Most temperate-zone species use photoperiod to coordinate breeding and ensure that offspring are born during favourable conditions. Although photoperiodic influences on the reproductive axis have been well characterized, the precise mechanisms by which photoperiodic information and other seasonal cues are integrated to regulate reproductive function remain less well specified. Two recently disc...

Seasonal appearance and seasonal disappearance of menstrual function Sir, Garai and co-workers' observation of seasonality in the occurrence of the first missed menstrual bleeding in perime-nopausal women (Garai et al., 2004) indicates that human menstrual function is influenced by seasonally varying environmental factors. As the authors assert, the influence of these seasonal triggers might be...

Seasonal variation in social behavior is often accompanied by seasonal variation in communication. In mammals, how seasonal environmental cues influence aggressive vocalizations remains underexplored. Photoperiod is the primary cue coordinating seasonal responses in most temperate zone animals, including Siberian hamsters (Phodopus sungorus), a species that undergoes reproductive inhibition and...

How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Be...

Seasonal control of reproduction is critical for the perpetuation of species living in temperate zones that display major changes in climatic environment and availability of food resources. In mammals, seasonal cues are mainly provided by the annual change in the 24-h light/dark ratio (i.e., photoperiod), which is translated into the nocturnal production of the pineal hormone melatonin. The ann...

Timing of breeding is a trait with considerable individual variation, often closely linked to fi tness because of seasonal declines in reproduction. Th e drivers of this variation have received much attention, but how reproductive costs may infl uence the timing of subsequent breeding has been largely unexplored. We examined a population of northern wheatears Oenanthe oenanthe to compare three ...

This study characterized the seasonal variation of the steroid hormones testosterone (T), 11-ketotestosterone (11-KT), 17beta-estradiol (E2), and cortisol (F) as they relate to the gonadal development and reproductive behavior of the plainfin midshipman fish, Porichthys notatus. The plainfin midshipman is a deep-water teleost that seasonally migrates into the shallow intertidal zone where type ...

age at first breeding on Ural owl lifetime reproductive success and fitness under cyclic food conditions. – Journal of Animal Ecology 67: 359–369. survival in a variable environment: Ural owls and the three-year vole cycle. – Auk in press. effort and reproductive values in periodic environments. – seasonal recruitment and the evolution of the seasonal decline in Ural owl clutch size. – Manuscri...

Vertebrates displaying seasonal shifts in reproductive behavior provide the opportunity to investigate bidirectional plasticity in sensory function. The midshipman teleost fish exhibits steroid-dependent plasticity in frequency encoding by eighth nerve auditory afferents. In this study, evoked potentials were recorded in vivo from the saccule, the main auditory division of the inner ear of most...

BACKGROUND Many vertebrates, including the goldfish, exhibit seasonal reproductive rhythms, which are a result of interactions between external environmental stimuli and internal endocrine systems in the hypothalamo-pituitary-gonadal axis. While it is long believed that differential expression of neuroendocrine genes contributes to establishing seasonal reproductive rhythms, no systems-level in...