Abstract
Sperm competition is a powerful selective force that has shaped sexual traits throughout animal evolution. Yet, how fertilization mode (i.e. external versus internal fertilization) influences the scope and potential for sperm competition to act on ejaculates remains unclear. Here, I examine how fertilization mode shapes ejaculatory responses to sperm competition in fishes, a diverse group that constitute the majority of vertebrate biological diversity. Fishes are an ideal group for this examination because they exhibit a wide range of reproductive behaviours and an unparalleled number of transitions in fertilization mode compared to any other vertebrate group. Drawing on data from cartilaginous and bony fishes, I first show that rates of multiple paternity are higher in internally than externally fertilizing fishes, contrary to the prevailing expectation. I then summarize how sperm competition acts on sperm number and quality in internally and externally fertilizing fishes, highlighting where theoretical predictions differ between these groups. Differences in how ejaculates respond to sperm competition between fertilization modes are most apparent when considering sperm size and swimming performance. Clarifying how fertilization mode influences evolutionary responses in ejaculates will inform our understanding of ejaculate evolution across the animal tree of life.This article is part of the theme issue ‘Fifty years of sperm competition’.
Highlights
Sperm competition, the competition between sperm from rival males to fertilize ova [1], is a widespread evolutionary force that has played a major role in shaping animal reproductive anatomy, physiology and behaviour throughout their evolution [1,2,3,4]
Anisogamy set the stage for a series of transitions in sexual strategies (a ‘sexual cascade’) that helped generate the extraordinary diversity in sexual traits observed in animals [3]
Sperm number predicts competitive fertilization success and fishes respond to sperm competition by increasing investment in sperm number
Summary
The competition between sperm from rival males to fertilize ova [1], is a widespread evolutionary force that has played a major role in shaping animal reproductive anatomy, physiology and behaviour throughout their evolution [1,2,3,4]. Ejaculate trait (a) sperm number testes size (relative testes size) sperm allocation sperm competition risk models sperm competition intensity models (b) sperm quality sperm size and swimming speed, and longevity predictions, fertilization mode and empirical patterns males are expected to increase their relative (i.e. correcting for body size) investment in testes size in response to increasing sperm competition risk and intensity both within and across species [17] This prediction is sensitive to the strength of raffle loading and costs of acquiring a mate [17], but does not differ between external and internal fertilizers. I identify commonalities and differences in patterns of selection on ejaculates between fertilization modes and highlight fruitful avenues for future consideration
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