Hedgehogs

Spiny insectivores, and why their numbers are falling.

Hedgehogs are small nocturnal insectivorous mammals of the subfamily Erinaceinae, found natively across Europe, Asia and Africa. They are not related to porcupines, which are rodents — the resemblance is convergent evolution, two lineages arriving at the same solution.

Spines

An adult carries roughly 5,000 to 7,000 spines, which are stiff hollow hairs of keratin. They are not barbed and do not detach, unlike porcupine quills. A specialised circular muscle draws the skin over the body like a drawstring bag, rolling the animal into a ball with head, legs and belly protected.

Hibernation

In cold climates hedgehogs hibernate, body temperature dropping close to ambient and heart rate falling from about 190 beats a minute to around 20. They must reach sufficient body mass first — an underweight animal entering hibernation will not survive it, which is why late-born young are the most vulnerable.

Decline

Numbers have fallen sharply across much of their range. The drivers are habitat fragmentation, particularly impermeable garden fencing that isolates populations; road mortality; loss of invertebrate prey to pesticides; and the removal of hedgerows and log piles that provide nesting sites. Small gaps at the base of fences are a disproportionately effective mitigation.