Behav Ecol Soclobiol (1989) 24:245-249 Behavioral Ecology and Sociobiology 9 Springer-Verlag 1989 Paternity assurance by "helpful" males: adaptations to sperm competition in burying beetles J.K. Miiller and Anne-Katrin Eggert Lehrstuhl ffir Evolutionsforschung der Universitfit, Universltfitsstrasse, D-4800 Bielefeld, Federal Republic of Germany Received July 18, 1988 / Accepted November 11, 1988 Summary. Male burying beetles invest parentally by participating in the burial of a carcass and in provisioning and guarding the larvae that come to the carcass. Since most of the females arriving at a carcass have stored fertile sperm within their spermathecae, sperm transferred by such "help- ful" males must compete with other males' ejacu- lates for the fertilization of the female's eggs. We showed that these males are able to achieve a high level of paternity (mean=92%). The mechanism they employ is a repeat-mating tactic, i.e., the fe- male is mated very frequently shortly before and during oviposition. Repeated matings are essential for a high reliability of paternity, since single copu- lations result in the fertilization of only a very small proportion of the female's eggs. Introduction Since Parker (1970), many publications have dealt with sperm competition and paternity assurance in animals. Trivers (1972) was the first to indicate that in species with substantial paternal invest- ment, males should have evolved adaptations to decrease their vulnerability to cuckoldry. In the following years, several authors restated that a cor- relation between male parental investment and cer- tainty of paternity should exist (e.g., Thornhill 1979). Yet this initially accepted correlation was doubted repeatedly during the following years. In 1978 Maynard Smith stated that a general correla- tion between paternity and male parental care could not be derived from theoretical considera- tions. Other authors specified the conditions under which such a correlation could be expected: (1) Offprint requests to: J.K. Mfiller the "sacrificing" of promiscuous copulations by parental males (Werren et al. 1980; Grafen 1980) or (2) an extreme time delay between broods due to a prolonged search by females for males (Parker 1984). The only model we know that does predict a correlation between male parental care and pa- ternity - which is achieved by the influence of pa- ternal investment on paternity and not vice versa - is the one developed by Knowlton and Greenwell (1984). Though this correlation can be observed in many species (see Gwynne 1984), the problem still remains controversial among theoreticians. Male parental care in insects is a relatively rare phenomenon; most cases of male parental invest- ment or nonpromiscuous mating effort consist in the transfer of nutrients or other substances to the female (see Gwynne 1984). Exceptions to this rule are, for example, male brooding water bugs (Smith 1979), woodroaches of the genus Cryptocercus (Seelinger and Seelinger 1983 ; Nalepa 1984, 1988), and burying beetles of the genus Necrophorus. Burying beetles utilize small vertebrate car- casses as a larval food source; the carcass is buried and prepared by either a pair or a single female. Aggressive contests occur if intra- or interspecific competitors are present on the carcass. The female lays the eggs near the carcass and afterwards waits for the larvae, which approach the carcass by themselves, in order to feed them regurgitated car- rion. The male, if present at the beginning, also stays with the brood for several days. More de- tailed descriptions of the natural history of burying beetles are given by Pukowski (1933) and Mfiller and Eggert (1988). Like many of the female insects investigated to date, female burying beetles can store trans- ferred sperm within a specialized storage organ, the spermatheca. Thus sperm competition for the fertilization of the female's eggs can occur any time a male mates with a non-virgin female whose sper-