Biology Reference
In-Depth Information
molecular level, presumably due to the difficulty in studying such a dynamic
phenotype in a laboratory setting. Variants in the circadian gene
Clock
have
been implicated in the seasonal timing of breeding in barn swallows (
Hirundo
rustica
) and are also predicted to be involved in migration (
Caprioli et al.,
2012
). Amicrosatellite approach inEuropeanblackcaps (
Sylvia atricapilla
)found
an association between migratory behavior and a microsatellite polymorphism
within the 3
0
UTR of the
ADCYAP1
gene locus (
Mueller, Pulido, &
Kempenaers, 2011
).
ADCYAP1
encodes pituitary adenylate cyclase activating
polypeptide 1 (PACAP) which stimulates adenylate cyclase and increases the
levels of cyclic adenosinemonophosphate (cAMP). PACAP is involved in con-
veying photoperiod information via the retinohypothalamic tract (RHT),
which directly connects the photosensitive ganglion cells of the retina to the
suprachiasmatic nucleus (SCN) of the hypothalamus (location provided in
Fig. 9.4
)(
Hannibal et al., 1997
). The SCN is considered the master circadian
clock in mammals and birds, and is entrained to the external environment by
light input fromtheRHT.Migrationoccurs on a circannual basis, but it is likely
that the circadian system plays a role in this timing as well. Photoperiod varies
according to the time of year, therefore the light signal received in the SCN
would also vary seasonally.
The SCN also regulates the hormone melatonin, which is produced and
released into the bloodstream by the pineal gland at night. Melatonin acts as
both a clock and a calendar in the body, because melatonin is only present at
night and the duration of melatonin is directly related to the length of the
night, which varies seasonally. PACAP stimulated melatonin synthesis in
pinealocytes (
Simonneaux&Ouichou, 1993
) and alsomodulated the expres-
sionof clockgenes in the pineal (
Nagy&Csernus, 2007
), and thus clearly plays
a role in circadian and/or circannual timing. PACAP could be important for
tracking photoperiod changes over seasons for the coordination of seasonal
activities in migrating birds and other animals with seasonal activity. Addi-
tionally, intracerebroventricular (ICV) injection of PACAP into chicks
(
Gallus gallus
) resulted in changes in energy metabolism, including increased
lipid utilization (
Tachibana, Oikawa, Adachi, Boswell, & Furuse, 2007
),
which indicates that PACAP could also be playing a role in migration prep-
aration, helping to facilitate a fuel switch to lipid metabolism.
4. HIBERNATION
Hibernation, like migration, is another survival adaptation to periods
of harsh conditions and low food availability. Obligate hibernation, the main
focus of this section, is an annual period of heterothermy where mammals
