Why does a rat's fur stand on end? - briefly
When a rat senses danger, sympathetic nervous activation releases norepinephrine, which contracts the arrector pili muscles and lifts the hair, creating a larger visual profile. The same mechanism can also trap air for extra insulation in cold conditions.
Why does a rat's fur stand on end? - in detail
Rats raise their hair when the sympathetic branch of the autonomic nervous system activates arrector pili muscles attached to each follicle. Norepinephrine released from sympathetic nerve endings binds to α‑1 adrenergic receptors on these smooth‑muscle fibers, causing rapid contraction. The contraction pulls the follicle upright, producing a visible puff of fur.
The response serves two adaptive functions. First, it creates a larger silhouette that can deter predators or rivals by making the animal appear more intimidating. Second, the erected hairs trap a layer of air, providing brief thermal insulation during sudden drops in ambient temperature. Both effects are advantageous in situations that trigger a fight‑or‑flight reaction.
Typical stimuli that elicit this reaction include:
- Sudden exposure to a predator or unfamiliar, threatening object
- Rapid temperature decline or direct contact with a cold surface
- Acute stress from handling, restraint, or painful stimuli
- Administration of sympathomimetic drugs (e.g., phenylephrine, epinephrine)
Physiological cascade
- Sensory receptors detect the threat and send afferent signals to the hypothalamus.
- The hypothalamus activates the medullary sympathetic centers.
- Preganglionic fibers release acetylcholine onto the spinal sympathetic ganglia.
- Post‑ganglionic fibers discharge norepinephrine at the hair follicle.
- Norepinephrine binds α‑1 receptors, inducing smooth‑muscle contraction and hair erection.
The phenomenon, termed piloerection, is conserved across mammals. In rats, the effect is especially pronounced because their fur is dense and the arrector pili muscles are highly responsive. The rapid, visible change provides researchers with a reliable behavioral indicator of acute stress or fear in experimental settings.