Mating competition among females: testing the distinction between natural and sexual selection in an insect

In species where females compete for mates, the male often provides the female with resources in addition to gametes. A recently suggested definition of sexual selection proposed that if females only benefit from additional resources that come with each mating and not additional gametes, female intrasexual competition for mating opportunities would result in natural selection rather than sexual selection. The nuptial gift-giving bushcricket Kawanaphila nartee has dynamic sex roles and has been a textbook example of sexual selection acting on females via mating competition. We investigated whether females of this species gain fitness benefits from nuptial gifts, additional ejaculates, or both, by controlling the number of matings and whether the female was allowed to consume the nutritious gift (spermatophylax) at mating. We found that egg production per day of life increased with the number of additional matings, both with and without spermatophylax consumption, but consuming the spermatophylax had an additional positive effect on the number of eggs. These effects were particularly strong in females with shorter lifespans. We discuss how the recently suggested definition of sexual selection applies to nuptial feeding insects, and conclude that both natural and sexual selection influence mating competition in K. nartee females.

Methods
Bushcrickets (Kawanaphila nartee) were collected from King's Park, Perth. Females were split into three treatments: a) ‘single mating’; b) multiple mating without spermatophylax: ‘sperm only’; and c) multiple mating with spermatophylax: ‘gift + sperm’. Females were allowed to continuously oviposit into a sand cup, and lifetime egg production was counted after the female perished naturally.

在雌性竞争配偶的物种中，雄性通常除配子（gametes）外，还会向雌性提供额外资源。近期有学者提出的性选择（sexual selection）定义指出：若雌性仅能从每次交配附带的额外资源中获益，而非额外配子，那么雌性为交配机会展开的种内竞争便属于自然选择，而非性选择。送交配彩礼的灌丛蟋蟀Kawanaphila nartee具有动态性别角色，一直是通过交配竞争作用于雌性的性选择经典教科书案例。本研究通过控制交配次数，以及雌性在交配时是否可取食富含营养的彩礼（精护垫spermatophylax），探究了该物种雌性是否能从交配彩礼、额外射精或两者中获得适合度收益。
研究结果显示：无论雌性是否取食精护垫，每日产卵量均随额外交配次数增加而提升，但取食精护垫会对产卵量产生额外正向影响；这类效应在寿命较短的雌性个体中尤为显著。我们探讨了新近提出的性选择定义如何适用于送彩礼的昆虫，并得出结论：自然选择与性选择共同影响了K. nartee雌性的交配竞争。

实验方法
研究所用灌丛蟋蟀（Kawanaphila nartee）采集自珀斯国王公园（King's Park）。我们将雌性个体分为三组处理：a）「单次交配组」；b）无精护垫的多次交配组：即「仅获取精子组」；c）伴随精护垫的多次交配组：即「彩礼+精子组」。允许雌性持续在沙杯中产卵，待雌性自然死亡后，统计其终生产卵总量。