Anim. Behav., 1997, 54, 305–312 Infanticide by males in a spider with suicidal maternal care, Stegodyphus lineatus (Eresidae) JUTTA M. SCHNEIDER*² & YAEL LUBIN* *M itrani Center for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev ² Max-Planck-Institut fu ¨r V erhaltensphysiologie, S eewiesen, Germany ( Received 21 June 1996; initial acceptance 31 July 1996; final acceptance 4 October 1996; M S. number: 5273) Abstract. Infanticide by males in species with high maternal investment may be the most extreme example of a reproductive conflict of interests between males and females. The subsocial spider Stegodyphus lineatus is semelparous, and the offspring eat their mother. Male and female maturation overlap, such that late maturing males encounter females that are already guarding the single clutch. A late male may try to remove the existing egg sac, thereby forcing the female to replace the clutch and use his sperm to fertilize at least some of the eggs. Males were responsible for 33% of all the egg losses in a population of 278 females with eggs. By losing her clutch, the female loses time equivalent to the age of the lost clutch plus the time it takes to replace it. The earlier in the season a female lost a clutch and the younger the eggs were, the longer was the interval between clutch loss and clutch replacement. Females that lost their egg sacs by infanticide replaced them after a significantly longer time than females that lost their egg sacs from other causes. The time loss of 18.5 days on average corresponds to a 23% reduction in female survival probability. In addition to reduced survival, females suffered a fecundity cost: replacement clutches were smaller than the original clutches and the reduction in egg number was greater with increasing intervals between the first and second clutch. Females defended their egg sacs aggressively against males. In field tests, the outcome of fights between males and females was determined by the relative size difference of the contestants. The mating system of S. lineatus provides an example of a male mating strategy that evolved despite being very costly for the females.  1997 The Association for the Study of Animal Behaviour Mating systems are the outcome of an evolution- ary arms race between males and females because of their conflicting interests over mating, fertiliz- ation and care of the offspring (Parker 1979; Thornhill & Alcock 1983; Andersson 1994; Rowe et al. 1994). The degree of sexual conflict depends on the asymmetry of male and female reproduc- tive investment (Trivers 1972): the greater the asymmetry in investment, the larger the difference between the costs and benefits of reproductive behaviour for the sexes and the greater the poten- tial for a conflict of interests (Parker 1979). The interests of males and females differ strongly in the case of sexually selected infanticide, where a male kills the offspring sired by another male in order to increase his own fitness. Males that kill the offspring of the preceding male are likely to impose substantial costs on the female whose progeny they kill (Clutton-Brock & Parker 1994). The costs of producing replacement young or eggs may be direct ones, such as energy and resources required, or indirect ones such as the time lost or increased risk of predation or parasitism. In addition, females resisting infanticide attempts by males may risk injury. Infanticide by males is known only for several mammal (Bertram 1976; Hrdy 1979) and a few bird species (Rohwer 1986). In most vertebrate examples, it has proven difficult to measure the costs and lifetime fitness consequences of infanti- cide because of the long life span of these species. Furthermore, infanticide occurs only on rare occasions and the conditions that favour its oc- currence are difficult to assess. Infanticide by Correspondence: Y. Lubin, Mitrani Center for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer 84990, Israel (email: lubin@bgumail.bgu.ac.il). 0003–3472/97/080305+ 08 $25.00/0/ar960454  1997 The Association for the Study of Animal Behaviour 305