Sunday, 21 September 2014

Shrews in my roof

18th August 2014

Today I took the opportunity to do some desk work out of doors, sitting on the terrace by the front door and proof-reading a book for a friend. Leaves were brushing the back of my neck from time to time as the wind stirred the vine there. It is a feature of the Bungalow garden: climbing up to the eaves and muffling the front of the house with a bright green jungle of stalks and questing tendrils.

The roof comes down to head height beside the front door. Twenty years ago we had a nest of hornets there. They were no bother - we saw them coming and going: they saw us doing the same; one of them was always on guard by the door to greet returning nest-mates and keep a weather eye open on the world; low-frequency murmurings heard at night, like Spitfires warming up on a distant aerodrome. I heard something different up there last week: a conversational twittering, scratchy and sporadic, like the sound of bats  - or pixies. I couldn't quite locate the source: was it coming from a chimney stack or a hole in the tiles? I had a vision of furry faces swivelling about in the semi-darkness of the roofspace, discussing something important.

My reading was disturbed by rustling among the leaf litter under the vine. Nothing to be seen. A few minutes later the rustling came from higher up among the thicket of leaves. A quick movement, and there - silhouetted by sunlight crossing a branch in the heart of the leafage - unmistakeably a shrew, climbing about, hunting and exploring; perhaps nervously taking its first foray out of the nest, its nose twitching about in frenetic hyper-mobility. The identity of my nattering neighbours became clear.

The locals call a shrew a 'ranny'. I'd like to think that the Latin name for a Common Shrew, Sorex araneus, owed something to the East Anglian dialect, or some obscure Viking root of it revived by Linnaeus who named them. Its name has another dimension for me: odour. It reminds me of the word 'rancid', and shrews have a reputation for smelling bad.

Shrews in my roof.... The twittering is apparently a 'low amplitude, broadband, multiharmonic and frequency modulated' sound used as a form of echolocation [1]. Coupled with a rosette of stiff but sensitive whiskers around their snout and a good sense of smell, that must be how they they find their way around in the roof. Their eyesight is said to be poor. Judging by size, I think mine are most likely to be the Common Shrew rather than the Pygmy Shrew.

I have always thought of shrews as being members of the Order Insectivora, along with hedgehogs. However recent genetic studies have shown they are part of an Order Soricomorpha (''shrew-forms') which they share with moles and a rare family from the West Indies called the solenodons. The latter are living fossils, with relatives dating back to Cretaceous times, over 65 million years ago, and teeth which can deliver a poisonous bite like a snake. Other shrews, including the Common Shrew, are said to have a poisonous bite, but this is due to a toxin present in the saliva [2, 3, 4]'It is a ravening beast, feigning itself gentle and tame, but being touched it biteth deep, and poysoneth deadly', says Topsell [5]. I have not yet tested the truth of this.

There is something primaeval about thise animal. How old is the species Sorex araneus? Scientists have traced shrew origins back to the Heterosoricidae, a family living in North America in Middle Eocene times, about 45 million years ago; behind them must lie some ranny-like Cretaceous mammal, as yet unknown. Later, the Soricinae family evolved, to which my Sorex shrews belong, and the earliest fossils of the genus are found in Germany, in the Middle Miocene, c.11 million years ago [6]. The earliest likely British record for S.araneus is from Hoxne, Suffolk, dated 400,000 BP [7]. The site is no more than two miles away from my house as the crow flies, and only yesterday in geological time. The species itself is not quite as ancient as I had imagined.

Strange to tell, shrews are one of the most studied animals on the planet. S.araneus is one of the few species whose genome has been completely mapped, and as a result we are able to carry out detailed studies of its subspecies and their distribution in time and space. Some surprising results have emerged. 30 years ago we thought it was just one species, but it now turns out to be five cryptic species, each with distinctive genetic signatures which directly contradict their apparent similarities in size, skeleton and bodily appearance [8]. Externally the look the same, but internally S.araneus is 'characterised by spectacular chromosomal variation' [8], and shows 'one of the most spectacular chromosomal evolutions ever recorded in mammals' [10]; it has evolved an extraordinary wealth of chromosomal races (karyotypes).

