Seasonal Update (September 2024)

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Autumn, the season of mists and mellow fruitfulness as Keats described it, marks the start of a change in the countryside as plants begin shutting down and animals are busy feeding up ahead of winter. In recent years, however, climate change has resulted in a seasonal weakening in Britain, with with more summer-type and less winter-type patterns during autumn. - Credit: Marc Baldwin

August was another broadly uninspiring summer month, with the remnants of ex-hurricane “Ernesto” bringing rain and strong winds to much of Ireland, Wales, northern England, and Scotland during the penultimate week. Still, we saw a last push of summer to end August and start meteorological autumn, with temperatures widely into the mid- or high 20s Celsius courtesy of warm, humid air straying across from the near continent. With the start of autumn comes the start of the pannage, or Common of Mast, season here in the New Forest, where 400 or so pigs are turned out onto Crown Lands to eat acorns. You can read more about this ancient Forest Right, in my Pigging Out in the Forest article.

Website news

The badger Speed Read has been revised and updated, and I'm working through reviewing the rest of the article, including filling a couple of gaps such as a description of habitat. Another section of the water deer article has also gone live recently, this one exploring the subject of play behaviour.

News and discoveries

Their use of complex bubble nets to trap prey may make humpback whales the largest tool-using mammals on earth. - Credit: Doug Kaye (CC BY-NC-ND 2.0)

Tooled-up? Researchers have known for a while that some whale species create a “bubble net” that they use to trap prey, making it easier to feed. In a paper to Royal Society Open Research last month, however, scientists at the Marine Mammal Research Program at UH Hawai'i Institute of Marine Biology and the Alaska Whale Foundation argue that the way these mammals manipulate the bubbles to maximize effectiveness should be considered a form of tool use. Watching humpback whales (Megaptera novaeangliae) feeding on krill off Alaska, the researchers observed how the cetaceans skilfully blew bubbles in patterns that formed nets with internal rings, precisely controlling the number of rings, size of bubbles, depth, and so forth. In their paper, the team calculate that this allows the whales capture up to seven times more prey in a single feeding dive than without the bubble nets.

Social media misfortune. We all know social media has terrific potential for good and positivity, but there's an equally dark side that many must deal with every day. When it comes to nature, social media is similarly a blessing and a curse; it can be used to reach a vast audience when support, preservation or education is needed, but new research suggests it can also have significant detrimental effects on wildlife. As a hobbyist wildlife photographer, I always try to ensure that my attempts to get a picture has no impact (and certainly no lasting impact) on the subject. A new study published in Science of The Total Environment suggests we're now seeing increased direct and indirect disturbance of wildlife, particularly by photographers, resulting in increased predation and disruptions to both breeding and feeding. The metanalysis indicates increased use of drones, baiting and use of call playback to try and facilitate the “wow!” shots that garner kudos on social media platforms.

Red spread? Despite all the conservation issues red squirrels (Sciurus vulgaris) face in Britain, there has recently been some cause for hope after a rare sighting of an animal in a garden in Scotland's central belt. Last month, squirrel conservation charity Saving Scotland's Red Squirrels (SSRS) announced that Lanette Perry photographed a squirrel in her garden in Bishopbriggs, East Dunbartonshire, near Glasgow. Speaking to local press, SSRS said: “This is one of the most southerly red squirrel sightings in Scotland's Central Belt in recent decades. We cannot say exactly why this pioneering red has turned up in this location.” It's thought grey squirrel culling and pine marten expansion through Scotland's Central Lowlands might be helping reds recolonize areas further south.

Seasonal highlight – Common shrews (Sorex araneus)

This month's feature takes a brief look at the natural history of an animal which is seldom seen, despite being widely distributed and abundant throughout mainland Britain. Indeed, it's only really during the late summer and early autumn that people start noticing this diminutive mammal, sadly usually as corpses by the side of paths.

