Repeatability of sperm number across multiple matings in three cricket species, Gryllodes sigillatus, Gryllus veletis, and Gryllus texensis (Orthoptera: Gryllidae)

Although studies of various taxa have shown that males can alter the number of sperm in their ejaculates according to the risk of sperm competition, few studies have examined the extent to which the number of sperm transferred by males across multiple matings is repeatable. We assess the within-male and between-male components of variation in sperm number by counting the sperm in multiple ejaculates of males of three cricket species and determining the repeatability of sperm number. Sperm number was highly repeatable across multiple matings in all three species, leaving open the possibility that variation in sperm number is based, in part, on heritable genetic variation. Résumé : Bien que des études de différents taxons aient démontré que les mâles peuvent modifier le nombre de spermatozoïdes dans leurs éjaculats en fonction des risques de compétition entre les spermatozoïdes, peu d’études se sont attardées à déterminer à quel point le nombre de spermatozoïdes transmis par les mâles au cours d’accouplements multiples est reproductible. Nous évaluons ici les composantes de la variation, chez un même mâle et d’un mâle à l’autre, par dénombrement des spermatozoïdes dans les éjaculats multiples des mâles chez trois espèces de grillons et par détermination de la répétabilité du nombre obtenu. Le nombre des spermatozoïdes s’est avéré très reproductible au cours des multiples accouplements des mâles des trois espèces, laissant entrevoir la possibilité que le nombre de spermatozoïdes soit en partie régi par la variation génétique héréditaire. [Traduit par la Rédaction] 585 Schaus and Sakaluk Introduction In many species of insects, females often mate with more than one male, resulting in competition between the sperm of rival males for the fertilization of a single female’s eggs. Such competition represents a potent selective force, and a wide array of male secondary traits in these insects has been attributed to selection on males to prevent females from remating or to incapacitate the sperm of their rivals (Parker 1970, 1984). Evolutionary biologists have begun to consider the possibility that male sperm-allocation strategies may also be tied to the risk of sperm competition. Under most circumstances, an increased risk of sperm competition favours an increase in the number of sperm transferred, but this increased gametic investment comes at the cost of a male’s ability to invest in future matings (Parker 1998). Comparative studies across a wide range of taxa have broadly supported the prediction that ejaculate expenditures should increase with an increase in the risk of sperm competition (Harcourt et al. 1981; Svärd and Wiklund 1989; Møller 1991; Stockley et al. 1997). In addition, a number of studies have shown that, within species, males exhibit considerable plasticity in ejaculate expenditure in response to varying sperm competition risk (e.g., Gage 1991; Gage and Baker 1991; Baker and Bellis 1993; Gage 1998). Notwithstanding the ability of males to customize their ejaculates, the average ejaculate expenditure of males of a given population should be subject to stabilizing selection imposed by both the social environment and the costs of producing sperm (see also Morrow and Gage 2001). Consequently, we might expect that variation between males in the number of sperm transferred at mating should account for less of the overall variance in sperm number than variation in the number of sperm transferred across multiple matings within males. Here we assess the within-male and between-male components of variation in sperm number by counting the sperm in multiple ejaculates of males of three cricket species, Gryllodes Received 25 July 2001. Accepted 10 January 2002. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 11 April 2002. J.M. Schaus1 and S.K. Sakaluk.2,3 Behavior, Ecology, Evolution and Systematics Section, Department of Biological Sciences, Illinois State University, Normal, IL 61790-4120, U.S.A. 1Present address: Missouri Botanical Garden, Education Department, P.O. Box 299, St. Louis, MO 63110, U.S.A. 2Corresponding author (e-mail: [email protected]). 3Present address: Zoologisches Institut (Biologie I) der Universität, Hauptstraße 1, D-79104 Freiburg, Germany; after 1 July 2002, Department of Biological Sciences, Illinois State University, Normal, IL 61790-4120, U.S.A. J:\cjz\cjz80\cjz-03\Z02-012.vp Monday, April 08, 2002 1:37:47 PM Color profile: Disabled Composite Default screen