Understanding why there is extensive variation in sperm form and function across taxa has been a challenge because sperm are specialized cells operating at a microscopic level in a complex environment. This comparative study collates published data to determine whether the evolution of sperm morphometry (sperm total length and separate component dimensions) is associated with sperm competition ...

Across many species, males exhibit plastic responses when they encounter mating rivals. The ability to tailor responses to the presence of rivals allows males to increase investment in reproduction only when necessary. This is important given that reproduction imposes costs that limit male reproductive capacity, particularly when sperm competition occurs. Fruitfly (Drosophila melanogaster) male...

A fundamental question in animal communication is whether the information provided is honest or deceptive [1, 2]. This problem has received much attention, both in theoretical [1, 3] and experimental [4] work. Here we show that male Atlantic mollies (Poecilia mexicana), when observed during mate choice by another male, reduce their mating activity and no longer prefer mating with one of two fem...

Female promiscuity is thought to have resulted in the evolution of male behaviours that confer advantages in the sperm competition that ensues. In mammalian species, males can gain a post-copulatory advantage in this sperm 'raffle' by inseminating females at the optimal time relative to ovulation, leading to the prediction that males should preferentially associate and copulate with females at ...

Sperm competition is generally thought to drive the evolution of sperm miniaturization. Males gain advantage by transferring more sperm, which they produce by dividing limited resources into ever smaller cells. Here, we describe the opposite effect of size on the competitiveness of amoeboid sperm in the hermaphroditic nematode Caenorhabditis elegans. Larger sperm crawled faster and displaced sm...

Costs incurred in the manufacture of ejaculates may constrain the number of sperm that males can produce, so males should show some economy in their allocation of sperm across multiple matings. In species in which females mate with multiple males and are capable of storing sperm for extended periods, sperm allocation of males should be tailored to the risk of sperm competition. Recent game theo...

Theory predicts that males adapt to sperm competition by increasing their investment in testis mass to transfer larger ejaculates. Experimental and comparative data support this prediction. Nevertheless, the relative importance of sperm competition in testis size evolution remains elusive, because experiments vary only sperm competition whereas comparative approaches confound it with other vari...

Sperm competition is a potent postcopulatory selective force where sperm from rival males compete to fertilize a limited set of ova. Considering that sperm production is costly, we expect males to strategically allocate sperm in accordance with the level of competition. Accordingly, previous work has examined a male's strategic allocation in terms of sperm number. However, the seminal fluid pro...