In accordance with our prediction, relatedness between mating partners influenced their mating behaviour: copulation duration and the risk of sexual cannibalism were significantly lower in sibling matings than in non-sibling matings.
One possible explanation for these results is that females react differently towards brothers, by interrupting copulations earlier to limit sperm uptake and/or by letting them survive because their brothers' future mating success would add to the females' inclusive fitness. However, this seems unlikely because females stereotypically attack any male independently of their degree of relatedness and independently of male courtship behaviour (Schneider & Lesmono 2009
). Males cling to the genital opening and can actively modulate the timing of their escape (Nessler et al. 2009
). The only reliable predictors of sexual cannibalism in A. bruennichi
are copulation duration and male mating status. The female's influence, by contrast, appears to be limited (Fromhage & Schneider 2005
The potential for polyandrous females to bias paternity cryptically towards a more compatible male (Welke & Schneider 2009
) may reduce the value of a sister as a mating partner for a male. Male investment in a mating event is reflected by two alternative mating tactics. They can either be monogynists (first copulation: long, second copulation: none or first copulation: short and second copulation: long with the same female) or bigynists (first copulation: short, second copulation: long, with another female (Fromhage et al. 2008
In our study, almost all of the non-sibling males were monogynous. Although re-mating with the same or another female would be possible, males sacrificed themselves to the female after a long first copulation. By prolonging their copulation, males increase their paternity share with a female but they also risk being cannibalized (Schneider et al. 2006
). High mortality during mate search in addition to an increased paternity share and benefits for the offspring could make it beneficial for males to be monogynists (Buskirk et al. 1984
; Andrade 2003
). Thus, if a male has found a compatible female, his optimal strategy may be to perform a long first copulation and invest maximally instead of copulating briefly and risking death during further mate search.
Half of the males mated to sibling females succeeded in cannibalism avoidance and thus in a natural setting would potentially be bigynous. But why do these males mate with their sisters at all? One possibility is that high male mortality during mate search selects for acceptance even of sub-optimal mating opportunities, but not at the cost of losing all other mating opportunities. This would explain why males tend to copulate briefly enough to survive (less than 5 s). Being monogynous with a sister would be disadvantageous because females might re-mate and post-copulatorily bias paternity towards a more compatible suitor and a sibling male would have sacrificed itself in vain.
In conclusion, our results support the idea that males adaptively modulate their mating tactic through selective self-sacrifice or escape behaviour.