To assess the possibility of thermal effects on reproductive function as a range-limiting mechanism in cottonmouths, we investigated the effects of temperature/photoperiod regimes on monthly male testosterone levels in three groups. The first was a field group, the second was a lab control group that received natural temperature/photoperiod conditions, and the third was an experimental lab group that received temperature/photoperiod conditions of a region north of the current range limit. The field and lab control groups exhibited a single testosterone peak in August (34.4 and 14.1 ng ml⁻¹, respectively) that coincided with observed reproductive activities in Northwestern Arkansas. The experimental group also exhibited a single peak (11.7 ng ml⁻¹), but the peak was delayed by one month. We coupled energetic calculations with environmental temperature, the timing of reproductive events, and the time available for foraging to demonstrate how pre- and postcopulation reproductive failure may serve as a sublethal range-limiting mechanism. North of their current range limit, the reduced time for energy acquisition may not allow for sufficient juvenile recruitment into the population to replace losses. Therefore, cottonmouths may be prevented from establishing a more northern range limit due to their reproductive physiology.