In communal dens of garter snakes, Thamnophis sirtalis, in Manitoba, males obtain copulations forcibly, by inducing hypoxic stress in females. Females emerging from their 8-month winter inactivity are weak and slow, but recover strength and speed within a day or two of emergence. Thus, males may benefit by targeting newly emerged females that are too weak to resist, and by courting females under conditions that maximize the male's physical advantage. Fieldwork at a Manitoba den supported these predictions. Both in the field and in outdoor enclosures, males focused courtship on to newly emerged (weak) rather than dispersing (postrecovery) females. Experimental trials showed that males based this choice on female skin lipids (pheromones) that predict time since emergence, rather than correlates of recent emergence that reflect female physiological state per se (low body temperature, low resistance to hypoxia, high breathing rates during courtship). Male superiority in locomotor speed was maximal at high temperatures, and males targeted courtship to hotter females. The postemergence recovery of females in this system substantially shifts the balance of power in physical encounters between males and females, and provides a fitness benefit to male ability to recognize and exploit more vulnerable females. Male ability to discern and exploit differentially vulnerable females is likely to be widespread, but less overt, in many other populations.