2.2 Species showing torpor or deep hibernation
Among the birds, torpor occurs in a number of species in the orders Apodiformes (hummingbirds and swifts), Caprimulgiformes (nightjars, nighthawks, goatsuckers and poor wills) and Coliiformes (mousebirds). In all of the hummingbirds (family Trochilidae) studied to date, torpor, if it occurs, takes place on a daily (or more usually nightly) basis. They are able to re-warm themselves independently of T a and show an increased thermogenesis if T a falls below 18° C during the time when the bird is not searching for food. The tiny rufous hummingbird (Selasphorus rufus; Figure 3), weighing only 3.0–5.5 g, has a summer range in North America that extends to Alaska, but it overwinters in Mexico. While undertaking this huge migration it undergoes overnight torpor, especially when breaking its journey for a few days, feeding on nectar to rebuild its energy reserves.
There is no evidence for long periods of unbroken torpor in the hummingbirds. The poor will (Phalaenoptilus nuttalli; Figure 4), which lives in the southern USA, is perhaps the only bird studied so far that shows bouts of torpor at all comparable to those of seasonally hibernating mammals. Poor wills kept in the laboratory at a T a of 1° C without food go into torpor, with a T b of 6° C, from which they arouse spontaneously about once every 4 days, showing an exceptionally large rise in their metabolic rate in the process. The pattern of torpor and change in T b that they undergo in the wild is not known.
Many birds, including doves and pigeons, enter shallow torpor (with the T b falling to about 32° C) when deprived of food. If food is available, the pigeon responds to low T a by a large (up to 55%) increase in basal metabolic rate (BMR). Indeed, a reduction in food supply seems to be a major factor in the induction of torpor in almost all the bird species studied. The white-throated swift (Aeronautes saxatilis) and the mousebird (Colius sp.) can tolerate a T b of −20° C and spontaneously re-warm at low ambient temperatures.
Among mammals, many groups contain species that undergo different degrees of adaptive hypothermia. Of the placental mammals, the largest number of hibernating species is found among rodents (see Table 2 in Section 2.2) and bats. All temperate-zone bats, including the 15 UK species, undergo daily torpor during certain seasons with some species remaining torpid for extended periods, as also does the European hedgehog (Erinaceus europaeus). Another famous example, which was a food valued by the Romans because of its habit of storing fat prior to hibernation, and was the sleepy guest of the Mad Hatter, is the hazel dormouse (Muscardinus avellanarius). Most research on hibernating mammals has focused on bats, hedgehogs, hamsters and in particular, the sciurid rodents (squirrels).
Table 2 The six (out of 30) families of rodents in which adaptive hypothermia is known to occur.
|Zapodidae||meadow jumping mouse, Scandinavian birch mouse||deep hibernation|
|Heteromyidae||pocket mice, kangaroo mice||erratic, seasonal hibernation|
|Gliridaed||dormice (including the native British species)||deep hibernation|
|Muridae||African fat mouse||daily torpor|
|white-footed mice||daily torpor|
|Sciuridae||chipmunks, marmots (woodchuck), ground squirrels (at least a dozen species)||deep hibernation|