Facultative decrease in mating resistance in hermaphroditic Caenorhabditis elegans with self-sperm depletion GUNNAR A. KLEEMANN* † & ALEXANDRA L. BASOLO * *School of Biological Sciences, University of Nebraska yDepartment of Molecular Genetics, Albert Einstein College of Medicine (Received 13 June 2006; initial acceptance 12 August 2006; final acceptance 28 February 2007; published online ---; MS. number: A10472) Facultative change in mating decisions can result from both exogenous and endogenous factors. The her- maphroditic roundworm, Caenorhabditis elegans, is a model system for many areas of biology, yet, aspects of its reproductive biology are not well understood. Here, we present data suggesting that the reproductive behaviour of hermaphrodites changes with self-sperm status. Self-sperm containing hermaphrodites are more likely to eject male ejaculate after insemination than self-sperm depleted hermaphrodites, presum- ably reducing the likelihood of fertilization by male sperm. Self-sperm depleted hermaphrodites are more likely to mate, and mate in a shorter period of time than self-sperm containing hermaphrodites. This increase in the willingness to mate is correlated with self-sperm depletion, but not hermaphrodite age. Hermaphrodites that have exhausted their self-sperm also show a lower rate of sprinting away from males than self-sperm containing hermaphrodites, indicating a lower level of mating resistance. The com- bined results suggest that self-sperm containing hermaphrodites are more likely to show precopulatory mate avoidance as well as postcopulatory mechanisms to avoid fertilization by males than sperm-depleted hermaphrodites, presumably to delay outcrossing until self-sperm is exhausted. Thus, there appears to be a shift in hermaphrodite mating behaviour based on reproductive status. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: Caenorhabditis elegans; behavioural plasticity; hermaphrodite; mate choice; mating decision; mating resistance; postcopulatory choice; sexual conflict; self-sperm depletion; sperm ejection Mating decisions can be influenced by a number of factors in animals. For example, fertility status (McLennan 1993; Sheldon 1994; Converse et al. 1995), the time since last mating (Wolfner 1997; Tram & Wolfner 1998; Sauter & Brown 2001), food availability (Gwynne 1990; Gwynne & Simmons 1990) and the relative safety of a location (Endler 1987; Magurran & Seghers 1990; Magurran & Nowak 1991; Berglund 1993; Godin 1995) have been shown to influence mating behaviour. Such changes are often in response to both internal and external conditions. For example, during their fertile period, both female brook sticklebacks, Culaea inconstans, and pygmy marmosets, Cebuella pygmaea, show reduced aggression to- wards males (McLennan 1993; Converse et al. 1995). Sim- ilarly female chaffinches, Fringilla coelebs, show an increased likelihood of soliciting and engaging in mating interactions when fertile (Sheldon 1994). After mating, fe- males may become refractory to mating (Wolfner 1997; Tram & Wolfner 1998; Sauter & Brown 2001), may display aggressive behaviour towards males (Wolfner 1997; Sauter & Brown 2001), may begin producing compounds that are repulsive to males (Tompkins & Hall 1981; Scott 1986), or may show reduced mating rates (Wolfner 1997; Sauter & Brown 2001). In response to changing environmental conditions, animals may alter the type and stringency of mate choice; increasing food availability, for example, causes katydids to reverse courtship roles (Gwynne 1990; Gwynne & Simmons 1990), and increased levels of preda- tion reduce choosiness and courtship effort in male Correspondence and present address: G. A. Kleemann, Lewis-Sigler In- stitute, Princeton University, 148 Carl Icahn Laboratory, Washington Rd, Princeton, NJ 08544, U.S.A. (email: kleemann@princeton.edu). A. L. Basolo is at the School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588, U.S.A. (email: basolo@cricket. unl.edu). 1339 0003e 3472/07/$30.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2007, 74, 1339e1347 doi:10.1016/j.anbehav.2007.02.031