December 2025

December is the month when dormancy is key for nature at our latitude. Humans are lucky if we have a heated house to live in and a warm car or public transport to travel in during winter. It must have been very hard for early people living in rock shelters and simple shacks. Temperature regulation is a key survival factor in the animal world. Humans are mammals and we regulate our temperature internally to some extent at about 37℃, i.e. we are endothermic. Plants cannot usually regulate their temperature but rely on physiological chemistry to resist succumbing to the cold. Most British plants become dormant, either by retreating underground into bulbs like daffodils or corms like crocuses. Or else die back to a woody skeleton like deciduous trees. Conifers, like other evergreen trees, resist frost by producing chemicals within their needles or leaves which act as antifreeze. Some moths, e.g. the winter moth (Operophtera brumata) have ethylene glycol in their blood. This is the same chemical that is put into combustion car engines to prevent cooling water from freezing. Indeed, we may see male winter moths flying at night in December, illuminated by car headlights as they search for an aromatic wingless female to mate with. Once mated, the female climbs up the closest oak tree and lays her eggs near the leaf bud on the tree that fed her as a caterpillar. She then dies. Many adult insects die as pray or freezing, leaving frost-resistant eggs dormant until spring. Or, if hatched as larvae during summer, have metamorphosed into pupa to sleep out the winter, as some adults also do in a frost-free nook or cranny before emerging as an adult in spring. A few insects migrate as some birds do. Like us, birds are endothermic, holding their body temperature at about 41℃.

Insects such as fleas spend the winter in the direct company of their endothermic host animal, which could be a bird or mammal. Reptiles such as lizards and snakes or amphibians such as frogs and newts, like invertebrates such as slugs and spiders are exothermic. They are entirely dependent on the external temperature for active life, so they will go into dormancy, i.e. hibernate. Some spiders have adapted to humans and are happy to live in peoples houses all winter such as the domestic house spider (Tegenaria domestica). This is the one that is sometimes seen running across the floor in a warm sitting room on a winter’s evening. Or else there is the cosmopolitan cellar spider (Pholcus phalangioides), sometimes called daddy long legs. These tiny animals with long spindly legs hide in ceiling corners or behind furniture and make long random cobwebs which catch small insects such as mosquitos and house flies.

Few mammals hibernate. Bears (Ursus sp.) do, reducing their body temperature from 37.7℃ to about 31℃. Dormice drop their body temperature to that of their surroundings optimally between 1 and 4 ℃, where the least amount of energy is consumed. Their heart and breathing rate drop by about 90%!