Despite looking like rodents, shrews are high octane relatives of hedgehogs. - Credit: Hanna Knutsson (CC BY-NC-ND 2.0 DEED)

Keeping it in the family

At first glance, you could be forgiven for thinking that shrews are closely related to other small, furry, scurrying mammals, such as mice and voles, and are therefore rodents - i.e., part of the Rodentia order. Early naturalists, however, spotted that (among other anatomical discrepancies) shrews lack the continuously growing incisor teeth that are characteristic of rodents. It was nonetheless apparent that these small mammals fed largely on insects, making them insectivorous (insect-eaters). So, based on their dietary preference, shrews were placed within a large taxonomic group called the Insectivora. Traditionally, the Insectivora had been something of a dumping ground for smallish insect-eating mammals that were rather unspecialised in their appearance; as well as the shrews, it included the hedgehogs, moles, tenrecs (sometimes referred to as “fake hedgehogs”), and colugos (gliding mammals found in Southeast Asia, also called flying lemurs). All-in-all, this group contained some 350 species classified into 60 genera. Use of the Insectivora as a taxonomic order has been largely abandoned in recent years as more rigorous study, combined with the advent of molecular genetics, has yielded phylogenetic arrangements quite different from those proposed by early naturalists. The first attempt to split up these mammals into groups that more closely reflected their relationships was made in 1864 and even now, 150 years later, we are still not entirely sure how all the members of this group fit together.

Currently, scientists place the shrews in a large taxonomic group called the Laurasiatheria (pronounced: Lor-asia-theer-a), which contains a huge variety of placental mammals from bats and hedgehogs to giraffes and lions. Indeed, the Laurasiatheria includes six different orders: the one we're interested in is the Eulipotyphla (pronounced: You-lip-o-tie-fla), which contains the hedgehogs, moles and our shrews. So, contrary to how it might appear at first glance, shrews are not closely related to mice or voles, instead sharing an ancestor with hedgehogs and, more distantly, moles. The shrews are grouped together in their own family, the Soricidae, and worldwide there are some 386 species grouped into 26 genera. Across Europe there are 15 species of shrew, three of which are found here in mainland Britain (although not Ireland and with patchy distributions in Scotland): the common shrew (Sorex araneus), the feature of this article; the pygmy shrew (Sorex minutes); and the water shrew (Neomys fodiens). A fourth species, Millet's shrew (Sorex coronatus), is found on the island of Jersey but nowhere else in Britain, and is almost indistinguishable from the common shrew. The scientific (Latin) name for the common shrew translates roughly to the “spider shrew”—Sorex is Latin for “shrew”, and araneus is Greek for “spider”—because both shrews and spiders were

What? Where? When?

Common shrews are small mammals, growing to a body length of 5 to 9 cm (2-3.5 in.) and with a 3 to 6 cm (1-2.5 in.) long tail. Adult shrews weigh between 6 and 12 grams (0.2-0.4 oz.)—about the weight of a pound coin—and are covered in short, dense chestnut brown or brown-grey fur; during autumn, they moult into a thicker, longer, denser winter coat. In young shrews, the tail is covered with short, bristly fur, but this is lost during adulthood (8 months or older). Shrews have very short stubby ears, small round eyes and a relatively long, thin snout covered in highly sensitive vibrissae (whiskers). Common shrews are part of a group known as red-toothed shrews, owing to the presence of reddish-brown tips to their teeth caused by deposition of an iron-based pigment in the tip of the enamel (where the most wear is generated) of all the teeth. The iron increases the acidic resistance of the enamel and thus serves to slow down the rate of wear, creating a sharp cutting edge where the pigmented enamel rubs against unpigmented enamel. This hardening of teeth is critically important to the shrew because, as we shall see, they live life on something of an energetic knife edge, being in constant search of food. Shrews shed their milk teeth while still in the uterus and so are born with a full set of permanent teeth. If those teeth start wearing down and breaking then the average mastication particle size increases (i.e., they end up having to swallow bigger lumps of food) and this leads to less efficient digestion, because smaller particles are more quickly broken down than large ones. The result is that shrews with worn teeth are less able to extract energy from their food and more prone to starvation than those with a full set of sharp teeth.

