The Native Pygmy Shrew (Sorex minutus) vs the Invasive Greater White-Toothed Shrew (Crocidura russula), who will win?
Did you know that the native pygmy shrew (Sorex minutus) is Ireland’s smallest mammal? Until recently it was thought to be the only species of shrew found in Ireland. In 2007 the remains of the invasive greater white-toothed shrew (Crocidura russula) was discovered in the pellet leftovers of birds of prey, indicating its presence on the Island of Ireland. It is thought that this invader could have been here since 2001, but due to their small size its’ no wonder how they were missed. Studies show the greater white-toothed shrew could colonise Ireland by 2050 and ultimately lead to the disappearance of the pygmy shrew. This is primarily thought to be as a result of competition of food sources.
The Irish name for pygmy shrew is Dallóg fhraoigh which means ‘blind animal of the heather’. This name was derived due to their poor eyesight, because of this they use a combination of touch and smell when detecting their prey. The pygmy shrew was thought to have been in Ireland a very long time, however recent studies of fossils and molecular data have now shown it may have been brought in from Britain. The pygmy shrew weighs between 3 and 6g while the greater white-toothed shrew can be up to three times bigger, weighing between 8 and 14g.
Pygmy shrews must consume 1.25 times their own body weight every day just to survive. This is because of their tiny size and extremely high metabolic rate. A short time frame of as little as two hours can lead to starvation for these little guys. They have a varied diet including woodlice, spiders, bugs, and beetles. Because of their need for a constant food supply, they hunt throughout the night. Greater white-toothed shrews feed on a variety of invertebrates and are in direct competition with the pygmy shrew.
It is thought that the invasive greater white-toothed shrew (Crocidura russula) currently covers an area of 7,600 km2. From Offaly to Cork, Limerick to Waterford, these tiny invaders are on a mission. At present it is thought that the Shannon River is impeding their spread into the West of Ireland, but experts believe it is only a matter of time…
The greater white-toothed shrew is thought to feed on a wide variety of invertebrates, similar to the native pygmy shrew, and studies have shown that it has a negative relationship on not only the pygmy shrew but also the abundance of wood mouse (Apodemus sylvaticus). In fact, pygmys appear to be absent in areas where the greater white-toothed shrews are found! (Montgomery et al., 2012).
If you see an, unfortunately, dead shrew in your garden, get a look at it’s teeth. The colouration of a shrew’s teeth, as the greater white-toothed shrew’s name suggests, is a tell-tale sign of the species! Due to iron deposits at the tips of the teeth of the native pygmy shrew, it’s teeth look as if they’ve just been dipped in some strawberry jam! The colours red iron-y colour is very vibrant and viable along the tips. This characteristic also helps protect the pygmy shew teeth from wear. The greater white-toothed shrew’s teeth lacks this colouration and are pure white!
Adult pygmy shrews have thick brown fur on their back which gets lighter as it reaches their underbelly. It also has a hairy and thick tail which can reach a length of 5cm. The fur of the greater, white-toothed shrew tends to be a grey/reddish colour with long white hairs on its tail.
The pygmy shrew can be found in a variety of habitats and is most likely found where there is rich ground cover in areas including ditches, grassland, peatlands, and woodlands. Although these shrews will not burrow, they will use existing burrows which other creatures have dug in order to rest (of which they don’t do a lot!). The greater white-toothed shrew can be found in hedgerows, woodlands, cultivated areas, and grasslands.
Pygmy shrews are an important link in the food chain with numerous animals including foxes, owls, pine martens, and maybe even a badger, or two, feeding on them. However, they are declining due to several factors. They are sensitive to changes in their environment including increased use of herbicides and pesticides, the increase in domestic and feral cats, and competition for food with the greater white-toothed shrew, which also have a greater reproductive capacity.
Further research is needed, and it is important to record sightings of both species. This only takes a few minutes and can be done on The National Biodiversity Data Centre website. Follow the link below for a step-by-step guide on how to report your sightings: https://docs.biodiversityireland.ie/biodiversity-maps/reporting
Written by Mary Moroney
Montgomery, W.I., Lundy, M.G. and Reid, N., (2012). ‘Invasional meltdown’: evidence for unexpected consequences and cumulative impacts of multispecies invasions. Biological Invasions, 14(6), pp.1111-1125.