Abstract:
Since the energy available to an animal for cell growth, thermoregulation, reproduction and other physiological functions is highly dependent on seasonal environmental changes many small mammals breed seasonally during times when environmental conditions are most favourable for growth and survival of the young. In the tropics and sub-tropics, seasonal rainfall appears to be the main reason for seasonal breeding. In order to maximize fitness, it is important for an animal to be able to anticipate these seasonal changes and to trigger reproductive events at the correct time. Photoperiod is used to time reproduction in many temperate mammalian species, but is also used by several seasonally breeding sub-tropical and tropical mammals. The neuroendocrine system is crucial in relaying these environmental signals to the reproductive system. A number of recent studies have suggested that kisspeptin may play a major part in the regulation of the hypothalamo-pituitary-gonadal axis and ultimately reproduction. The exact role of kisspeptin signalling in seasonal breeders is, however, unclear.
The present study investigated the seasonality of reproduction and the reproductive photoresponsiveness of two southern hemisphere species, the spiny mouse (Acomys spinosissimus) and the Eastern rock elephant-shrew (Elephantulus myurus) from South Africa. Furthermore, it compared hypothalamic kisspeptin expression in males and females of both species between the breeding and non-breeding seasons to ascertain if kisspeptin had any potential role in seasonal breeders under natural conditions.
Both A. spinosissimus and E. myurus breed seasonally in South Africa. The breeding season extended through the spring and summer months, whereas the gonads were regressed and steroid hormone levels of both sexes were low during the autumn and winter months of the southern hemisphere. Testes mass and volume of A. spinosissimus were smaller and plasma testosterone concentrations lower under short-day than long-day photoperiods which implies that male spiny mice are reproductively photoresponsive. In contrast, male E. myurus did not appear to be responsive to changing photoperiods and testes size and seminiferous tubule diameter did not differ between photoperiods. It may be possible that other environmental factors such as changes in food availability and/or social factors influence seasonal reproduction in E. myurus.
In E. myurus and male A. spinosissimus, kisspeptin-immunoreactivity was significantly lower during the non-breeding than the breeding seasons suggesting that kisspeptin may be important for the regulation of the reproductive axis to seasonal environmental changes. In contrast, kisspeptin-immunoreactivity did not differ between the breeding and non-breeding seasons in female A. spinosissimus, but was higher in pregnant than in non-pregnant females implying a differential regulation of the reproductive system of males and females of this species by kisspeptin.
In conclusion, seasonal environmental changes trigger similar reproductive responses in these two phylogenetically very distinct species. However, A. spinosissimus may be energetically more restricted than E. myurus because of its small body size and judging from the lack of responsiveness to photoperiod, E. myurus may follow a more opportunistic breeding strategy. In both species, kisspeptin seems to be important in the regulation of the reproductive system to environmental changes.
