Which of the following is true about sperm competition and male physiology?

Question

BlackTom AI · Accepted Answer

High levels of sperm competition select for larger testes and higher sperm production per ejaculate, not the opposite.

1) Relevant knowledge points and concepts
- Sperm competition: when females mate with multiple males, male gametes compete to fertilize ova. Evolutionary responses in males often involve increased sperm production (larger testes relative to body size) and sometimes behavioral adaptations. Well-known comparative examples: chimpanzees (multi-male mating) have very large testes relative to body size, whereas gorillas (single dominant male mating system) have small testes. Penis size is not a simple inverse indicator of sperm competition and is influenced by other selective pressures.

2) Step-by-step reasoning
- Option (a): Claims low sperm competition → relatively large penises. Empirical comparative data do not support a general rule that low sperm competition leads to larger penises; if anything, penis size is not a consistent compensatory trait for low sperm competition. So (a) is incorrect.
- Option (b): Claims high sperm competition → relatively small testicles. This contradicts the core prediction of sperm competition theory: high sperm competition favors larger testicular volume to produce more sperm. So (b) is incorrect.
- Option (c): Claims low sperm competition → relatively high sperm count in ejaculate. This is the reverse of predictions and observations; species with low sperm competition typically have lower sperm counts per ejaculate because there is less selection pressure to flood the female reproductive tract. So (c) is incorrect.

3) Calculations/intermediate steps
- No numeric calculation is required; logic follows from direction of selective pressure: increased sperm competition → selection for increased sperm production (larger testes) → higher sperm counts. The three given statements all contradict that direction.

4) Why the correct answer is correct
- Because all three specific statements (a, b, c) are inconsistent with established evolutionary and comparative evidence about sperm competition and male reproductive physiology, the only correct choice is that none of the other answers are true.

5) Why other options are incorrect (brief)
- (a) Penis size does not reliably increase under low sperm competition; there is no general rule supporting this.
- (b) High sperm competition is associated with larger, not smaller, testes.
- (c) Low sperm competition is associated with lower, not higher, sperm counts.

Conclusion: Therefore the correct answer is d. None of the other answers.

Which of the following is true about sperm competition and male physiology?Single choice

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