Britain's shrews - living on a knife edge

by Fiona Mathews, Mammal Society Chair and Professor of Environmental Biology at University of Sussex

This article first appeared in British Wildlife magazine in April 2021

A new European Mammal Atlas is currently being assembled. This extraordinarily complex undertaking, collating data on 242 terrestrial species from across the continent, is a brilliant collaborative enterprise led by Tony Mitchell-Jones (who will be known to many readers through his former role as Natural England’s senior mammal specialist). In reviewing the data to be submitted by the UK, I was struck by the brevity of our species list compared with most other countries, particularly those in eastern and southern Europe. While we lay claim to just 62 species (and that includes our two seal species: Halichoerus grypus and the Harbour Seal Phoca vitulina), Ukraine has 117, Italy 118, Bulgaria 98 and North Macedonia 89. The better news is that we have an extraordinary army of skilled volunteers willing to collect, submit and verify records. The Mammal Society’s recent Atlas (Crawley et al. 2020) was based on more than 1.3 million records submitted since 2000. We are so familiar with biological recording, wildlife charities and the expanding environmental consultancy sector, that it is easy to forget that many other European countries — including some of the most biodiverse — have very few people studying wild mammals in either a voluntary or a professional role. We hope that the Atlas will stimulate capacity-building mammal research and monitoring, and we will be supporting initiatives to share expertise across Europe.

Despite the UK’s comparatively large network for mammal recording, there are still many species that we know very little about. Take the red-toothed shrew for example. Whilst the Mammal Atlas could draw on 135,000 records of hedgehogs Erinaceus europeaus, there were only 8,500 common shrew Sorex araneus records, 4,200 pygmy shrew Sorex minutus records and 3,300 water shrew Neomys fodiens records (for the period 2000-16). Our best estimate of population size for the water shrew Neomys fodiens is 700,000, but there could be as few as 240,000 or as many as 1.9 million. The plausible population sizes for our other shrew species are similarly wide. It doesn’t take a statistical  genius to work out that it probably matters where on this range the true number lies. Shrews are vital components of ecosystems. They are also potentially good bioindicators, given their tropic level and voracious appetites. Recent research on common shrews Sorex araneus has revealed that they are almost incapable of storing fat: it is metabolised within 4-5 hours (Keicher et al. 2017). They also have an extraordinary capacity to shrink their body size, which seems to reduce their energy demand during the cold winter months (Shaeffer et al. 2020). This reversible shrinkage, which is known as Dehnel’s Phenomenon, has only been described in a few taxa, including the red-toothed shrews. After growing rapidly as juveniles in the summer, animals can undergo substantial shrinkage in the autumn and over the winter (this is not just loss of muscle mass: the size of the skull can reduce by up to 25%). They then grow again in spring, achieving a final body mass that is higher than that reached as juveniles, though some tissues, including the brain, only partially regrow. It would seem that the reduced energy demand of smaller body size more than compensates for the additional heat that they lose as a consequence of increasing their surface area to volume ratio.

Despite these metabolic gymnastics, shrews are mammals living on a knife-edge. This makes them especially difficult to study using conventional techniques. Live-trapping can often be sufficient to push them over the edge, despite all best efforts to supply suitable food and bedding (and this is the reason why anyone trapping shrews must be in possession of a licence from the relevant Statutory Nature Conservation Body). Therefore, many people undertaking small mammal surveys chose to use traps fitted with shrew-holes which permit the animals to escape, taking important data with them.  While trapping surveys are valuable in generating information on species density, simple presence/absence data will go a long way towards filling our current knowledge gaps. We need to know where each species occurs, as records are very patchy in many regions. Provided that we have high resolution grid references (which are attached to records automatically if you use the MammalMapper app), we can use the information on species presence/absence to work out both the geographical range and the occupancy rates for different habitats.

