Tenrecs are small
mammals of variable body form. The smallest species are the size of
shrews, with a body length of around , and weighing just , while the largest, the common or
tailless tenrec, is in length, and can weigh over . Although they may resemble shrews, hedgehogs, or opossums, they are not closely related to any of these groups, their closest relatives being the
otter shrews, and after that other African insectivorous mammals including
golden moles and
elephant shrews. The common ancestry of these animals, which are classified together in the
clade Afrotheria, was not recognized until the late 1990s. Continuing work on the molecular and morphological diversity of afrotherian mammals has provided ever increasing support for their common ancestry. Tenrecs are among the few terrestrial mammals that
echolocate. Unusual among
placental mammals, the
rectum and
urogenital tracts of tenrecs share a common opening, or
cloaca which is a feature more commonly seen in
birds,
reptiles, and
amphibians. They have a low
body temperature, sufficiently low that they do not require a
scrotum to cool their
sperm as do most other mammals. All species appear to be at least somewhat
omnivorous, with
invertebrates forming the largest part of their diets. One species,
Microgale mergulus, is
semiaquatic (similar to the lifestyle of their closest relatives, the otter shrews). All of the species, semiaquatic or not, appear to have evolved from a single, common ancestor with the otter shrews comprising the next, most-closely related mammalian species. While the fossil record of tenrecs is scarce, at least some specimens from the early
Miocene of Kenya show close affinities to living species from Madagascar, such as
Geogale aurita. Most species are
nocturnal and have poor eyesight. Their other senses are well developed, however and they have especially sensitive
whiskers. As with many of their other features, the
dental formula of tenrecs varies greatly between species; they can have from 32 to 42 teeth in total. Unusual for mammals, the permanent dentition in tenrecs tends not to completely erupt until well after adult body size has been reached. This is one of several anatomical features shared by
elephants,
hyraxes,
sengis, and
golden moles (but apparently not
aardvarks), consistent with their descent from a common ancestor. Tenrecs have a
gestation period of 50 to 64 days, and give birth to a number of relatively undeveloped young. While the
otter shrews have just two young per litter, the tailless tenrec can have as many as 32, and females possess up to 29
teats, more than any other mammal. Some tenrec species are social, living in multigenerational family groups with over a dozen individuals. ==Interaction with humans==