Dancing drives evolution of sexual size dimorphism in manakins

Body size mediates life history, physiology, and inter- and intra-specific
interactions. Within species, sexes frequently differ in size, reflecting
divergent selective pressures and/or constraints. Both sexual selection
and differences in environmentally-mediated reproductive constraints can
drive sexual size dimorphism, but empirically testing causes of dimorphism
is challenging. Manakins (Pipridae), a family of Neotropical birds
comprising ~50 species, exhibit both male- and female-biased size
dimorphism and are distributed across gradients of precipitation and
elevation. Males perform courtship displays ranging from simple hops to
complex aerobatic manoeuvres. We tested associations between sexual size
dimorphism and (a) agility and (b) environment, analysing morphological,
behavioural, and environmental data for 22 manakin species in a
phylogenetic framework. Sexual dimorphism in mass was most strongly
related to agility, with males being lighter than females in species
performing more aerial display behaviours. However, wing and tarsus length
dimorphism were more strongly associated with environmental variables,
suggesting that different sources of selection act on different aspects of
body size. These results highlight both the strength of sexual selection
in shaping morphology—even atypical patterns of dimorphism—while
demonstrating the importance of constraints and ecological consequences of
body size evolution.