Several methods are suitable for generating presence/absence data. Shrews will visit bait tubes provisioned with castors or mealworms, and the droppings they leave behind can be identified to species using molecular techniques. Faecal samples collected in lofts and submitted for bat species identification also not infrequently turn out be from pygmy shrews! However, genetic testing is prohibitively expensive for most volunteer surveys. Water shrews Neomys fodiens can be identified on the basis of aquatic prey remains in faecal samples, a technique that was developed by Sarah Churchfield and the Mammal Society in the late 1990s (Churchfield et al. 2000). Clearly, this is only suitable in riparian habitats, rather than in places where water shrews are eating terrestrial prey.  My own experience of surveying of water shrews is that they turn up with reasonable frequency in hedgerows well away from water. Focusing surveys entirely in riparian habitats therefore risks generating biased data.

Given these difficulties, it is always exciting to discover new techniques. Last month’s British Wildlife featured the possibility of using ultrasonic recordings to survey shrews and other small mammals (Newson, Middleton & Pierce, 2021). Recording their lower frequency calls within the audible range also appears promising, with different shrew species producing diagnosic ‘trills’ (Zsebők et al. 2015). While analysing acoustic data has its own rewards, it’s hard to beat camera trapping as an engaging method for surveying mammals. I noticed that early in the COVID-19 lockdown, several popular types of camera traps went out of stock from the major suppliers, perhaps reflecting the new-found interest of finding out about wildlife close to home. While camera trapping has long been used to survey large mammals, it has been used much less commonly for small mammals. This is because the animal needs to pass within sufficient proximity of the camera to trigger a photograph, and the focal lengths of trail-cams are often not compatible with sharp photographs of small animals scurrying past. Nick Littlewood and colleagues have come up with the ingenious two-part solution (Littlewood et al. 2020). First, an inexpensive macro-lens is attached to the front of the trail-cam, and then the camera is attached to a wooden box with bait supplied at the correct focal length for good images. If the one I have in my garden is anything to go by, small mammals feel secure entering the box and the result is hundreds of brilliant images. I’ve still to find a water shrew, but I have easily been able to distinguish between common and pygmy shrews Sorex minutus. So if you’ve got a camera-trap lurking in a cupboard, why not fish it out and give it a go?  

Churchfield S, Barber J, Quinn C. A new survey method for water shrews (Neomys fodiens) using baited tubes. Mammal Review. 2000 Dec;30(3‐4):249-54.

Keicher L, O'Mara MT, Voigt CC, Dechmann DK. 2017 Stable carbon isotopes in breath reveal fast incorporation rates and seasonally variable but rapid fat turnover in the common shrew (Sorex araneus). J. Exp. Biol. 220, 2834-2841. https://doi.org/10.1242/jeb.159947

Lázaro J, Hertel M, Sherwood CC, Muturi M, Dechmann DKN. 2018 Profound seasonal changes in brain size and architecture in the common shrew. Brain Struct. Funct. 223, 2823-2840. https://www/doi.org/10.1007/s00429-018-1666-5

Littlewood NA, Hancock MH, Newey S, Shackelford G, Toney R. Use of a novel camera trapping approach to measure small mammal responses to peatland restoration. European Journal of Wildlife Research. 2021 Feb;67(1):1-10.

Newson SE, Middleton N, Pearce H. 2021. The acoustic identification of small terrestrial mammals in Britain. British Wildlife 32: 186-194.

Shaeffer P, O’Mara MT, Breiholz J, Keicher L, Lazaro J, Muturi M, Dechmann K.   Metabolic rate in common shrews is unaffected by temperature, leading to lower energetic costs through seasonal size reduction. Royal Soc. Open Sci., 7, 191989. https://doi. org/10.1098/rsos.191989

Zsebők S, Czaban D, Farkas J, Siemers BM, von Merten S. 2015. Acoustic species identification of shrews: twittering calls for monitoring. Ecological Informatics. May 1;27:1-10.

Crawley D, Coomber F, Kubasciewicz L, Harrower C, Evans, J, Waggitt J, Smith B, Mathews F. 2020. Atlas of the Mammals of Great Britiain and Northern Ireland. Pelagic Press: Exeter.