Biology Reference
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(
Fig. 9.5
A;
Carey et al., 2003
). These bouts of torpor are interspersed with
brief returns to normal body temperature and metabolic rate, called inter-
bout arousals (IBAs) that typically last less than 1 day. The reason for these
energy costly returns to normothermia is not known, but the IBAs occur
regularly throughout the hibernation season, suggesting another specific
timing mechanism is in place to coordinate them.
Hibernation of small mammals is more feasible to study in a lab environ-
ment than migration, so more is known about the molecular changes under-
lying the physiology of this phenotype. Several examples are described below.
4.1. Satiety and feeding behavior
Obligate hibernation is circannually timed, with a period of preparation pre-
ceding the onset. During this preparation, mammals are hyperphagic
(
Barnes & Mrosovsky, 1974
) and body weight increases dramatically due
to the accumulation of white adipose stores (
Jameson&Mead, 1964
). Roche
454 sequencing of the white adipose tissue transcriptome in thirteen-lined
ground squirrels revealed several genes in various tissues important in this
preparation period. These include Acyl CoA desaturase (
ACOD
) and adipo-
cyte fatty acid binding protein 4 (
FABP4
), which are involved in lipogenesis
and are highly expressed in August when the squirrels are increasing their fuel
stores in preparation for hibernation (
Hampton et al., 2011
). In October,
there is an increase in leptin (
LEP
) in white adipose tissue, which encodes
a satiety signal received by receptors in the hypothalamus (
Fig. 9.5
B).
In the hypothalamic transcriptome of the same species, orexigenic
hypocretin (
HCRT
), agouti-related protein (
AGRP
), and neuropeptide
Y(
NPY
) are highly expressed during April when the ground squirrels have
emerged fromhibernation and are starting to increase their food consumption
(
Fig. 9.5
B;
Schwartz et al., 2013
). Anorexigenic cocaine- and amphetamine-
regulated protein prepropeptide (
CARTPT
) is lowduringApril, but is highly
expressed during October, when food consumption is declining (
Fig. 9.5
B).
HCRT
remains elevated during October, while
AGRP
and
NPY
levels drop
at this time (
Fig. 9.5
B). This preparatory period is extremely important for
survival during hibernation, because the accumulated white adipose stores
are used as the primary fuel source for the entire hibernation season.
Food availability is a very strong entrainment stimulus during the active
season in brown bears (
Ursus arctos
), a hibernating species that is metabolically
suppressed, but only exhibits decreases in body temperature to 32-34
C, and
thus is not considered a deep hibernator (
Ware, Nelson, Robbins, & Jansen,
