What Is Polyandry?

Polyandry is a mating system in which a single female mates with multiple males in a breeding season, while each male typically mates with only one female. This system is relatively rare but well-documented in certain birds (such as phalaropes and jacanas), some fish, and a few insects.

Why Does Polyandry Lead to Higher Female Competition and Fitness Variance?

• Female Competition: In polyandrous systems, females compete for access to males who will incubate eggs or care for offspring. The limited availability of males for parental care makes them a valuable resource, intensifying female-female competition.

• Variance in Female Fitness: Not all females are equally successful in securing mates or having their eggs cared for. Some females may mate with several males and have more offspring survive, while others may fail to secure mates or lose out in competition, resulting in greater variance in female fitness.

• Reversal of Typical Patterns: In contrast to polygynous or promiscuous systems—where males compete and have higher fitness variance—polyandry shifts these pressures onto females.

Scientific Context

Most studies and models of sexual selection observe that males usually show higher variance in reproductive success due to female choice and male-male competition. Polyandry is the notable exception, where these dynamics are flipped, and females become the competitive sex.

Why Not the Other Mating Systems?

• Monogamy: Both sexes have similar opportunities for reproduction, so variance in fitness and competition for mates are generally low and equal.

• Polygyny: Males compete for multiple females, leading to higher male fitness variance.

• Sequential Monogamy: Partners change over time, but at any one time, both sexes typically have similar reproductive opportunities, keeping variance relatively equal.

Conclusion

Polyandry is the mating system where competition for mates and variance in fitness are higher among females than among males. This unique reversal of typical sexual selection patterns highlights the diversity and adaptability of animal reproductive strategies.