Pygmy shrew

The tiny pygmy shrew is the smallest land mammal in Britain. Although their vision is poor, their long noses are perfectly designed for sniffing out whichever invertebrates they can find.

Taxonomy chart

Animalia - Chordata - Mammalia - Eulipotyphla - Soricidae - Sorex - Sorex minutus

Conservation status: UK Red List

GB: Least Concern

England: Least Concern

Scotland: Least Concern

Wales: Least Concern

Global: Least Concern

Summary

Diet: Insectivorous. Primarily beetles and woodlice, but also other insects and arachnids – unlike common shrews, they rarely eat earthworms. 

Habitat: Almost anywhere with good ground cover – gardens & urban areas, deciduous & mixed woodland, grassland, peatland, arable land & even mountains! 

Size: Weight: highly variable, between 2.4 and 6.1g; shrews can lose up to 28% of their body mass in winter. Size: 40-60mm with a 32-46mm tail. 

Lifecycle: Maximum lifespan is around 13 months, with a spike in mortality between 2-4 months old. Breeding takes place between April and October, and the young shrews will be ready to breed the following year. 

Conservation concerns: The non-native greater white-toothed shrew, recently detected in England, is thought to be causing declines in Irish pygmy shrew populations. British pygmy shrews are also threatened by habitat loss, pollution and pesticides. Pygmy shrews can be reliably surveyed by trapping under license, monitoring cat predation, or analysing skeletal remains from owl pellets. 

Terminology

Dehnel’s Phenomenon: The ability to shrink and re-grow the whole body, including brain and skeleton, as a response to harsh winter conditions. 

Geography 

Pygmy shrews are widespread in Britain, found across most of the mainland and Ireland in any habitat providing suitable food and cover. They are also established on the Isle of Man, Orkney and the Outer Hebrides. 

They aren't fussy about habitat and will live happily alongside humans in gardens and urban areas, making good use of hedgerows and compost heaps. They can be found in woodland, grassland, peatland, moors, bogs and arable land, and have even been recorded on Ben Nevis! British pygmy shrews seem to be more numerous in grassland than woodland – a preference not shared by their Irish counterparts. 

Biology 

As the name suggests, the pygmy shrew is tiny, with a body length of about 4-6cm and a tail measuring 3-4cm. Although they can weigh up to 6.1g in the summer, the pygmy shrew's clever physiology allows it to lose up to 28% of its body mass in winter. During the colder months, an adult pygmy shrew could weigh as little as 2.4g. Known as Dehnel's Phenomenon, this isn't just fat loss – even the brain and the bones shrink! There are several theories as to the exact reasons behind this evolutionary trait. These include helping shrews to cope with lower temperatures or helping them to use less energy, so they don't need to eat as much.

Like all native British shrews, the pygmy shrew has a distinctive long nose with very small eyes and ears. Their fur is short and two-tone, grey-brown on top and grey or white on the underside. The tail is hairy and about 70% the length of the body, which can be an easy way to tell it apart from the common shrew, whose tail tends to measure 50% of the body length. All native British shrews except the lesser white-toothed shrew also have red-tipped teeth, caused by the deposits of iron in the enamel. It is thought this might strengthen the enamel to prevent wearing. 

Pygmy shrews breed between April and October, with a peak around June and July. The mating is brief, and females may mate with several consecutive males, with the resulting litter containing embryos from multiple fathers. They will have 4-6 young in a litter, each weighing as little as 0.25g, 

and can produce up to 3 litters in one breeding season. By 22 days old, the juveniles are weaned and will leave to establish their own territories, where they will overwinter. They remain solitary until April, when males will disperse to find females. The lifespan of a pygmy shrew is short – up to 13 months, although there is another spike in mortality between the ages of 2-4 months. 

Ecology 

Pygmy shrews are insectivores, and despite their small size, they are voracious predators. Their fast metabolism means they need to feed every few hours just to avoid starvation, and they can eat over 250 prey items in a single day – up to 125% of their body weight! Their preferred invertebrates are beetles and woodlice, but they will eat almost anything that they can physically overpower; many invertebrates are just too big for the tiny pygmy shrew to eat! 

Pygmy shrews are solitary outside of their breeding season and aggressive towards other shrews, maintaining home ranges between 500 to 2000 square metres in an area. In some areas, they seem more abundant at higher altitudes than common shrews, and exhibit more arboreal tendencies, which may be a strategy to avoid direct competition with common shrews. They have been recorded climbing trees and bushes, and even using dormouse nestboxes! However, they also spent plenty of time on the ground, and have been observed using vole surface tunnels, as well as making their own. 

Conservation 

The pygmy shrew is listed as Least Concern on the UK Red List, both globally and throughout its range across Britain and Ireland. Under Schedule 6 of the 1981 Wildlife & Countryside Act, it is illegal to kill or trap shrews without a license from the relevant Statutory Nature Conservation Organisation. This is because their metabolism puts them at a high risk of starvation, and they may die after just 3 or 4 hours without appropriate food. 

Pygmy shrew populations are threatened on a local level by habitat loss, degradation and fragmentation, and use of pesticides. They are naturally predated by owls, weasels, stoats and foxes, and additionally by non-native domestic cats, which tend to kill shrews without eating them. In Ireland, where the pygmy shrew is the only native shrew species, they are threatened to the point of local extinction by competition from the invasive greater white-toothed shrew. The greater white-toothed shrew was first discovered in Britain in 2021, and now the ‘Searching for Shrews’ project by the Mammal Society is closely monitoring its spread and potential impact on the pygmy shrew. 

History

The pygmy shrew is the most ancient species of European shrew, with a fossil record dating back as far as the Pliocene, 5.3 million years ago. It's also the most widespread, ranging across most of Europe, but only found its way into Britain at the end of the last Ice Age. Although all UK pygmy shrews are the same species, their genes show that they colonised the British Isles independently at least twice. From there, with accidental human help, pygmy shrews hitched rides as far as the Outer Hebrides and even Ireland, where they earned the Gaelic name dallóg fraoigh (“blind one of the heather”). It was only in the 19th century that they were recognised as a separate species, having been previously mistaken for young common shrews. 

Historically, shrews have been associated with evil, diseases and bad luck – unsurprisingly to their detriment. Blame for a sick child or lame cow fell on the harmless shrew, and a live shrew would be caught and sealed up inside an ash tree known as the Shrew Ash. This ritual was thought to cure the afflicted, but inevitably killed the shrew. 

Identification and surveying 

Visual: Two-tone, with brown or grey fur on the back and a white or grey belly. Long, pointed snout, with small ears and very small eyes. Smaller than the common shrew: 4-6cm body length with a 3-4cm tail. Tail length is about 70% of the total body length. 

Surveying Period: Pygmy shrews can be surveyed year-round. 

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