Abstract1_15 (Hasumi)

Hasumi, M.: Sexual behavior in female-biased operational sex ratios in the salamander Hynobius nigrescens. Herpetologica (USA, International) 57(3): 396-406, September 2001. Stable URL

Abstract: I assessed the effects of female-biased operational sex ratios (OSR) on timing of oviposition, fertilization duration, and any other behavioral interaction, and on fertilization success during spawning behavior of a hynobiid salamander (Hynobius nigrescens) with external fertilization. In female-biased OSRs in the absence of scramble competition from other males (one male:1-5 females), a monopolist male and each female engaged in successive pair spawns in which the monopolist folded himself over a pair of egg sacs from the female for insemination. The mean duration of fertilization was 206 s (SE = 10), and occupied one-third of that (633 s, SE = 58) in a group spawn with scramble competition (8-10 males:one female). Fertilization duration appeared to be constant within each OSR treatment and independent of the highly variable fertilization success, suggesting an adaptive tactic "optimized" in the pair spawn for economical sperm allocation. Thus, I hypothesize that scramblers adopt a conditional tactic for sperm depletion of a monopolist by forming a mating ball (sperm depletion hypothesis). I further propose a poor mate hypothesis that scramblers are allowed to fertilize many of the eggs by the presence of poor mating monopolists, exhibiting disadvantageous behaviors or low fertilization success. These two hypotheses seem to account for the adaptive significance of mating balls formed by scramble competitors.