Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 215

Recent evidence suggests that a number of other neural and nonneural tissues
exhibit circadian rhythms in gene clock expression, hormone secretion, and electri-
cal activity in mammals ( Guilding and Piggins, 2007 ). These brain and peripheral
circadian clocks are self-sustained and may be reset by restricted feeding ( Challet
et al., 2003 ). However, changes in the peripheral clocks result from signals generated
in response to light in the SCN, rather than any direct effect of light.
That the SCN serves as pacemaker and synchronizer implies the transmission of
relevant information to other brain and peripheral clocks. Indeed, SCN neurons pro-
ject to other brain clock regions and via circulating hormones, and the autonomic
nervous system controls the circadian rhythms of physiology in all the organs includ-
ing the pineal gland and the function of the adrenal cortex ( Dibner et al., 2010 ). This
brain center serves as “a master pacemaker for the organism to drive rhythms in
activity and rest, feeding, body temperature, and hormones” ( Mohawk et al., 2012 ).
In birds, the timing system appears more complicated than that of mammals. This
is important especially for migratory birds, such as arctic birds that need to adjust
their timing system quickly to accommodate changing conditions during migration
( Gwinner and Brandstatter, 2001 ).
Most multicellular organisms live more than a year to experience another dis-
turbing geophysical phenomenon, the circannual cycle, which is accompanied by
changes in temperature, photoperiod, sources of food, humidity, and predation that
are all more drastic than those associated with the circadian cycle.
So living longer came not for free. To survive and thrive, plants and animals
evolved a “long-term” circannual temporal structure, or calendar, alongside the cir-
cadian timetable. This new temporal structuring device would allow them to antici-
pate and be prepared for extreme conditions in the environment, by developing
adaptive seasonal changes in physiology, behavior, morphology, and life cycle.
All plants and animals that outlive the year have evolved a circannual photoperi-
odic calendar, which lets them program the phenotypic changes necessary to adapt to
seasonal environmental changes.
Despite the wealth of information on the functions of this circannual clock, little
is known of its mechanics compared to the circadian clock. Day length is the envi-
ronmental cue that multicellular organisms use to set up their photoperiodic calendar.
There is sufficient reason to believe that the structures responsible for establishing
the circannual calendar are localized in the brain ( Figure 4.10 ).
What is certain is that the input and output pathways of the photoperiodic cal-
endar end in the brain and begin from the brain. This makes it obvious that the cir-
cannual calendar structure is located in the brain. The input pathway starts with the
perception of the photoperiodic cue (i.e., the length of the day and night in retinal
neurons) and transmission of this photic information to the SCN, or with direct brain
photoreception in some insects. The output pathway for photoperiodic changes in
animal physiology, behavior, and life history starts from brain neurosecretory cells
that send instructions for adaptive phenotypic changes to effectors.
The photoperiodic calendar's crucial role is revealed in the course of the individ-
ual development of diapausing insects and some other arthropods. Depending on the
photoperiod and related temperature, at particular stages of their development, these