Research
Resource competition drives an invasion‐replacement event among shrew species on an island {Photo: ©Alan Poole}
Species Pygmy shrew
Facts about the pygmy shrew
- English/Irish name: Pygmy shrew/Dallóg fhraoigh
- Scientific name: Sorex minutus
- Number of young: Between 1-9 (but mostly 4-6) with 2-3 litters between April and October.
- Diet: Beetles, spiders, bugs, woodlice and insect larvae.
- Habitat: Habitats rich in ground cover. Found in hedgerows, grasslands, and woodlands and is particularly abundant in peatlands.
- Photo: ©Alan Poole
Identification
Females:
Length 4-6cm; Tail length 3-4cm; Weight 6gMales:
Length 4-6cm; Tail length 3-4cm; Weight 6g- Extremely small size
- Long hairy tail
- Red-tipped teeth
Photo: ©Alan Poole
History of the pygmy shrew in Ireland
The pygmy shrew is Ireland’s smallest mammal and, up until 2007, was the only shrew species found on the island. Fossil and molecular evidence suggest that the pygmy shrew likely arrived in Ireland from Britain, accidentally introduced by early human settlers. Due to its non-burrowing nature, it may have been transported to islands in hay stooks during the movement of livestock.
Before the accidental introduction of the greater white-toothed shrew, the pygmy shrew was believed to be widespread across the entire island of Ireland.
Photo: ©Alan Poole
Distribution and status
Once considered common and widespread across Ireland, the pygmy shrew is now disappearing from areas where the invasive greater white-toothed shrew has become established.
Beyond Ireland, the pygmy shrew is the most widely distributed shrew species in Europe and also one of the most ancient, with a fossil record dating back to the early Pliocene (5.3–3.6 million years ago). It is absent from southern Iberia and much of the Mediterranean region but has a range extending from western Iberia and Ireland to Siberia.
The pygmy shrew is found throughout the British Isles and is the only shrew species present on the Isle of Man, Orkney, and the Outer Hebrides. Much of its distribution in these regions is believed to be the result of accidental transportation by early human settlers.
The pygmy shrew can be easily distinguished from other small mammals in Ireland by its extremely small size, weighing no more than 6 grams. In contrast, the recently introduced greater white-toothed shrew is approximately three times larger.
The pygmy shrew has distinctive, red-tipped teeth due to iron deposits, which help resist wear, a feature absent in the greater white-toothed shrew, as its name suggests. Adult pygmy shrews have thick, bi-coloured fur with a brownish hue on the back that lightens along the flanks and underbelly. They also have a relatively thick, hairy tail in proportion to their body size.
It is found in a wide variety of habitats in Ireland, particularly those rich in ground cover like hedgerows, grasslands, woodlands and peatlands.
Pygmy shrews do not burrow themselves but will use the burrows of other animals.
The pygmy shrew mainly feeds on small invertebrates, including beetles, spiders, bugs, and woodlice. Most of its prey items measure less than 10mm long, as larger invertebrates are often too difficult to overpower. In captivity, pygmy shrews consume up to 1.25 times their body weight daily. This extreme food intake is necessary due to their small size and high metabolic rate. A period of just two hours without food can lead to starvation.
To meet these intense energy demands, pygmy shrews hunt continuously, both day and night, with only brief periods of rest. They move rapidly across the ground, using their snout to probe through vegetation while relying on smell and touch to detect prey, as their eyesight is poor. Unlike other shrew species, they do not emit audible twitters while foraging, a primitive form of echolocation. Their poor vision is reflected in their Gaelic name, Dallóg fhraoigh, meaning 'blind animal of the heather'.
The breeding season extends from April to October, with a peak in activity occurring in June and July. While little is known about their specific breeding behaviour, it is believed to be similar to that of the common shrew.
Females are in oestrus for approximately 24 hours at a time, and courtship and mating are brief. Like common shrews, female pygmy shrews are thought to be promiscuous, often mating with multiple males. Genetic analysis of one pregnant female and four embryos captured in County Waterford provided evidence of multiple paternity within a single litter.
Females do not generally breed until their second calendar year. The gestation period lasts approximately 22–25 days, with litters consisting of four to six young. Each female may produce up to three litters per season. Newborn pygmy shrews are exceptionally small, weighing just 0.25g at birth. The lactation period lasts around 22 days, after which the young disperse to establish their own territories.
The introduction of the greater white-toothed shrew to Ireland in 2007 has raised significant concerns for the native pygmy shrew population. Having existed in isolation from its main competitors for at least 5,000 years, the Irish pygmy shrew is now experiencing rapid declines in areas where the invasive species has become established, often disappearing within a year of first encountering the new shrew.
As the greater white-toothed shrew spreads into new regions, it initially preys on larger insects before shifting to smaller invertebrates, the primary food source of the pygmy shrew. Given that pygmy shrews must consume more than their body weight in insects daily to survive, competition for these vital resources makes it increasingly difficult for them to coexist with the invasive species.
This species is protected under the Wildlife Act (1976) and Wildlife (Amendment) Act 2000.
Further reading