Scent marking and resource defence by male coypus (Myocastor coypus)

The alternative ideas that scent marking functions mainly in intrasexual competition or for attracting or stimulating mates were tested using data from a 16-year study of coypus, Myocastor coypus . Male coypus have a large anal gland, specialized for marking. Despite year-round breeding, the anal gland shows regular seasonal variation with marked enlargement in OctobeDecember. The analysis supported the hypothesis that this seasonal increase in glandular activity was linked to intrasexual competition: the size of the autumn peak was correlated with the increase in male numbers from the previous season and with the frequency of fighting scars. There were no relationships between gland size and any measures of the availability of mates. Increased scent marking may have been part of the response by resource-holding males to male recruits as these competed for existing territories. The timing of the autumn peak in marking and fighting may have been a response to the accumulation of non-resource-holding males over the summer (when recruitment exceeded mortality in the population) and to increasing accessibility within previously dense wetland habitats; together, these factors may cause a switch from scramble to despotic competition and an increase in scent marking to help keep the costs of resource defence within economic limits.