Behavioural repertoire and the onset of precocious sexual behaviours in juvenile ruffs (Calidris pugnax)

Adult phenotypes are shaped by developmental processes during early life. This also applies to reproduction and dominance-related behaviours before maturation, called precocious sexual behaviours. However, beyond largely anecdotal reports, the onset of such behaviours is rarely studied. Here we document the development of precocious sexual behaviours in the ruff Calidris pugnax, a lekking shorebird with pronounced sexual dimorphism and three genetically determined morphs that correspond to alternative reproductive tactics. To document how behaviours develop and diversify, we compiled a comprehensive ethogram in ruff chicks and examined age-related changes in behaviours. Using 6-min video recordings of interactions with a stuffed dummy chick, we analysed occurrence and duration of behaviours in 24 hand-raised chicks (3-15 days post-hatching, every other day). We documented 34 distinct behaviours belonging to five categories: ‘maintenance’, ‘movement’, ‘stationary’, ‘social non-sexual’, and ‘social precocious sexual behaviour’. Behavioural expression in chicks changed markedly with age, largely through shifts in the occurrence of a behaviour. Notably, ‘precocious sexual behaviours’ such as ‘circling’, ‘mounting’, and ‘strutting’, display and mating behaviours typical to adult ruffs, were already expressed from day 5 onwards, increasing in occurrence with age. This suggests that ruff chicks start to express motor coordination of these behaviours long before sexual maturity. The ethogram will function as framework for future analyses, for example to quantify sex, morph, and individual variation, and the developmental origins of complex social behaviours.