Shrews are small, insectivorous mammals often mistaken for mice, but they are not rodents; they belong to the order Eulipotyphla. They are characterized by a long, pointed snout. Shrews are aggressive, but this behavior stems from biological necessity rather than temperament. Their constant need for food drives a relentless pursuit of prey and a highly competitive nature. While aggression is rarely directed at humans, shrews are among the most fiercely territorial and active small mammals.
The Metabolic Engine Driving Shrew Behavior
The root cause of a shrew’s aggressive behavior is its incredibly high metabolic rate. As some of the smallest mammals, shrews lose heat rapidly and must maintain a fast metabolism to stay warm. This high rate of energy expenditure means they must eat almost constantly; a shrew can starve to death in just a few hours without food.
This physiological demand translates directly into a relentless, aggressive hunting style. Shrews often consume an amount of food equal to or exceeding their own body weight daily. This constant need for resources makes them fearless hunters, often attacking prey larger than themselves, such as mice or voles. Aggression is a survival strategy, ensuring they secure enough food to meet their energy requirements.
Distinguishing Intraspecies and Interspecies Aggression
Shrew aggression is categorized as intraspecies (toward their own kind) or interspecies (toward other species, including humans). Intraspecies aggression is particularly intense because shrews are solitary and fiercely territorial. They view other shrews as direct competitors for their limited food supply, which is critical due to their high metabolism.
Fights between shrews are frequent and violent, often resulting in death for the loser. Studies show that an established shrew will likely kill a new shrew introduced into its territory within a few hours. This intense territoriality is driven by resource competition, where larger shrew species are often found to be dominant and more aggressive than smaller species.
In contrast, a shrew’s aggression toward humans is usually defensive. Shrews are naturally bold but prefer to avoid confrontation with large threats. Bites to humans are rare and typically only occur when a shrew is trapped, handled, or attacked by a pet.
The Unique Threat of Venomous Shrews
A small number of shrew species, such as the Northern Short-tailed Shrew (Blarina brevicauda), are venomous, a rare trait among mammals. The venom is produced in modified salivary glands and flows through grooves in the lower incisor teeth when the shrew bites its target. The purpose of this venom is to aid in hunting, not defense against large predators.
The venom contains a paralytic toxin known as Blarina Toxin (BLTX). It is highly effective at subduing smaller animals like earthworms, insects, and small rodents by causing paralysis and loss of muscle movement. This allows the shrew to paralyze and “live-hoard” its victims for later consumption, ensuring a fresh meal.
A bite from a venomous shrew to a human is painful and can cause swelling, but it is generally not life-threatening. The effect is similar to a minor, painful sting. Medical attention is advisable for any animal bite to prevent secondary infection. The venom’s potency is relatively mild for large mammals compared to its effect on the shrew’s small prey.
Handling Encounters and Reducing Risk
The best advice when encountering a shrew is to leave it alone and avoid direct contact. Handling a shrew is risky due to its aggressive nature and the possibility of a venomous bite. Although rare, any bite can potentially transmit diseases or cause infection, so non-confrontation is the safest approach.
Managing a shrew presence near a home involves habitat modification rather than eradication. Since shrews are attracted to areas with ample insects and cover, homeowners can discourage them by trimming overgrown vegetation and keeping the lawn well-mowed. Removing clutter like woodpiles and debris eliminates potential hiding and nesting spots for the shrews.
Sealing small cracks and gaps in the foundation or basement of the home is a practical way to prevent shrews from entering structures. Reducing outdoor food sources, such as by promptly cleaning up pet food and birdseed, also helps make the area less attractive to these voracious hunters. These methods focus on exclusion, allowing the shrews to continue their beneficial role of controlling insects and small rodents elsewhere in the yard.