Strategic ejaculation and level of polyandry in Callosobruchus chinensis (Coleoptera: Bruchidae)

Abstract

Variation in the level of polyandry of females produces a difference in the risk of sperm competition among males. As a consequence, investment in ejaculate expenditure by males should vary. We compared the number of sperm ejaculated by males into the female reproductive organ of six strains of the adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae), when males were reared at different larval densities in a bean. A significant positive correlation was found between the remating frequency of females and the ratio of the ejaculate sizes of high-density and low-density males as a measure of the response to the risk of sperm competition among males. The measure was estimated by dividing the number of sperm ejaculated by males reared at high larval density in a bean with the number of sperm ejaculated by males reared alone. The number of sperm transferred by a male to a female was not correlated with the duration of copulation. The results suggest an evolutionary relationship between ejaculatory expenditure and the level of polyandry in C. chinensis.

A trade-off between pre- and post-copulatory sexual selection in a bean beetle

“…When it is easy for males to locate females, females are likely to encounter and mate with many males leading to high risk and intensity of sperm competition (Kokko and Rankin 2006;Pomfret and Knell 2008). It is likely that males compete by scramble competition for paternity in this species (Parker and Ball 2005), and it has been shown in a previous study in C. chinensis that males from populations with a higher risk of sperm competition invest more in ejaculation, whereas males from low-risk populations invest less (Yamane and Miyatake 2008). In the current study, although there was no difference in the ability of males from the hindwing and testes strains to secure mates under the easy environmental treatment, males with greater allocation to testes did transfer more sperm to the females in our assay of flight ability versus sperm transfer.…”

“…The average density of sperm within a fixed area (0.7×0.7≒0.5 mm 2 ) was estimated, across 15 subsamples per female, under an optical microscope (×200 magnification). By multiplying the area of water on the slide by the average density of sperm, the total number of sperm ejaculated by the male into the spermatheca was estimated (Yamane and Miyatake 2008).…”

“…As C. chinensis females mate with multiple males Miyatake 2005, 2008), post-copulatory sperm competition is also an important determinant of male fitness and should select for increased investment in testis and ejaculate size (Parker 1990;Okada et al 2007;Yamane et al 2010). Previous studies of this species have demonstrated that the distance which males fly to disperse (Ohno and Miyatake 2007) and the number of sperm transferred into females (Yamane and Miyatake 2008) differ across populations. Here, we therefore first examined whether there is a trade-off in resource investment to hindwings versus testes in male C. chinesis from across different geographic strains.…”

Mating Frequency and Fecundity in Agrilus anxius (Coleoptera: Buprestidae)

“…In most cases where the fecundity of single and multiply mated females have been compared, females beneÞted from mating more than once (Arnqvist and Nilsson 2000). Multiple mating by females may have several beneÞts; direct material beneÞts (e.g., nuptial gifts, fresh sperm, and gonadotropins) (Arnqvist and Nilsson 2000, Dunn et al 2005, Edvardsson 2007, Yamane and Miyatake 2008, Avila et al 2011, avoiding costly harassment from males (Arnqvist and Rowe 2005) or genetic beneÞts Petrie 2000, Slatyer et al 2011).…”

It’s all in your head: the role of quantity estimation in sperm competition

“…Clearly, if males use innate means, they need not estimate the number of their rivals, and automatic response to perceptual amount cues (such as concentration thresholds) suffice. Indeed, some species rely upon an a priori value, embedded either prenatally or during a critical period in development [60,61]. This option may apply to males who lack plasticity in accommodating for changes in competition size [23,62].…”