Hello and welcome to another Daily Fun Fact From Flora ,

Today we are going to have a look at the evolutionary origins (and advantages) of polyandry vs monogamy.

We spoke about sexual selection last week, where females picking the sexiest male leads to what we like to call pre-copulatory selection. The lady can pick which male she thinks is best after he’s fought off some of the other less good males (either through literal direct fighting or through other sexually selected traits like vocalisations of those frogs or ornaments like those crazy long-tailed widow birds).

Now, we are going to have a look at post-copulatory selection in the form of polyandry.

Polyandry occurs when a female mates with multiple males. In the animal kingdom this is the rule rather than the exception in sexually reproducing organisms. It happens in angiosperms (those are flowering plants), invertebrates and the vast majority of vertebrates as well.

This is interesting and is something of a conundrum , because it is not entirely clear ** why females are polyandrous.

It makes evolutionary sense for males to be polyandrous — the more ladies they mate with the more offspring they have the more genes they propagate into the next generation. Winner, winner, sexy dinner.

But females, largely, once they have mated with a male and had their eggs fertilised — well what’s the point in having any more sex? You’re done. Box ticked. Move on.

But this isn’t what we see.

There are both non-adaptive and adaptive explanations for why this may be. Let’s start with non-adaptive.

NON-ADAPTIVE

The trait is male-driven

In a finite population where males want to mate with multiple females (as we mentioned above) maybe it’s just the case that eventually many males will mate with the same female. Those males don’t know that she’s already been fertilised so they might just go for it.

Correlated evolution

If there is a genetic component to mating with multiple partners then this will be strongly selected for in males. This is because any males with genes that say ** HAVE SEX WITH LOADS OF GIRLS** are probably going to have a lot of offspring. And guess what — those offspring are all also likely to have genes that say HAVE SEX WITH LOADS OF GIRLS . And on it goes until all the males in a population want to ** HAVE SEX WITH LOADS OF GRILS** sounds a bit like boarding school tbh.

ADAPTIVE

Convenience

This one is super depressing. It might be that

levels of

Acquisition of resources.

This one is MUCH more feminist. A lot of the time in the animal world, males will provide their sexual partners with presents, known as nuptial gifts. If females are enjoying this gift-giving it might be in her interest to have a lot of sexual partners.

There is a wonderful example of this seen in butterflies. Butterflies undergo a form of behaviour called “puddling” (I know it’s adorable). They sip moisture from puddles to extract salt and minerals.

It’s mostly males that do this because they incorporate the salts into their sperm and transfer it to the lady in a little bundle called a spermatophore. Now females can mate every 2 or 3 days and so can basically exist entirely off these gifts. Pretty ideal really.

Sometimes this benefits the males as well and in a type of grasshopper known as a katydid , females eat their present while the male inseminates her. The bigger the present, the longer the male gets to put as much sperm in her as he can, increasing his odds of successful fertilisation.

But sometimes that means that males will trick females , like empidid flies, which put seed fluff in their food packets to distract her while he mate with her.

Sorry — got off topic.

Sperm replenishment

I guess if you’re a female you don’t want to (quite literally) put all your eggs in one basket. What if the male you mated with is infertile or low in sperm? Then your eggs aren’t actually so fertilised are they hmmm might as well have a back-up.

When you have polyandry in a system it introduces a new form of sexual competition. In monandry — each male is guaranteed full fertilisation of the eggs that the female produces. He can have a lot of sex with her and do a lot of fertilising. That means he fights other males for access to females , but then the fighting is done.

In polyandry things are a bit different. If a female is mating with multiple males so that their ejaculates overlap both spatially and temporally then those sperm cells are going to be competing with each other for access to her eggs.

NOW you have sexual selection AFTER mating as well as BEFORE. There is now post-copulatory selection. There is sperm competition. Females can even bias the outcome of sperm competition to favour the ejaculates of some males over others if she thinks they’re better options. This is known as cryptic female choice.

Hope I"ve converted all of you to polyandry (apart from one of you — you know who you are).

Love to you,

Flora xxx