Thus, it may be also affected by the time of birth during the reproductive season.
Īccording to the theory of delayed life history effects, phenotype of an adult individual results from environmental conditions experienced at birth, and later during growth and maturation. Although these pathways are often interrelated, winter traits may be regulated independently. Melatonin, the hormonal signal of day length, enters multiple molecular pathways which control molting, torpor expression or gonadal regression. The diversity of winter phenotypes may result from complexity of physiological and molecular mechanisms underlying photoresponsiveness. These adjustments allow for energy savings and are considered beneficial for winter survival, yet individuals insensitive to changes in day length (nonresponding individuals), or individuals presenting only some of winter traits (partial-responding individuals) also exist in many populations. In winter, small mammals, which are mostly long-day breeders, regress gonads and cease reproduction, decrease body mass ( m b), some molt to white fur, and heterothermic species use torpor. Response to shortening photoperiod consists of morphological, physiological and behavioral adjustments and results in development of winter phenotype. The ability to respond to day length (photoperiodism) allows animals to change phenotype across the annual cycle. This strategy could have evolved in response to living in stochastic environment. Existence of littermates presenting different phenotypes suggests a prudent reproductive strategy of investing into offspring of varied phenotypes, that might be favored depending on environmental conditions.
Our data indicate that duration of postnatal exposure to LP may define propensity to photoresponsiveness, regardless of the litter in which animal was born.
We also found that over 10% of individuals presented late response to short photoperiod. Although one phenotype usually predominated within a litter, littermates were often heterogeneous. Moreover, individuals that did not molt had significantly higher BMR in SP than those which molted to white fur. Litter order or duration of LP acclimation had no effects on torpor use or seasonal body mass changes, but prolonged acclimation to LP inhibited winter molting both in first and third litters. Individuals born in third litters had faster growth rates and were bigger than individuals from first litters, but these differences vanished before transfer to SP. To assess effect of litter order, duration of acclimation to long days, and phenotype on basal cost of living we measured basal metabolic rate (BMR) of hamsters. We predicted that, irrespective of the litter order, individuals exposed to long photoperiod for a short time have less time to gather energy resources and consequently are more prone to developing energy-conserving phenotypes. We experimentally distinguished the effect of litter order (first or third) from the effect of exposure to long photoperiod (LP) before winter (3 months or 5 months) by manipulating the duration of LP acclimation in both litters. We used Siberian hamsters Phodopus sungorus, a model species characterized by high polymorphism of winter phenotype. We hypothesized that late-born individuals are more prone to respond to short photoperiod (SP) than early born ones. In seasonal environments, being born late in the reproductive season affects timing of puberty, body condition, longevity, and fitness. The theory of delayed life history effects assumes that phenotype of adult individual results from environmental conditions experienced at birth and as juvenile.
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