Common shrews are part of a group known as red-toothed shrews, owing to the presence of reddish-brown tips to their teeth caused by deposition of an iron-based pigment in the tip of the enamel (where the most wear is generated) of the teeth. - Credit: Hanna Knutsson (CC BY-NC-ND 2.0 DEED)

Shrews are widespread throughout England and Wales, with a patchier distribution in Scotland where they are more abundant in coastal regions. They're found in a variety of habitats, including thick grassland, deciduous woodland, coastal dune systems, wetlands, hedgerows, bushy scrub, bracken, heathland and even among mountain scree, being particularly abundant in grassy peripheral habitats such as road verges. They also range over surprisingly large areas, with tagged individuals having been observed to move over between 100 sq. metres and one sq. km (between 120 and 1,200 sq. yards), although most home ranges seem to fall between 37 and 67 sq. metres (44-80 sq. yards). The total British population is estimated to be about 41.7 million individuals, with 26 million of those in England, 11.5 million in Scotland, and 4.2 million in Wales.

A shrew's home range is largely exclusive, and they will generally not tolerate the presence of another adult in their vicinity. Direct observations in captivity suggest, however, that shrews don't protect a specific territory, but instead defend a small area around their current location. The shrew's energy demands mean that it cannot afford to waste time patrolling borders looking for potential interlopers - looking for food is the number one concern. If a shrew encounters another adult, a fight will erupt, involving much squeaking, biting and scratching, often with each rearing up on its hind legs to drive the other back. Such fights are intense and violent and may result in the death of one, or in some instances both, combatants.

Unlike some other insectivores, shews don't hibernate, but they do appear to shrink during the winter! - Credit: Hanna Knutsson (CC BY-NC-ND 2.0 DEED)

A fascinating study published in the Biology Bulletin during 2013 by Tumas'yan and Shchipanov, biologists at the Russian Academy of Science, reported that any given population of common shrews includes some animals that distance themselves from their neighbours, and others that live on ranges that overlap widely with their neighbours; this ratio may change in different years. The researchers found that the shrews employed scent marks around their core areas, the bits of the plots they visited most frequently, which served to passively avoid confrontation and reduce overcrowding. The study also identified two 'types' of animals: those sensitive to the scent-marks left by others, and those that largely ignored them (“reactive” and “indifferent” animals, respectively). Interestingly, whether a shrew was indifferent or reactive affected how it used an area, with reactive individuals frequently nipping outside the boundaries of their foraging area, while indifferents stayed within their familiar borders. This implies some shrews may study the social milieu by making repeated, long-distance trips into the territories of other shrews. Indeed, other studies have demonstrated that scent plays an important role in shrew society and shrews have specific glands that secrete a strong-smelling discharge, which makes them distasteful to many predators (explaining why they are often abandoned after capture). The flank (side) glands are the most highly developed, but the secretion is also discharged by glands on the neck, under the tail, around the lips, behind the ears, and on the soles of paws. Studies in captivity have reported that when shrews scratch their sides, the secretion from the side gland inevitably stains the paws and other shrews can find and even trace the track taken by a single passing shrew.

