Development of intraspecific size variation in black coucals, white‐browed coucals and ruffs from hatching to fledging

Lina M Giraldo‐Deck,Clemens Küpper,Terry Burke,Ignas Safari,David B Lank,Deborah A Dawson,Michael Stocks,Wolfgang Goymann

doi:10.1111/jav.02440

Lina M Giraldo‐Deck, Clemens Küpper + Show 6 more

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https://doi.org/10.1111/jav.02440

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Abstract

Most studies on sexual size dimorphism address proximate and functional questions related to adults, but sexual size dimorphism usually develops during ontogeny and developmental trajectories of sexual size dimorphism are poorly understood. We studied three bird species with variation in adult sexual size dimorphism: black coucals (females 69% heavier than males), white‐browed coucals (females 13% heavier than males) and ruffs (males 70% heavier than females). Using a flexible Bayesian generalized additive model framework (GAMM), we examined when and how sexual size dimorphism developed in body mass, tarsus length and bill length from hatching until fledging. In ruffs, we additionally examined the development of intrasexual size variation among three morphs (Independents, Satellites and Faeders), which creates another level of variation in adult size of males and females. We found that 27–100% of the adult inter‐ and intrasexual size variation developed until fledging although none of the species completed growth during the observational period. In general, the larger sex/morph grew more quickly and reached its maximal absolute growth rate later than the smaller sex/morph. However, when the daily increase in body mass was modelled as a proportion, growth patterns were synchronized between and within sexes. Growth broadly followed sigmoidal asymptotic models, however only with the flexible GAMM approach, residual distributions were homogeneous over the entire observation periods. These results provide a platform for future studies to relate variation in growth to selective pressures and proximate mechanisms in these three species, and they highlight the advantage of using a flexible model approach for examining growth variation during ontogeny.

Highlights

Intraspeciic variation in body size is ubiquitous, often manifested as sexual size dimorphism (SSD) and linked to diferent sex roles (Fairbairn 1997)
Sex diferences in body size developed in all three species before ledging (Fig. 1, Table 1) and were more pronounced for body mass (Fig. 1) than for tarsus length (Fig. 2) or bill length (Supplementary material Appendix 1, Fig. A1)
The smaller sex had reached a larger proportion of its adult body mass than the larger sex (Fig. 1, Supplementary material Appendix 1, Table A3). hese results are consistent with growth patterns described for other sexually size-dimorphic species (Ricklefs 1968, Teather and Weatherhead 1994)

Summary

Introduction

Intraspeciic variation in body size is ubiquitous, often manifested as sexual size dimorphism (SSD) and linked to diferent sex roles (Fairbairn 1997). In most sexually reproducing animals, females are larger than males, a pattern explained by fecundity selection. In many birds and mammals, males are larger than females (Fairbairn et al 2007), a pattern explained by sexual selection (Székely et al 2000, 2004). To understand the evolution of variation in body size within species, most previous studies have focused on adults, correlating SSD with a variety of factors such as mating system, habitat preference, activity pattern, body size and phylogenetic relationships (CluttonBrock et al 1977, Székely et al 2004). Species with ARTs are typically characterized by two or three diferent male mating morphs with nearly discrete size variation (Oliveira et al 2008a). Small male morphs are similar – or even smaller – in size to females and usually adopt alternative mating strategies such as sneaking (Shuster 1987, Taborsky and Brockmann 2010) or female mimicry (Dominey 1980, Shuster 1987, Gonçalves et al 2005, Jukema and Piersma 2006)

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