Shrews, often mistaken for rodents due to their small size and similar appearance, are actually insectivores with a voracious appetite. These tiny mammals play a crucial role in their ecosystems, primarily by controlling insect populations. This article delves into the dietary habits of shrews, focusing on the Common Shrew ( Sorex araneus) and also exploring the diet of shrews in the Greater Boston area, particularly in Billerica, Massachusetts.
What is a Shrew?
Shrews belong to the taxonomic order Insectivora and the family Soricidae, which encompasses over 385 species. They are found on five continents in a variety of habitats. Superficially, many shrew species look similar, with most having pointy snouts, a streamlined body and a grayish coloration. The Common Shrew, also known as the Eurasian shrew, is the most common shrew throughout Northern Europe, including Great Britain, but excluding Ireland. They are small mammals, typically 55 to 82 millimeters (2.2 to 3.2 inches) long and weighing 5 to 12 grams (0.2 to 0.4 oz). They have velvety dark brown fur with a pale underside. Juvenile shrews have lighter fur until their first moult. The common shrew has small eyes, a pointed, mobile snout and red-tipped teeth.
General Dietary Habits
Shrews are known for their high metabolism and must eat frequently to survive. Some species need to consume 200% to 300% of their body weight in food each day to meet their energy requirements. Without food for more than a few hours, they can starve. Shrews generally rely on smell and hearing to perceive their environment, find prey and escape threats due to their poorly-developed sense of vision.
What do shrews eat?
Shrews are primarily carnivorous. The majority of what they consume is hardly vegetarian, with their diet mostly made up of insects and other invertebrates. Food habit studies have revealed that shrews eat beetles, grasshoppers, butterfly and moth larvae, ichneumonid wasps, crickets, spiders, snails, earthworms, slugs, centipedes, and millipedes. Shrews also eat small birds, mice, small snakes, and even other shrews when the opportunity presents itself. Seeds, roots, and other vegetable matter are also eaten by some species of shrews.
Feeding Behavior and Hunting Strategies
Shrews are in constant motion, rarely stopping to sleep. They have been recorded making 12 body movements per second. Shrews explore and sniff the area with their snout and whiskers while giving out a characteristic high-pitched sound when looking for food on the ground.
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Reliance on Whiskers
Shrews have poor eyesight and often live in thick cover filled with obstacles. Instead, shrews rely on their long, highly sensitive whiskers, also known as vibrissae. According to a study published in Philosophical Transactions B, the Etruscan shrew hunts in an environment where crickets are particularly abundant. It moves its whiskers constantly - a motion called, appropriately enough, whisking - until it brushes its prey. “Experiments with dummy prey objects showed that shrews attacked a plastic replica of a cricket but not other plastic objects of similar size.
Venom
Many shrew species are venomous. Research has found that an individual shrew stores enough venom to kill 200 mice. The shrew lacks hollow fangs (as in venomous snakes) but instead has a gland that allows saliva to flow with the venom. For the prey, this is the beginning of a very bad day. The venom paralyzes the creature, but keeps it very much alive. The shrew can then move it to a cache, available for whenever hunting is not going so great. Shrew bites on humans are reportedly painful but fade in a few days.
Diet of Shrews in Billerica, Massachusetts
Shrews are found in almost all parts of the Greater Boston area during all seasons. In Billerica, Massachusetts, these animals can be seen all throughout the year, although their populations and food sources can vary depending on the weather.
Seasonal Diet Changes
The winter months in Billerica bring with them a drastically different food supply for shrews. While insects are virtually nowhere to be found, small mammals and their nesting grounds can serve as a much-needed meal for these tiny animals. As a result, mice and voles can be a substantial part of a shrew’s diet during the cold winter months.
Opportunistic Feeding
Shrews eat small, frequent meals that are neither rich nor abundant. This means that the shrew’s diet can be quite varied and spread out. In Billerica, this can range from worms, grubs, small rodents, and slugs to dropping beetles, grasshoppers, and fruits.
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Impact of Insecticides and Fertilizers
Insecticides used around the home and garden can drastically reduce the insect populations in the area. While this may help reduce the presence of certain bugs, it can also lead to a decrease in the number of available food sources for shrews. Organic fertilizers can be a great food source for animals like shrews, as they are often filled with essential nutrients that shrews are looking for and can easily obtain.
Availability of Mice and Voles
Available year-round, mice and voles can provide a readily accessible food source for shrews both in the residential neighborhoods of Billerica as well as in the more wild sections of the town. Small mammals like these can be found in walls, yards, and even within tree stumps or gardens.
Adaptations for Winter Survival
Common shrews have evolved adaptations to survive through the winter. Their skulls shrink by nearly 20% and their brains get smaller by as much as 30%. Their other organs also lose mass and their spines get shorter. As a result, total body mass drops by about 18%. When spring returns, they grow until they reach roughly their original size. Scientists believe that low temperatures trigger their bodies to break down bones and tissues and absorb them. As temperatures start to rise with the onset of spring, their bodies start to rebuild the lost bones and tissues. Shrews eat much less food in winter, and are capable of shrinking their bodies in order to survive. While shrews aren’t known to hibernate, physically shrinking their bodies in the winter allows them to survive with less food during these colder months.
Importance of Shrews in the Ecosystem
Shrews are incredibly important to the environment, helping to keep the insect population in check. Understanding what shrews eat is key in determining successful wildlife management techniques. As such, it is important to create wildlife management techniques that account for the food needs of these tiny mammals.
Predation
Shrews may be fierce predators, but they’re small, which means they in turn become prey. Many mammalian predators, including red foxes, raccoons and cats, will attack them but rarely actually eat them. This does not deter other predators, like owls and snakes. But my favorite incidence of shrew predation is a rainbow trout caught at Alaska’s Togiak National Wildlife Refuge that had 19 shrews in its stomach. These were not water shrews, but other species that fell into the water and became prey.
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Additional Facts About Shrews
- The Common shrew is a solitary creature. This animal will fiercely defend its home range against outsiders.
- Although generally nocturnal, the Common shrew can be active during both day and night. It usually has alternating periods of rest and activity during the 24-hour day. Each period of activity lasts for 1 - 2 hours on average, after which the shrew will rest, generally in its nest. However, this animal can take a nap wherever it find suitable.
- Being a very small mammal, it cannot store enough winter fat and hence, doesn't undergo hibernation. The Common shrew is known to create system of runways in the vegetation.
- The Common shrews are polyandrous animals, which means that one female mates with multiple males. They usually mate from April to August. Females may yield up to 4 litters per year, although they generally produce 1 - 2. About 6 - 7 babies are born after 19 - 21 days of gestation. As a result of their reproductive system, young from one litter may have 2 - 3 different fathers.
- Newborn shrews start venturing from the nest at 16 days old. During this period, they are known to follow their mother, forming a 'caravan'. This often happens, when their nest is disturbed. When moving around in a caravan, one of the babies holds its mother's tail while each baby grabs the tail of the one in front of it. Weaning occurs after 26 - 30 days old.
- According to IUCN, the Common shrew is common and widespread throughout its range but no overall population estimate is available. According to the People’s Trust for Endangered Species recourse, the UK population of this species is 41,700,000 individuals.
- The temperature of their body is directly related to their feeding frequency. Shrews are known for their good appetites.