SPERM COMPETITION AND THE EVOLUTION OF NONFERTILIZING SPERM IN MAMMALS

Area of Science:

• Reproductive Biology
• Evolutionary Biology
• Mammalian Reproduction

Background:

• The existence of nonfertilizing sperm with specialized morphologies is known in invertebrates.
• Abnormal sperm in mammals were historically viewed as production errors.
• Recent hypotheses propose mammalian sperm are polymorphic, serving nonfertilizing roles in sperm competition, such as forming copulatory plugs.

Purpose of the Study:

• To challenge the hypothesis that mammals produce nonfertilizing sperm.
• To argue that mammalian sperm are unlikely to evolve nonfertilizing roles due to biological and evolutionary constraints.
• To re-evaluate evidence supporting nonfertilizing sperm and sperm competition in mammals.

Main Methods:

• Critically analyze the "kamikaze" sperm hypothesis proposed by Baker and Bellis.
• Examine metabolic costs and selection pressures on sperm production in mammals.
• Evaluate the role of accessory gland secretions in semen coagulation.
• Compare sperm morphology and production in polyandrous versus monandrous primates.

Main Results:

• Mammalian reproduction may not afford the evolution of nonfertilizing sperm due to fertilization uncertainties and metabolic costs.
• Selection appears to act against the production of deformed sperm in mammals.
• Accessory gland secretions are sufficient for copulatory plug formation, suggesting no need for "kamikaze" sperm.
• Quantitative data from primates do not support the "kamikaze" sperm hypothesis; polyandrous genera do not produce proportionally more deformed sperm.

Conclusions:

• Current evidence suggests all mammalian sperm are adapted for fertilization.
• Sperm competition in mammals likely operates through scramble competition, not contest competition.
• Selection favors accessory glands over sperm morphology for enhancing semen coagulation.