1. Polyandry

2. 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

4. 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

5. 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

6. Zoo animals, primates

7. 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.

8. 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

10. 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

11. 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.

12. Increase heterozygosity (genetic diversity)

13. 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

14. 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.

15. 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

16. 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