Polyandry

Polyandry Female is receptive to and/or actively mating with multiple males Mate guarding may prevent in some cases, but clearly not all Why is polyandry adaptive for females? They invest much more in the egg and the parental care They dictate the terms of mating in most cases

The team approach A single female assembles a team of males which aid in defending the territory and/or gathering food, building nest, rearing young Obvious advantage for the female Males have better chance to mate by being part of a team than not

Extra-pair matings Female has a stable social partner, but engages in extra pair matings with other males Risks: Detection by the social partner and rejection? Loss of parental care by this social partner Time and energy spent finding a suitable male Sexually transmitted disease

Zoo animals, primates

Fertility insurance hypothesis Supported by evidence in birds Eggs more likely to hatch in polyandrous blackbirds In species where sperm degrades quickly, multiple matings prolong fertility And certain prairie dogs 100% of polyandrous females become pregnant, only 92% of monogamous Polyandry protects against infertile males, ensures investment in egg production does not go to waste.

Good gene hypothesis Polyandry leads to better offspring survival Behavioral difference Guppies born to monogamous mothers did not form schools but wandered off alone (dangerous) Survival of infancy Wild guinea pig pups born to polyandrous mothers were less likely to be stillborn and to survive through weaning

Proximate explanations for the good gene hypothesis Increased chances of acquiring sperm of exceptional quality In polyandrous guppies, the male with the better courtship display fertilizes more eggs than the other Females that seek extra-pair copulations choose males that molt early, a sign of exceptional condition Older males that demonstrate the ability to stay alive also are a source of good genes in some songbirds These traits in fathers can be passed on to sons

S-successfully mated, U-unsuccessfully mated Expt 1: two males competed for the same female, loser given a different female to mate with so that sons could be compared Expt 2: two males competed for a mate, then both were given a different, random female to mate with. Rule out any contribution of the courtship ritual on the female side to the success of the sons.

Increase heterozygosity (genetic diversity)

Gene matching: not all alleles are compatible Chickens in vitro fertilization experiment Sperm of two males is mixed and used to fertilize multiple females’ eggs No clear “winner” in these experiments Conclude that chickens possess an internal mechanism to pick the sperm that are most complementary or lead to the most heterozygosity Avoid embryonic lethality Scorpions reject the spermatophore of males she has previously mated with but not new males

Polyandrous queens advantage Also note that single insemination queens’ colonies did not survive the winter while about 25% of poly-inseminated queens did. Bigger hives from more diverse work force. Also, better disease resistance and larger broods.

The MHC locus Major histocompatibility locus is important in immune function May be the proximate mechanism for detecting genetic compatibility Humans, primates, reptiles, and possibly birds who mate with partners having the same MHC haplotype suffer reduced fertility and offspring survival

How can one ascertain the MHC compatibility? Pre-mating: The region contains genes encoding polymorphic olfactory receptors Animals and humans prefer the odor of MHC-dissimilar individuals At/near fertilization: MHC similarities/homozygosities can be non-viable so eggs fertilized by genetically similar sperm don’t survive No preference for any one haplotype, just a difference