Data from: Old-male paternity advantage is a function of accumulating sperm and last-male precedence in a butterfly

Old-male mating advantage has been convincingly demonstrated in Bicyclus anynana butterflies. This intriguing pattern may be explained by two alternative hypotheses: (i) an increased aggressiveness and persistence of older males during courtship, being caused by the older males' low residual reproductive value; and (ii) an active preference of females towards older males what reflects a good genes hypothesis. Against this background, we here investigate postcopulatory sexual selection by double-mating Bicyclus anynana females to older and younger males, thus allowing for sperm competition and cryptic mate choice, and by genotyping the resulting offspring. Virgin females were mated with a younger virgin (2–3 days old) and afterwards an older virgin male (12–13 days old) or vice versa. Older males had a higher paternity success than younger ones, but only when being the second (=last) mating partner, while paternity success was equal among older and younger males when older males were the first mating partner. Older males produced larger spermatophores with much higher numbers of fertile sperm than younger males. Thus, we found no evidence for cryptic female mate choice. Rather, the findings reported here seem to result from a combination of last-male precedence and the number of sperm transferred upon mating, both increasing paternity success.

在安尼娜矍眼蝶（Bicyclus anynana）中，老龄雄虫的交配优势已得到令人信服的证实。这一引人关注的现象可通过两种对立假说加以解释：（i）老龄雄虫因剩余繁殖价值较低，在求偶过程中表现出更强的攻击性与持续求偶意愿；（ii）雌性对老龄雄虫存在主动偏好，这契合优质基因假说（good genes hypothesis）。
在此背景下，本研究通过让安尼娜矍眼蝶处女雌虫先后与老龄、年轻雄虫进行双交配，以模拟精子竞争与隐蔽配偶选择（cryptic mate choice），并对所得子代进行基因分型，以此探究交配后性选择过程。实验设置两种交配顺序：先与2~3日龄处女雄虫交配，再与12~13日龄处女雄虫交配；或反之。
结果显示，老龄雄虫的父权成功率（paternity success）高于年轻雄虫，但仅当老龄雄虫作为第二只（即末次）交配配偶时成立；若老龄雄虫为第一只交配配偶，则二者父权成功率无显著差异。此外，老龄雄虫可产生更大的精包（spermatophore），且其携带的可育精子数量远高于年轻雄虫。
综上，本研究未发现雌性存在隐蔽配偶选择的证据。相反，本次研究结果似乎由最后雄性优先效应（last-male precedence）与交配时转移的精子数量共同作用所致，二者均能提升雄虫的父权成功率。