As the species is notoriously difficult to observe in the wild, most of what we know about the activity patterns and territoriality of the common shrew has been gained from observations in captivity. One such study, conducted by Peter Crowcroft and published in the Proceedings of the Zoological Society of London during 1953, looked at the activity of common, pygmy and water shrews in a purpose-built enclosure. Crowcroft found that common and pygmy shrews were significantly more active than water shrews; he calculated what's called the coefficient of activity by dividing the number of hours of activity by the number of hours of rest (so the bigger the number, the more active the animal) and arrived at values of 0.70, 1.36 and 1.46 for water, pygmy and common shrews, respectively. So, common shrews were the most active of them all, being active for twice as long as the water shrews. Crowcroft observed that common shrews tended to be active for periods of three or four hours continuously, although this varied by individual. He also observed that they never slept for more than two hours in one go, bouts typically lasting between 12 and 36 minutes; several bouts lasted only a few minutes. Crowcroft's common shrews showed activity throughout the 24-hour cycle, with peaks in activity during early morning, mid-morning, and again at about 10 pm; there was a relative lull in activity during the afternoon (from around noon until dark). One very interesting observation was that, while moving about, a shrew would frequently stop, close its eyes and draw its feet underneath them and rock from side-to-side, apparently dozing, for a few minutes. This “fitful dozing”, as Crowcroft referred to it, was different from the true sleep, which always occurred in the nest, with the head tucked under the body. So, this suggests that even during a shrew's active period it may not be active all the time. Other studies have noted shrews to be active for as many as 10 bouts per day, with particular peaks at dusk, during the night, and at dawn, and that much of this activity happens below ground.

Live fast, die young

Shrews are almost exclusively invertivorous (i.e., they feed on invertebrates), with most prey falling in the size range of 6 to 10 mm. One study on the diet of shrews in Slovenia found that common shrews in the south fed on 15 taxa (groups) of animals, while those in the north fed on half as many, suggesting that the breadth of the diet varies with habitat. The study also found that the most frequent prey taken in both locations were Araneae (spiders, particularly lace-webs of the genus Amaurobius), Lumbricidae (earthworms), and Coleoptera (beetles). Diplopoda (millipedes), which are abundant in both locations, were avoided. Similar studies elsewhere have found that common shrews occasionally take molluscan prey (i.e., slugs and snails), but they're generally not favoured, while certain species of woodlice are apparently relished. Common shrews take more subterranean prey (particularly earthworms) than our other shrew species, and typically attack prey head-first to immobilise it.

Shrews have such a high metabolism that they must feed almost continuously. - Credit: Hanna Knutsson (CC BY-NC-ND 2.0 DEED)

Shrews have an exceptionally high metabolism, meaning that they're almost constantly on the search for food. During normal operations, they must eat 80-90% of their body weight per day, although this varies with ambient temperature and sex, with colder conditions resulting in higher a metabolism and lactation increasing a female's metabolism to 150%. One study estimated that a typical shrew needs to eat 4 g (about 1/7th oz.) of beetle per day, which equates to about 1,800 small staphylinid beetles. By contrast, they appear very efficient at digesting insect pupae and cocoons, requiring only about 2 g per day. This high metabolic rate is coupled with very low energy reserves (i.e., shrews carry very little fat), meaning that they are highly prone to starvation when deprived of food for even a few hours, explaining why they feed every 1.5 to 3 hours. Indeed, a study published in 1994 calculated that the maximum time a shrew could survive without food was eight hours. Given that invertebrate prey is often difficult to find and/or get at during the winter months, it may be surprising that shrews do not hibernate. Instead, they've evolved an amazing mechanism for reducing their energy demands during the winter: they shrink!

Across Europe, common shrews have brain and body masses between 9% and 28% lower in winter than they have during the summer months, a peculiarity known as Dehnel's phenomenon. Shrews sequester minerals from their spinal column, causing a flattening of the vertebral discs and an overall shortening of the spine, and there is some resorption of the parietal and occipitointerparietal bones at the edge of sutures in skull, resulting in a shrinking of the skull. These skeletal changes are also often associated with a decrease in the size of many of the internal organs, as well as less fat and water being retained. A study by Jan Taylor at University of Białystok and colleagues found that this reduction in body size resulted in an overall decrease in basal metabolic rate of 18%. Normally, a reduction in size for a mammal would result in a higher metabolism because their surface area to volume ratio increases, but this is more than offset by the shrew's dense winter coat. Taylor and her co-workers found that heat loss dropped by about one-fifth (19%) thanks to the insulation of their winter fur. So, this allows the shrew to reduce its body size and thus its nutritional requirement to help it through leaner times.

