Forced Copulation in Waterfowl
Authors: McKinney Frank , Derrickson Scott R. , Mineau Pierre 
Published: April 2008

Abstract
1. Although almost all waterfowl (ducks, geese and swans comprizing the family Anatidae) have basically monogamous mating systems, males of 39 species have been observed to perform or attempt forced copulation (FC) (previously called "rape"). 2. Suggestions that FC is an artifact of crowding in urban mallards, an outlet for thwarted male sex drive, a component of territory defence, or an alternative reproductive strategy of unpaired males are not supported by available evidence. 3. In the case of the mallard, HEINROTH'S (1911) interpretation of FC as a secondary insemination method whereby paired males increase their reproductive success by fertilizing some of the eggs laid by females other than their mate is supported by evidence of various kinds. FCs are performed by paired males; they occur during periods in the breeding season when eggs are being fertilized; they are directed mainly at females in prelaying and laying condition; paired males usually defend their mates against males attempting FC (suggesting defence of genetic paternity); paired males have been seen to copulate forcibly with their own mates after the latter have been subjected to FC (suggesting an antidote insemination strategy). 4. Experiments on captive mallards have shown that eggs can be fertilized by sperm delivered during FC and, since females can store viable sperm for up to 17 days, sperm competition must be taking place. Artificial insemination experiments have shown that (a) the second of 2 competing inseminations 6 hours apart overlays the former insemination and is 70% more potent, (b) there is an insemination "window" within 1 hour of oviposition when the next egg in the clutch is fertilized. The extent to which males time their copulations (FCs, pair copulations, forced pair copulations) to take advantage of these patterns of sperm competition is not known. 5. Studies of the lesser snow goose, northern pintail and lesser scaup indicate that FC is a secondary reproductive strategy of paired males in these species also. Observations on 13 species of dabbling duck provide additional evidence for the selection of fertile females as FC targets and the development of male tactics for achieving and combatting FC. 6. It is difficult to rule out the possibility that females could derive certain benefits from FC inseminations (e.g. genetic diversity of offspring) but the costs and risks of being involved in FC assaults can be high and the females apparently try to avoid FC. Female dabbling ducks can be damaged, or even killed, during assaults and female escape behaviour (high flights, diving, hiding in cover) entails energetic costs and wastage of time that are presumably detrimental to females during the period when they are producing eggs. 7. As HEINROTH suggested, FC probably does not occur in swans, most geese and shelducks because it is incompatible with the major roles that paired males play in defence of breeding territories, mates, nest-sites and broods in these groups. The occurrence of FC in snow and Ross goose may be facilitated by colonial nesting.. 8. Among dabbling ducks, male territoriality is associated with strong mate-guarding and these characteristics appear to conflict with male FC activities. Involvement of paired males in extrapair courtship, polygyny and brood-care may produce additional conflicts with FC behaviour in certain species. 9. In at least 4 species of waterfowl (mallard, northern pintail, lesser snow goose, lesser scaup) behavioural evidence indicates that FC is part of a mixed male reproductive strategy of the type postulated by TRIVERS (1972). Rigorous testing of the hypothesis will require studies to (a) assess the incidence of multiple paternity in wild clutches, (b) distinguish species (or populations within a species) in which paired males actively pursue FC tactics from those in which they merely capitalize on incidental opportunities for FC.