Since 1987 S.araneus has been the subject of a specialist research group called the International Sorex Araneus Cytogenetics Committee. Through the work of Jeremy Searle and others, it has been possible to divide the British population into six racial types and plot their geographies; from this we can work out the story of how the Common Shrew recolonised Britain after the last Ice Age, having spent it sheltering in a refugium area on the Continent, most likely in central Europe [8]. Some of the genetic diversity within the species seems to have adaptive value, as slightly different karyotypes are found in different habitats, for example shrews in bogs are different from those in dry grasslands [11]. Apparently shrews are a rapidly evolving species, and this helps adapt them to changing ecological niches and ultimately leads to the evolution of subspecies and - on a much longer time scale - species. I have no idea which race my shrews belong to, but quite possibly they are of the Oxford race discovered by Searle which is typical of Eastern England [12]. It now seems likely the fossil S.araneus from Hoxne is one of the cryptic subspecies. David Polly's research holds out the hope that we may eventually be able to tell them apart by the details of their molar teeth [8]. There is a lot more to shrews than meets the eye.

Tonight I saw one climb straight up a brick wall and disappear under the eaves. They are evidently bold and enterprising animals. They are also fevered and vulnerable beings, who live with death as a constant companion. Shrews have an incredibly fast metabolism, and when frightened their heart rate can shoot up to over 1200 beats per minute [13]; their digestion lives on a two-hour turnaround, so if they go without food for more than a few hours they will starve and quickly die [14]. I imagine they suffer greatly, passing their short and passionate lives in a ferment of hunger and competition for food and living space. I have often heard them shrieking at each other, hidden deep in the undergrowth of some grassy forest - 'a rapid succession of shrill cries, which pierce the ears like needles of sound'  [15]. Gladiatorial screams of defiance, triumph and submission. If I wanted to tame one I would have to bring it offerings of woodlice, beetles and spiders, and promise to keep it in solitude. 'They love the rotten flesh of ravens', says Topsell, but I think I'd have to draw the line there.

From Topsell, 1658.



1 -
2 - Buczacki, S. 2002: Fauna Britannica; Hamlyn.
3 - Poisonous Shrews, at: Shrew Culture, Myths, Stories and Poisonous Facts; The Shrew (ist's) Site aka 'The Shrew Shrine' - online at: [accessed Aug 2014].
4 - The poison is a paralysing organic peptide called Soricidin - see Wikipedia  [accessed Aug 2014].
5 - Topsell, E. 1658: History of Four-footed Beasts and Serpents; London; p.406. Online at: [accessed Sept 2014]. 
6 - Rzebik-Kowalska, B. 1998. Fossil history of shrews in Europe. In: J. Wojcik, J. & Wolsan, M. (eds.): Evolution of shrewsMammal Research Institute, Polish Academy of Sciences, Białowieza, Poland. 
7 - Schreve, D.C. 2000: The Vertebrate Assemblage from Hoxne, Suffolk. In: Lewis, S.G. et al (eds) 2000: The Quaternary of Norfolk and Suffolk Field Guide; Quaternary Research Association.
8 - Polly, D.P. 2003 - Paleophylogeography of Sorex araneus (Insectivora, Soricidae): molar shape as a morphological marker for fossil shrews; Mammalia 68.2.
9 - Borodin, P.M. et al 2008: Recombination Map of the Common Shrew, Sorex araneus (Eulipotyphla, Mammalia); Genetics 178.2 - [accessed August 2014]
10 -Taberlet, P. et al 1994: Chromosomal versus mitochondrial DNA evolution: tracking the evolutionary history of the southwestern European populations of the Sorex araneus group (Mammalia, Insectivora); Evolution, 48.3
11 - Wojcik, J.M. 1991: Chromosomal polymorphism in the common shrew Sorex araneus and its adaptive significance; Mémoires de la Société Vaudoise des Sciences Naturelles, no.19.
12 - Searle, J.B. & Wilkinson, P.J. 1987: Karyotypic variation in the common shrew (Sorex araneus) in Britain – a 'Celtic Fringe'; Heredity, no.59.
13 - Crowcroft, P. 1963: Shrews; Animals of Britain Series  no.17, Sunday Times Publications, London.
14 - Churchfield, S. 1988: Shrews of the British Isles; Shire Natural History, Princes Risborough.
15 - Wood, J.G. 1865: The Illustrated Natural History. Mammalia; George Routledge & Sons, London; p.434.
16 - Topsell, E. 1607: The History of Four-footed Beasts