We tested adaptive hypotheses for eunuch phenomenon (i.e. males with damaged genitalia) in nephilids. Genital amputation, i.e. genital damage or loss, seems maladaptive because it renders the amputee functionally sterile, but is nevertheless common in sexually dimorphic spiders. There, male genital amputation correlates with plugging of female genitals and with sexual cannibalism. We tested two adaptive hypotheses about eunuch behavior in an orb-web spider, Nephilengys malabarensis, 1. plugging hypothesis (i.e., broken male genitals (palps) effectively plug female genitals) and 2. better fighter hypothesis (i.e., eunuch males are better fighters compared with their intact rivals). By staging mating trials, we precisely documented genital amputation, sexual cannibalism, and genital organ reuse, morphologically examined plugs to infer their effectiveness, and conducted a series of male-male contests to determine whether eunuch males would fight better. Copulations always resulted in palpal amputation: 87.5% males became eunuchs directly during copulation and plugged females, while 12.5% males first damaged palps partially, then severed them after copulation. Sexual cannibalism and plugging effectiveness both reached 75%. Eunuchs guarded females, were highly aggressive and active, initiated and won contests more often, whereas intact males and half-eunuchs showed significantly lower levels of guarding behavior, aggression and general activity. Thus, both hypotheses were supported and we concluded that the eunuch phenomenon is adaptive.
COBISS.SI-ID: 32493101
This research delves into sexual cannibalism in the sexually size dimorphic spider Nephilengys livida. Through mating trials and assessment of female personalities, we tested aggressive spillover and two alternative hypotheses, i.e. mate choice and mate size dimorphism hypotheses, that were proposed to explain sexual cannibalism. The aggressive spillover hypothesis explains precopulatory sexual cannibalism as spillover aggression from the juvenile foraging to the adult mating context, and predicts positive correlations of aggression levels across contexts. Our study found female general aggressiveness, body size measures and rates of sexual cannibalism to be independent, and thus provides no support for aggressive spillover hypothesis in Nephilengys livida. We never observed precopulatory sexual cannibalism, whereas postcopulatory sexual cannibalism was linked to male aggressiveness and boldness during male-male contest. Hence, mate quality remains the best explanation for sexual cannibalism in N. livida.
COBISS.SI-ID: 33159725