Behav Ecol Sociobiol (2006) 60: 212–219 DOI 10.1007/s00265-005-0158-9 ORIGINAL ARTICLE Diego Rubolini . Paolo Galeotti . Gabriele Ferrari . Michele Spairani . Franco Bernini . Mauro Fasola Sperm allocation in relation to male traits, female size, and copulation behaviour in a freshwater crayfish species Received: 13 October 2005 / Revised: 8 December 2005 / Accepted: 18 December 2005 / Published online: 7 February 2006 # Springer-Verlag 2006 Abstract Sperm competition is a well-recognised agent in the evolution of sperm and ejaculate structure, as well as variation in female quality. Models of the evolution of ejaculate expenditure predict that male body condition, female fecundity and the risk and intensity of sperm com- petition may be the ultimate factors shaping optimal ejaculate size. We investigated sperm allocation in Austro- potamobius italicus, a freshwater crayfish exhibiting a coercive mating system and external fertilisation, in relation to male and female traits and copulation behaviour under laboratory conditions. We found that mating males were sensitive to female size and produced larger ejaculates when mating with larger females, which were more fecund in terms of number of eggs produced. We found no evidence for female egg production being sperm-limited, as the number of eggs was not dependent on male sperm expenditure. Copulation duration and number of ejacula- tions reliably predicted the amount of sperm transferred, and both these behavioural measures positively covaried with female body size. These results indicate that male freshwater crayfish can modulate their sperm expenditure in accordance with cues that indicate female fecundity. In addition, a novel finding that emerged from this study is the decrease in sperm expenditure with male body size, which may either suggest that large, old male crayfish are better able than small males to economise sperm at a given mating to perform multiple matings during a reproductive season, or that they experience senescence of their repro- ductive performance. Keywords Arthropoda . Decapoda . Fecundity . Senescence . Sperm competition . Sperm economy . Sperm limitation Introduction The traditional notion that male reproductive success is only limited by the number of mates he can encounter and fertilise, due to the huge amount of sperm cells produced (Bateman 1948), has been questioned in recent decades by recognising that sperm production inevitably entails some costs (Dewsbury 1982; Simmons 1988; Van Voorhies 1992; Gage and Cook 1994; Olsson et al. 1997; see also Wedell et al. 2002). Indeed, there is ample evidence that males have evolved strategies for allocating a finite number of sperm to each mating attempt to maximise their lifetime reproductive success (Dewsbury 1982; Parker 1990a,b; Shapiro et al. 1994; Parker et al. 1996). As a consequence, male ejaculate expenditure has been found to be dynamic in both space and time and sensitive to the risk and intensity of sperm competition (e.g. Gage and Baker 1991; Gage and Barnard 1996; Oppliger et al. 1998; Evans and Magurran 1999; Mougeot et al. 2001; Pilastro et al. 2002; Evans et al. 2003). If females vary in reproductive quality, males may achieve a greater reproductive success by strategically al- locating their sperm to females that provide the greatest fertilisation returns. Female egg production or fecundity may depend on condition or age (Trivers 1972; Arak 1988). Therefore, males in several species prefer larger or younger mates (e.g. in insects, see review in Bonduriansky 2001), and there is evidence that males also provide larger ejaculates to heavier females in insects, molluscs, and fish (e.g. Yusa 1994; Gage and Barnard 1996; Marconato and Shapiro 1996; Gage 1998; MacDiarmid and Butler 1999; but see Simmons and Kvarnemo 1997). Additionally, the energetic costs associated with sperm production may result in males being sperm-limited, which may in turn translate in female reproductive output being sperm-limited as well, if females can vary clutch size to match male Communicated by P. Backwell D. Rubolini (*) . P. Galeotti . G. Ferrari . M. Spairani . F. Bernini . M. Fasola Dipartimento di Biologia Animale, Università degli Studi di Pavia, p.zza Botta 9, 27100 Pavia, Italy e-mail: diego.rubolini@unipv.it Fax: +39-0382-986290