ZOOLOGY Zoology 112 (2009) 362–369 Sexual size dimorphism in caecilian amphibians: analysis, review and directions for future research Alexander Kupfer Institut fu¨r Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich Schiller-Universita¨t Jena, Erbertstrasse 1, D-07743 Jena, Germany Received 24 June 2008; received in revised form 22 December 2008; accepted 30 December 2008 Abstract Sexual dimorphism, widespread in the animal kingdom, describes differences between the sexes in size, shape and many other traits. Sexual size dimorphism (SSD) plays a significant role in understanding life history evolution and mating systems. The snakelike morphology of limbless caecilian amphibians lacking obvious secondary sexual characters (in contrast to frogs and salamanders) impedes accurate intrasexual comparisons. In this study, sexual size dimorphism in the oviparous caecilian Ichthyophis cf. kohtaoensis, a phylogenetically basal caecilian, was analysed. Females were larger in all body and head characters tested. However, when adjusted to body size (total length), females differed only in their cloacal shape. Clutch volume was positively correlated to female body size, thus female fecundity increased with body size supporting the hypothesis of a fecundity-selected SSD in the oviparous Ichthyophis cf. kohtaoensis. A review of the present SSD data for caecilians shows that many species are monomorphic for body size but show dimorphism in head size, while other species demonstrate female-biased SSD. Male-biased SSD has not been reported for caecilians. To understand life history evolution in caecilians, further studies on the reproductive biology of other taxa are urgently needed, in particular for rhinatrematids and uraeotyphlids. New data will allow phylogenetically controlled comparative analyses to fully explore the pattern of SSD among caecilian lineages. r 2009 Elsevier GmbH. All rights reserved. Keywords: Caecilians; Life history evolution; Sexual selection; Fecundity selection; Niche divergence hypothesis Introduction Sexual dimorphism describes intraspecific differences in size, shape or traits between males and females ( Fairbairn, 2007). Sexual differences in body size are widespread in animals and several hypotheses aim to explain the existence of sexual size dimorphism (SSD). Fecundity selection is proposed to be the driving force behind female-biased SSD (Darwin, 1871). Female clutch size and sometimes also egg or offspring size, and therefore reproductive success, typically increase with female body size (e.g. Roff, 1992; Honek, 1993), at least in many oviparous poikilotherms such as in most fish (e.g. Wootton, 1979; Elgar, 1990), in most reptiles (e.g. Shine, 1994; Cox et al., 2007) and amphibians (e.g. Shine, 1979; Kupfer, 2007; Wells, 2007). Male-biased SSD is proposed to be influenced by sexual selection, i.e. large male body size often increases mating success due to intra-sexual (largely male–male) competition or female choice ( Andersson, 1994). Famous examples of sexual selection within amphibians are found in newts (e.g. Lissotriton vulgaris ) where males display their crests during courtship (e.g. Darwin, 1871; Kupfer, 2007; Wells, 2007). ARTICLE IN PRESS www.elsevier.de/zool 0944-2006/$ - see front matter r 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.zool.2008.12.001 E-mail address: alexander.kupfer@uni-jena.de.