Sex and morph variation in activity from early ontogeny to maturity in ruffs (Calidris pugnax)

Intraspecific variation provides the substrate for the evolution of
organisms. Ruffs show exceptional phenotypic variation in physiology,
appearance and behaviour linked to variation between sexes and male
alternative reproductive tactics (ARTs). The male ARTs are associated with
the evolution of separate morphs, which are encoded by an autosomal
supergene. However, the effects of the supergene on females and chicks are
much less well understood. In particular, it is still unknown, whether
females also show morph-specific behavioural variation, when behavioural
differences emerge during ontogeny, and whether behavioural differences
can be detected outside of the breeding context. To address these
knowledge gaps, we repeatedly measured the activity in an unfamiliar
environment, also known as exploration behaviour, of 109 hand-raised young
ruffs throughout their first two years of life. We used automated tracking
in an open field arena, and quantified the distance moved within 10
minutes to examine behavioural differences between sexes, morphs and
individuals. After their crouching reflex, which is a response to
potential threats, subsided during the first month of life, the activity
of young ruffs rapidly increased. Repeatability of individual activity was
initially low but increased throughout juvenile ontogeny and was high
(R = 0.5) from day 21 onwards. Variation in activity was clearly
sex-linked with females moving more than males, indicating potential
energetic trade-offs accompanying the strong sexual size dimorphism. In
contrast, morph differences in activity remained inconsistent and elusive,
both in females and in males. Our results indicate that in species in
which much of the known behavioural variation is linked to mating tactics,
a non-reproductive behaviour can show between-individual variation and
clear sex differences, whereas morph differences appear less
pronounced.