Dead shrews everywhere: a shrewicide pact?

Shrews are not long-lived mammals. Their size makes them a target for many predators, while their metabolism makes them prone to starvation. Typically, a common shrew could expect to live for 15 to 18 months, with half of shrews dying before they reach two months old. Indeed, only about 20 to 30% of shrews survive to breed in their first spring. James Carey and Debra Judge, in their 2000 monograph Longevity Records, gave the record age for a wild common shrew as two years old, while Richard Weigl, in his 2005 Longevity of Mammals in Captivity, gives three years and two months as the oldest captive individual, held at Helsinki Zoo in the mid-1980s. So, does this short life span explain why we suddenly start finding dead shrews littering the countryside during late summer and early autumn? Well, nobody is entirely sure, but scientists believe this is probably part of the story.

During late August and early September, dead shrews start turning up in the countryside. - Credit: Marc Baldwin

The shrew breeding season runs from around mid-April until mid-August, so at this time of year we've reached the end of the frenetic mating season, and this has taken a huge toll on an animal that is already living on an energetic knife-edge. Consequently, many shrews simply die from exhaustion, while others starve to death if they have devoted too much time searching for females or failed to secure a home range. Indeed, as early as 1935 it was demonstrated that almost all adult common shrews die off during the late summer and early autumn after breeding, and that the overwintering populations is, therefore, almost entirely made up of young born that year. Of course, not all shrews die of overexertion - many are predated. Despite having a potent scent gland that releases a secretion most predators (particularly domestic cats) find distasteful, this doesn't do them much good because most of their mammalian predators hunt by sound rather than sight and so don't know what they've caught until they've pounced; only then do they find out it's not a mouse or vole and often discard it. Foxes commonly take shrews, often caching them for later use (they prefer mice and voles), while badgers, stoats and weasels take them occasionally. A significant source of predation comes in the form of birds of prey, with barn owls, tawny owls and kestrels frequently taking and consuming shrews. Some reptiles also take shrews with Chris Reading and Gabriela Jofre's study on smooth snakes in Dorset between 2004 and 2012 finding 32% of snake faecal samples contained shrew remains.

So, some of the dead shrews you find will just have died of exhaustion or starvation, while others will have been discarded or accidentally dropped by predators. (It is worth pointing out that it is not always easy to see wounds on shrews, and a close inspection in the hand is often required.) There are, however, some more interesting suggestions for these sudden deaths. In his 1950 book, Wild Animals in Britain, Oliver Pike found the idea that shrews starve to death or die of old age an unlikely explanation for the ubiquitous bodies to be found at this time of year. Pike's reasoning was that, in his experience, sick animals go off and find a secluded spot to die, rather than dying out in the open as the shrews appear to have. Pike, instead, mentioned a suggestion made by nature photographer Douglas English who observed how three shrews died in their beds, one during a thunderstorm and two when “the atmosphere was in an unsettled thundery state”. Pike wrote:

I believe that a sudden clap of thunder can kill certain creatures. I have seen a coal tit fall dead from a tree directly after a gun was fired underneath, but pointing in the opposite direction to the bird. Many of these small animals are far more sensitive to sounds than we humans, so it is quite possible that a sudden loud noise close at hand would have a fatal effect.

Being dropped or discarded by a predator may explain some of the shrew carcasses found at this time of year, but most have no obvious puncture marks, even on close inspection. - Credit: Steven Mcgrath (Glesgastef)

As intriguing as Pike's theory is, it seems unlikely that stochastic events such as thunderstorms could account for the consistent appearance of dead shrews during these few weeks, even if they're more common during the late summer and early autumn. I'm of the opinion that the evidence points firmly to the exhaustion/starvation hypothesis, but perhaps thunder takes its toll too!

For a round-up of Britain's seasonal wildlife highlights for early autumn, check out my Wildlife Watching - September blog.

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