's organs (minute sensory receptors). The star-nosed mole (Condylura cristata) has a snout tip featuring 22 fleshy pink appendages or tentacles consisting of 25,000 sensory receptors used to help it locate food. When the star-nosed mole eats, the tentacles curl up out of the way. The snout of the desman is such that it can be used as a snorkel or periscope that continously monitors the air for prey or predators.

Many variations on the dental formulae can be found among Insectivora but one of the most frequent is (I3/3 C1/1 P 4/4 M3-4/3-4) X 2 = 44, 46, or 48. All insectivores have rooted, primitive teeth. Deciduous teeth, the first set that develops in mammals, are shed early on and rarely serve a purpose. Insectivores have unspecialized sharp teeth and often a crown pattern typical of primitive placentals. Some have front teeth modified by specialized or at times enlarged incisors and canines with a varying morphology, sometimes shaped like incisors or premolars. Shrews and moles often have dilamb-dodont upper molars (W-shaped crest pattern). Tenrecs, golden moles, and solenodons have zalambdodont upper molars (V-shaped crest pattern). The upper molars in hedgehogs and gymnures are quadrate meaning they have four main cusps. The Haitian solenodon (solenodon means "grooved tooth") and European water shrews—both omnivorous but preferring an animal diet—have unusual dentition: a large upper incisor points slightly backward and a deeply grooved channel in the lower incisor at the base of which is a duct that transports venomous saliva.

The majority of insectivores have plantigrade feet meaning that, like humans, they place the full length of their foot on the ground during each stride. Fossorial insectivore limbs are specialized for digging tunnels and burrows. The fore-limbs are short, powerful, and shovel-like, rotated so that the elbows face upwards with strong large-clawed paws facing

Characteristics Mammals
An eastern mole (Scalopus aquaticus) emerging from its hole. (Photo by L. L. Rue III. Bruce Coleman, Inc. Reproduced by permission.)

backwards. Moles (Talpidae) have five claws and a falciform bone, which expands the palms and supports the digits. It is sometimes referred to as a "sixth digit." Tenrecs and golden moles have four claws. Talpids also have an elongated olecranon process (projection on the proximal, or elbow, end of the ulna). The aquatic desmans (Desmaninae) do not dig much and have small forelimbs with webbed feet. Moles, but not desmans, use their weaker, less developed posterior limbs for propulsion only. Shrew moles (Uropsilus) seldom dig as they spend most of their time under leaf litter. As a result, their forelimbs and claws are not as well adapted for digging. Several species of semi-aquatic insectivores have long stiff hairs (fibrillae) on their feet to help propel them in the water like paddles. Several aquatic forms of family Soricidae (shrews) have webbed feet with hairy fringes that increase the surface area of the foot. This allows air bubbles to be trapped making it possible for the shrew to run on the water's surface for up to several seconds at a time. No insectivore forelimbs are adapted for jumping or leaping.

Insectivore tail length and texture vary like most other physical characteristics belonging to this order. Most talpids and hedgehogs have relatively short tails, some covered with vibrissae. Several of the semi-aquatics, the desman for example, have stiff flat rudder-like tails. Golden moles (Chlrysochloridae) have no external sign of a tail. The star-nosed mole has a tail that is almost as long as its body. Mole tails brushing against tunnel walls and roofs are able to pick up a variety of information including ground vibrations. The tail of the Madagascar hedgehog (Microgale longicaudata) is 1.5 to 2.6 times longer than its body and has 47 vertebrae. It is one of the longest mammal tails, second only to the pangolin

Characteristics Mammals

A greater white-toothed shrew (Crocidura russula) feeds on an earthworm. (Photo by Jose Luis Gomez de Francisco/ Reproduced by permission.)

Mole Tunnel Characteristics

The Pacific mole (Scapanus orarius) has claws that are useful for tunneling. (Photo by R. Wayne Van Devender. Reproduced by permission.)

A greater white-toothed shrew (Crocidura russula) feeds on an earthworm. (Photo by Jose Luis Gomez de Francisco/ Reproduced by permission.)

The Pacific mole (Scapanus orarius) has claws that are useful for tunneling. (Photo by R. Wayne Van Devender. Reproduced by permission.)

(scaly anteater). Microgale also have a tail that is modified for prehension (grasping). The lesser hedgehog tenrec (Echinops telfairi), on the other hand, has no external tail whatsoever. Several shrew names denote the nature of their tails—short-tailed, pen-tailed—the other 300 plus have tails that range from short to long, thick to thin, hairy to sparse, and tubular to flat depending on the species. The giant otter shrew (Pota-mogale velox) has an eel-like, lateral tail that allows it to swim like a fish while its keeps its hind feet glued to its flanks.

One of the mammals' most unusual skeletal structures is found among Insectivora. The West African armored, or hero, shrew (Scutisorex somereni), is believed to possess amazing strength. It has vertebrae with many interlocking spines as well as dorsal and ventral spines. The spines appear to allow it to bend considerably as well as bear phenomenal amounts of weight. The way in which the muscles are attached is complex and it seems that the extra joints allow additional flexibility.

Insectivores have fur or, in the case of some tenrecs (Ten-recinae) and hedgehogs (Erinacidae), a spiny coat, or even a combination of hairs and blunt spines that serve as protection against predators. When threatened, these mammals activate a set of muscles that cause their spines to become erect. Hedgehogs can protect themselves further by rolling up into a tight ball by activating their panniculus carnosus (a powerful orbicular muscle). Most aquatic insectivores have water-repellent pelage. Fur colors go from light to black, covering all the earth tones in between. Insectivores can be mono-, bi-, or multi-colored. Many feature lighter shades on their undersides. Fur texture can be glossy on golden moles to short, dense and velvety in least shrews and thick and woolly in hero shrews. Water shrews have been reported to have iridescent fur when exiting the water. Other species with iridescent fur include the ground-dwelling Cape golden moles (Chrysochlo-ris). Some of them are reported to display greenish, violet, or purplish tinges.

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