Confidence of paternity and paternal care: covariation revealed through the experimental manipulation of the mating system in the beetle Onthophagus taurus

Abstract

Theoretical models of paternal care predict that facultative reductions in male care may occur under certain conditions. One important parameter that has been shown to influence the outcome of these models is a male's confidence of paternity. In this study, we tested whether the amount of care provided by horned males in the dimorphic beetle, Onthophagus taurus, varied with his confidence of paternity. Male care results in an increased weight of dung provided in the brood masses produced by the pair. Using the sterile male technique we showed that a horned male's paternity declined with the number of sneak males in the population. The relationship was nonlinear, with paternity declining most rapidly between a frequency of one and three sneaks, and stabilizing thereafter at about 50%. A horned male's paternity was directly related to the number of copulations with the female, relative to the number of copulations achieved by sneaks. Horned males were shown to reduce their care in relation to their declining paternity. Video analysis demonstrated that reductions in male care occurred through a combination of male desertion and a trade-off between caring and paternity assurance behaviours. The number of fights with sneak males was negatively related to the amount of care provided by a horned male. These results suggest that by gauging his expected paternity through the number of fights with sneaks, a horned male is able to assess his paternity and reduce his investment accordingly. Our data thus provide strong empirical support for the proposed link between paternity and paternal care. IntroductionAccording to Hamilton's rule (rb ) c > 0), the evolution of parental behaviour should depend exclusively on the costs to the parents (c) and the benefits to the offspring (b), because the relatedness (r) between parent and offspring should be the same for all offspring and for both parents (Hamilton, 1964). However, sperm competition because of multiple mating by females can reduce the average relatedness between males and the young produced by their mates, and is a widespread phenomenon across animal taxa (Smith, 1984;Birkhead & Møller, 1998). Thus, males often face the uncertainty of investing in offspring to which they are not genetically related. Given that males cannot increase their fitness by investing in unrelated offspring, Triver's (1972) proposed that uncertainty of paternity would favour male desertion because failure to assure paternity prior to investment would put a male at a selective disadvantage in competition with more reproductively selfish individuals. Consequently, variation in a male's confidence of paternity has been proposed to explain the observed inter and intrasexual differences in the magnitude of paternal care provided (e.g. Ridley, 1978;Alexander & Borgia, 1979). Theoretical models have sought to establish the influence of confidence of paternity, the average probability that a male is the father of a given set of offspring (Alexander, 1974; also referred to as certainty of paterni...

The behavioral ecology of threshold evolution in a polyphenic beetle

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Genital morphology and fertilization success in the dung beetleOnthophagus taurus: an example of sexually selected male genitalia

“…Minor males outnumber majors in the dung pad so that both morphs face a high risk of sperm competition. Indeed, the two morphs invest equally in ejaculates (Simmons et al 1999b), spend equivalent durations in copula and gain fertilizations in direct proportion to the relative number of copulations they perform with a female (Tomkins & Simmons 2000;Hunt & Simmons 2002). The average pattern of sperm use strongly suggests that sperm mix randomly in the female's genital tract.…”

Sex ratio bias in the dung beetle Onthophagus taurus: adaptive allocation or sex-specific offspring mortality?

“…In contrast to males, female fitness components (i.e. fecundity and survival) increase linearly across the natural range of female body sizes when females provision in isolation (Hunt and Simmons 2002). We do not currently know, however, whether the increase in fitness with size is greater in males or in females when measured under the same conditions, but we suspect the latter is more likely due to intense sperm competition in this species (Simmons et al 1999).…”