Male alternative reproductive tactics and sperm competition: a meta-analysis

Area of Science:

• Evolutionary biology
• Animal behavior
• Reproductive strategies

Background:

• Males in many species use alternative reproductive tactics (ARTs) to achieve fertilization.
• ARTs often involve trade-offs between pre-mating investment (e.g., competition) and post-mating investment (e.g., ejaculate quality).
• Theoretical predictions suggest males facing higher sperm competition should invest more in post-mating traits like sperm and ejaculate quality.

Purpose of the Study:

• To quantitatively summarize the evidence for differences in sperm and ejaculate investment between male alternative reproductive tactics (ARTs).
• To investigate if males facing high sperm competition risk invest more in post-mating traits like testes size, sperm number, and sperm traits.
• To examine factors influencing post-mating investment differences, such as tactic flexibility and population frequency of certain tactics.

Main Methods:

• Conducted a systematic literature review and meta-analysis of 92 studies across 67 animal species.
• Analyzed differences in testes size, sperm number, and sperm traits (e.g., sperm ATP content) between males exhibiting ARTs with high vs. low sperm competition risk.
• Controlled for body size in testes size analyses and considered sperm measurements from stores and post-ejaculation.

Main Results:

• Male fish with ARTs facing high sperm competition risk had larger testes, but this was likely an artifact of using the gonadosomatic index.
• No significant differences in sperm number were found between males with different ARTs.
• No significant differences in most sperm traits were found, except for sperm adenosine triphosphate (ATP) content in fish.

Conclusions:

• Despite theoretical predictions, empirical evidence shows limited substantial differences in sperm and ejaculate investment across male alternative reproductive tactics (ARTs).
• Incongruence may stem from theoretical models not accounting for resource levels or trade-offs, or from studies using suboptimal measures of post-mating investment.
• Further research is needed to accurately assess post-mating investment and its impact on fertilization success in different ARTs.