The mallard (Anas platyrhynchos) and its domesticated varieties have been used for well over a century in studies designed to understand physiological defence against submergence asphyxia. The mallard is a dabbling (non-diving) duck yet it is able to survive forcible submergence for at least as long as other aquatic species, including penguins. This paper discusses    both  the  proximate  and  ultimate  causes of this  remarkable  underwater endurance capacity. A brief summary is presented of the cardiovascular and metabolic responses that serve to maximize the time taken for endogenous oxygen stores to be depleted. A mechanism by which these adaptations may have evolved is then proposed. Noting that foraging behaviours are unlikely to explain differences in underwater endurance capacity between species, it is hypothesized that a behavioural characteristic of many Anatidae – forced copulation – may represent the selective pressure for evolution of the physiological adaptations that confer increased underwater endurance capacity. Evidence is presented that this mating strategy may be a significant cause of drowning during the breeding  season.  The relevance of forced immersion of dabbling ducks to our understanding of physiological responses in voluntary dives is discussed.