Abstract
Social monogamy predominates among birds in general and waterfowl (Anatidae) in particular. The Muscovy duck (Cairina moschata) is one of few waterfowl species thought to be non-monogamous, but its mating system has not been described previously. I studied wild Muscovy ducks in the northern Pantanal of Brazil in order to characterize mating patterns and spacing systems during the breeding season. Behavioral observations of known individuals (n = 33 in 2000, n = 40 in 2001) showed that the same individuals did not consistently comprise male-female pairs. The duration of male-female association periods was highly variable, and simultaneous association with multiple partners, as well as switching between partners, was observed. A benefit to females of male attendance, in terms of increased feeding time or efficiency, could not be conclusively demonstrated. There was extensive overlap in spatial use between and within sexes, and territoriality did not occur. The weak, short-term nature of male-female relationships and the occurrence of multiple mating indicate that the social mating system of Muscovy ducks is best classified as promiscuous. I propose that the benefits to females of pair formation are reduced in Muscovies compared to north temperate waterfowl, and that resource unpredictability and an extended breeding season make it uneconomical for males to monopolize mates or resources. The association of females with multiple males suggests a role for post-copulatory sexual selection, which I investigated in a captive population of wild-type Muscovies. I developed DNA microsatellites to facilitate paternity assignment and conducted sperm competition trials to assess the degree of last male sperm precedence. I found that sperm are lost from the female reproductive tract at an exponential rate. The passive rate of sperm loss is species-specific and has not been measured previously in Muscovy ducks. There was not a clear last male advantage in fertilization success, which is usually reported to be the outcome of sperm competition in birds. Instead, I found a bimodal pattern of sperm precedence, which highlights the need for attention to variation within and between individuals in terms of their sperm competitive ability.