Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the births at a time optimal for the survival of the young in terms of factors such as ambient temperature, food and water availability, and even changes in the predation behaviors of other species. Related sexual interest and behaviors are expressed and accepted only during this period. Female seasonal breeders will have one or more estrus cycles only when she is "in season" or fertile and receptive to mating. At other times of the year, they will be anestrus. Unlike reproductive cyclicity, seasonality is described in both males and females. Male seasonal breeders may exhibit changes in testosterone levels, testes weight, and fertility depending on the time of year.