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Shrew ecology-metabolic limitations to small size
Soricidae = Shrews |
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The Common Shrew has a brown back, darker than that of the Water Shrew and has a short tail in proportion to its body. It lives on the mainland but not the islands of Britain. The average adult weighs 12-14g.
A Pigmy Shrew
The Pigmy Shrew has a dorsal coloring not as dark as that of the Common Shrew. It has a longer tail than the common shrew, but is smaller-the average adult weighing 8-12g. Found in Ireland, Outer Hebrides, as well as thoughout Great Britain. |
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White toothed shrews, Crocidurinae, DO NOT occur on the British mainland, only on a few
of the islands |
Territories... |
Individuals are territorial over winter-they have patches of ground divided up between individual shrews. Males and females have different territories and when a neighbour dies, its territory is usually taken over by other shrews. Both Common and Pigmy hrews behave in this way, but Pigmy Shrew territories overlap those of Common Shrews.
Territory sizes get bigger over winter for Common shrews (500m2) however the smaller Pigmy Shrews have larger territories (1500m2). The increase in territory size over winter is explained by surviving shrews taking over the territories of shrews that have died.
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Diet... |
Due to the difference in size between the Pigmy and Common Shrew, the two focus on different prey. For example the smaller Pigmy Shrew eats a lot of small spiders and beetles whereas the Common Shrew eats a lot of earthworms and molluscs-these are either too big for the Pigmy Shrews or simply not attractive to them. As a consequence, Pigmy Shrews are common on moorland (plenty of small spiders and beetles) but Common Shrews are scarce (no earthworms).
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Competition... |
Despite there being no direct competition between the two, there are points to be noted. Where both Common and Pigmy Shrews are present, skull measurements have shown that the pigmy shrews are smaller. Where there are no common shrews, the pigmy shrews are bigger, as are the common shrews when the pigmy shrews are absent. Does this mean there is a small amount of indirect competition?
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Life cycle... |
Sorex and Neomys shrews usually only live for about 18 months. Deposition of iron in the tooth enamel gives their teeth a red tip. After 18 months this wears away and the shrew cannot feed any more so dies. In a breeding season, females will have up to three litters with 6-8 offspring per litter. Both male and female shrews are very promiscuous - a litter of 8 young will have an average of 3.3 fathers!
White-toothed shrews
White-toothed shrews don't occur as far north as Sorex, they are more Afrotropical in their habitats living mostly in southern Europe and Spain. White toothed shrews are very social animals - they aren't territorial, when the weather is cold, they huddle together in nests to reduce 'heating energy' expenditure, which Sorex don't do. As they live in warmer climates, the white-toothed shrews have lower metabolic rates than the red-toothed shrews, but cannot tolerate cold climates.
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Crocidura russula = Greater White-toothed Shrew |
The Greater White-toothed Shrew is larger than the lesser, but apart from that they are very similar looking both with grey/brown hair and long bristles on the tail. The Greater White-toothed Shrew weighs 4.5-14g. Found on Guernsey, Herm, Alderney
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Crocidura suaveolens = Lesser White-toothed Shrew |
The Lesser White-toothed Shrew weighs 3-7g. found on Jersey, Sark, Scilly Isles
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Neomys fodiens = Water shrew |
The Water Shrew has a white belly and ears, black dorsal fur and has a long fringe of silver hair on its feet and underside. It has a large tail for swimming and the average adult weighs 12-18g. Found thoughout Great Britain.
A Water Shrew
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Activity... |
Shrews have a strange cyclic pattern of behaviour over the 24hr day. They have short alternating periods of sleep and activity, sleep and activity. During the active periods they go out to feed-it is important that they feed regularly. Shrews need about 2/3rds of their body weight in food a day due to their exaggeratedly high metabolic rate-it is about 300% that of other mammals of a similar size.
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Dehnel's Phenomena... |
During the winter, the Common Shrew's body shrinks, especially the brain and liver. In late winter/spring there is a sharp increase in weight as the shrew becomes sexually mature. This isn't as prominent in Pigmy Shrews as they are smaller animals.
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Distribution... |
There are Pigmy Shrews in Ireland but no Common Shrews. The Pigmy Shrew occurs as far north in Britain and Europe as does the Common Shrew.
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The Water Shrew |
The Water Shrew is the largest but rarest of all the shrews. It eats anything that it can overcome like earthworms, beetles and spiders, however as it is also an aquatic mammal it has extras in its diet that common and pigmy shrews can't eat such as crustaceans, shrimps and aquatic larvae.
Why is the Water Shrew the rarest?...
Being the largest shrew it has the biggest energy demands. Good feeding grounds and habitat for the Water Shrew are few and far between and the smaller shrews demand less in the form of energy so can thrive more successfully.
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References |
Genoud 1988. Mammal Review 18: 173 (comparison of physiological ecology of Sorex and Crocidura).
Stockley et al. 1993. Proc. Roy. Soc. Lond. 254: 173-179 (promiscuity in S. araneus)
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