ORIGINAL PAPER The role of body size in early mating behavior in a simultaneous hermaphrodite, Chelidonura sandrana Dennis Sprenger & Rolanda Lange & Nico K. Michiels & Nils Anthes Received: 5 December 2008 / Revised: 23 January 2009 / Accepted: 17 February 2009 / Published online: 4 March 2009 # Springer-Verlag 2009 Abstract Intraspecific variation in mating behavior is widespread among simultaneous hermaphrodites but its underlying sources remain largely unexplored. In the sea slug Chelidonura sandrana, most matings are reciprocal. However, despite non-conditional sperm exchange and potential polygamy-mediated benefits, 30% of matings end after unilateral insemination. To resolve this apparent inconsistency, we here investigated the effect of body size on the frequency of reciprocal matings by testing the following two hypotheses. First, sex-allocation theory predicts that the likelihood of reciprocity depends on the size difference between mating partners. Second, if both sex functions temporally differ in reaching maturity, reciprocal matings should be more frequent with increasing absolute body size of the smaller partner. The likelihood of reciprocity increased with body size of the smaller partner. Moreover, smaller individuals acted more often as males among unilateral matings. These findings suggest that the ability to donate sperm develops prior to female function- ality in C. sandrana. Keywords Opisthobranchia . Reciprocal matings . Sex allocation . Simultaneous hermaphrodite . Slight protandry . Unilateral matings Introduction Simultaneous hermaphrodites show large variation in mating behavior at both interspecific and intraspecific level (Michiels 1998). For example, in cephalaspid sea slugs, the degree of reciprocity differs enormously between members of different families as well as between species of the same genus, showing a full cline from exclusively unilateral to exclusively reciprocal matings (Anthes and Michiels 2007). While such variation may partly be due to independent evolution of different mating strategies as shown by phylogenetic analysis (Anthes et al. 2008), sources of intraspecific variation in mating behavior, in particular in the degree of reciprocity, remain largely unexplained. In Chelidonura sandrana, a member of the cephalaspid sea slug family Aglajidae, about 70% of all observed matings are reciprocal and the remaining are unilateral (Anthes et al. 2006a; Anthes and Michiels 2007). Reci- procity in this species is not conditional, i.e., sperm donation does not depend on sperm receipt. Instead, as revealed by experimental manipulation of sperm transfer, reciprocal matings appear to be the result of a mutual willingness between two partners to mate in both sexual functions (Anthes and Michiels 2005). Consistent with this, recent experimental work indicated that C. sandrana benefits from mating multiply in the female function by producing larger offspring at an intermediate mating rate (Sprenger et al. 2008a, b). Likewise, reproductive success of the male function is likely to increase with the number of matings, as previously shown in a freshwater flatworm (Pongratz and Michiels 2003). Hence, both sex functions likely benefit from mating multiply within a reproductive season. While such potential polygamy-mediated benefits may help explaining the prevalence of unconditional reciprocity Behav Ecol Sociobiol (2009) 63:953–958 DOI 10.1007/s00265-009-0738-1 Communicated by T. Moore D. Sprenger (*) : R. Lange : N. K. Michiels : N. Anthes Animal Evolutionary Ecology, University of Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany e-mail: dennis.sprenger@uni-tuebingen.de