Animals living in seasonally changing environments must accurately time yearly events, like migration, moult, hibernation and reproduction in order to survive, and successfully rear their young. Animals resident in highly seasonal environments with short growing seasons are more likely to evolve seasonal phenotypes than conspecifics whose niches provide more evenly distributed food resources throughout the year (Concannon et al 2009). In some cases these events occur in direct response to seasonal changes in the external environment such as alteration in ambient temperature or day length. This adaptive reactive strategy allows the necessary physiological changes to occur rapidly. In many instances physiological adaptations need to begin well in advance of the environmental change. This means there will be for many mammals an internal seasonal adaptation, that is; they occur in the absence of changes in environmental time and are endogenously generated (Thrun et al 1995).
In most vertebrates, neuroendocrine functions are organized in regular cycles of different periodicity, which may range from minutes to months. Cycles of approximately 12 months are known as circannual rhythms, while periods of approximately 24 h are called circadian rhythms. Cycles that occur more frequently than once daily are termed ultradian cycles (Johnsonn 2009).
The seasonal adaptation is controlled by the biological pacemaker, but it can be difficult to distinguish between externally and internally regulated rhythms. However, if the animal is isolated and deprived of all external signals including the daylight variations that can provide time information, and the physiological rhythm then shows a period deviating from the 24 hour cycle it is possible to conclude that the rhythm is internally controlled (Johnsonn 2009; Mustonen2003 ).
Seasonal changes in photoperiod adjust the period duration of circannual cycles, which is typically 9–10 months under constant conditions, to 365 days. Thus, the expression of circannual rhythms involves an interaction between an endogenously generated signal and environmental cues (Duncan 2007 ).
Hormones serve as a mechanism for mediating seasonal changes in physiology. Two photo-dependent hormones are Prolactin and Melatonin. Prolactin is synthesised in the anterior pituitary gland but also in the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviours and it plays a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role (Freeman et al 2000). Melatonin is primarily synthesised in the pineal gland. Prolactin and melatonin have a synergistic effect, the melatonin secretion pattern is an inverse function of day length, and circulating melatonin levels correlate inversely with Prolactin levels. Melatonin secretion occurs during the hours of darkness, so the amount of melatonin biosynthesis is directly related to the length of the daily dark period. The quantity continues to increase in autumn until winter solstice (Mandel 2012).
Most studies on circannual rhythms in mammals is done on humans or wild animals, there are very few of the domestic dog, however studies that have been done, have shown a circannual rhythm which shows seasonal fluctuations in line with wild and human studies ( Kreeger et al 1992; Corrida et a 2003; Gobella et al 2002).
Mink exhibit a seasonal fur growth cycle regulated by photoperiod. As the length of the photoperiod decreases; an increase in melatonin concentration and a decrease in Prolactin results. Which initiate growth of the winter pelage, which begins in September in northern latitudes. In spring, the opposite occurs: the length of the photoperiod increases, resulting in an increase in Prolactin concentration and a decrease in melatonin, initiating the spring moult (Valtonen 1995). Mink treated with implants of melatonin in June, when natural melatonin production is low; are exposed to an artificial melatonin they have suppressed Prolactin production and exhibit an early onset of autumn moult and produce prime winter pelage 6 to 8 weeks earlier than normal (Rose 1984).
Belyaev (1979) demonstrated a destabilizing effect on the loss of strict circannual rhythm through the domestication process. Food generalists like the domesticated pet dog that exploit diverse diets, and generally are kept sheltered from the weather tend to be less subject to seasonal pressures than specialists reliant on a few seasonally variable foods (Concannon et al 2009. But despite that domestic dogs do moult in autumn and develop a winter a coat, and they have been shown to have a circannual rhythm. It seems likely that the increasing endogenous melatonin secretion after the summer solstice triggers the autumn moult of the domestic dog .
Therefore the start of the winter coat in domestic dogs will begin after the summer solstice predicting the oncoming winter, due to the decrease of Prolactin as a result of increased melatonin secretion.
References
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