Original article Foraging behaviour of coypus Myocastor coypus: why do coypus consume aquatic plants? M.L. Guichón *, V.B. Benítez, A. Abba, M. Borgnia, M.H. Cassini Grupo de Estudios en Ecología y Etología de Mamíferos, Departamento de Ciencias Básicas, Universidad Nacional de Luján, Rutas 5 y 7, 6700 Luján, Argentina Received 27 March 2003; accepted 20 August 2003 Abstract Foraging behaviour of wild coypu was studied to examine two hypotheses that had been previously proposed to explain the species’ preference for aquatic plants. First, the nutritional benefit hypothesis which states that aquatic plants are more nutritional than terrestrial plants. Second, the behavioural trade-off hypothesis which states that coypus avoid foraging far from the water because of the costs associated with other types of behaviour. In order to test the nutritional benefit hypothesis, we studied the diet composition of coypus in relation to the protein content of the diet and of the plants available in the environment. Fieldwork was conducted seasonally from November 1999 to August 2000 at one study site located in the Province of Buenos Aires, east central Argentina. Behavioural observations showed that coypus remained foraging in the water and microhistological analysis of faeces indicated that their diet was principally composed of hygrophilic monocotyle- dons (Lemna spp. and Eleocharis spp.) throughout the year. We did not find support for the nutritional benefit hypothesis: nutritional quality (based on nitrogen content) of hygrophilic plants was not higher than that of terrestrial plants, and seasonal changes in diet quality did not match either fluctuations in vegetation quality or proportion of hygrophilic plants in the diet. Although not directly tested, the behavioural trade-off hypothesis may explain why coypus prefer to forage in or near the water as a mechanism for reducing predation risk. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Diet selection; Diet quality; Foraging ecology; Nutria 1. Introduction The coypu (Myocastor coypus) is a semi-aquatic herbivo- rous rodent indigenous to southern South America (Gosling and Baker, 1991). This species was introduced into several regions around the world early this century, including North America, Europe, the Middle East, Africa, and Japan (Carter and Leonard, 2002). In some of these regions the coypu is considered a pest because of the damage caused to crops, native flora and fauna, and drainage systems (Kinler et al., 1987; Gosling and Baker, 1989; Reggiani et al., 1993; Ver- heyden and Abbas, 1996). In order to predict the potential impact of an invasive herbivore species like the coypu on crops or native vegeta- tion, it is important to understand the behavioural mecha- nisms underlying diet selection (Gosling, 1981). Previous studies have indicated that foraging is the major activity of coypus when outside their burrows (D’Adamo et al., 2000; Guichón et al., 2003), and that their diet has a strong depen- dence on aquatic and semi-aquatic plants (Shirley et al., 1981; Kinler et al., 1987; Wilsey et al., 1991; Borgnia et al., 2000). This high consumption of hygrophilic vegetation was also evident in areas where terrestrial plants constituted a non-limiting, high quality resource (i.e. crops) located a few meters from the water body (Gosling, 1981; Borgnia et al., 2000; D’Adamo et al., 2000). Gosling (1981) suggested three hypotheses that could explain the feeding pattern of coypus: (1) nutritional benefit: aquatic plants are better sources of nutrients than terrestrial plants; (2) innate preference: coypus have digestive mechanisms to deal with this type of vegeta- tion as one of several adaptations to live in aquatic environ- * Corresponding author. E-mail address: lalifer@vianw.com.ar (M.L. Guichón). Acta Oecologica 24 (2003) 241–246 www.elsevier.com/locate/actoec © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/j.actao.2003.08.001