Field signs
Being so small the water shrew leaves behind very few signs. Small plastic tubes, baited with casters (blowfly pupae), can be placed close to the water’s edge to obtain water shrew droppings. These droppings look greyish in colour as they contain fragments of aquatic invertebrate exoskeletons. Droppings of pygmy shrews appear brown as they contain mainly spiders, whereas those of common shrews are black from beetle remains. The contents can be checked under magnification.
Burrow entrances are two centimetres in diameter and have a round cross-section.
Habits
The water shrew is primarily found in wetland habitats: unpolluted rivers and streams, ponds, lakes, ditches, reedbeds and marshes. In Kent it often inhabits watercress beds. It can travel up to three kilometres from water, particularly when dispersing, and has been found in gardens, woodland, hedgerows and grassland. It lives in extensive burrow systems in the river bank, building nests from moss, dried leaves, grass and sometimes stones. It is found at a lower density than common and pygmy shrews but also has a smaller home range. Unless searching for a mate, it is usually solitary.
The water shrew can dive to depths of 30 to 200 cm, staying submerged on average for three to 10 seconds but can be as long as 24 seconds. After a swim it dries off by shaking, scratching or squeezing through narrow burrows. The main prey items are aquatic invertebrates such as water slaters, freshwater shrimps, insect larvae, snails, water beetles and bugs. It also takes terrestrial invertebrates such as earthworms, beetles and millipedes. Our only venomous mammal in Britain, the water shrew uses neurotoxins in its saliva to paralyse larger prey such as small fish. Its high metabolic rate means that it needs to feed every few hours to consume 50% of its body weight in prey per day. It is therefore active both day and night, but mostly after dark.
Predators include owls, kestrel, buzzard, pike, stoat, weasel, fox, cat, and occasionally mink and otter. The shrew’s scent glands exude a deterrent musty odour which often causes it to be abandoned after being killed.
Reproduction and life cycle
Water shrews breed from April to September. Females rear one to two litters of up to 15 young per year, with a 19 to 21 day gestation period. The young are born blind and naked, developing fur by 10 days of age. Whilst independent at 28 days they often stay with the mother for 40 days. Juveniles usually overwinter, becoming sexually mature the following spring. Their lifespan is 14 to 19 months, with most adults dying at the end of the breeding season.
Distribution, status and conservation
The water shrew is widely distributed across Kent, mostly associated with water courses. It is probably under-recorded in the county as it lives at a lower density than the common and pygmy shrew and so is less frequently caught during trapping surveys. Also traps are much more effective when set very close to or within (on objects protruding from the water) rivers or streams, which is not often the case on general small mammal surveys. In addition, it is a small species which leaves few obvious field signs.
The main threat is habitat change, particularly from water pollution and modification of river banks, over-grazing or over-management of bankside vegetation and loss of wetland habitat. The water shrew can quickly accumulate pesticides from the prey it ingests.
The water shrew is partially protected under the Wildlife and Countryside Act 1981 (as amended) regarding trapping, killing and using invasive procedures.
