More sex partners means more grandkids … if you’re a guppy

If you’re a female guppy it pays to take many mates … sort of. André "Drekas" Correia/Flickr

Here’s some good news: having more sexual partners makes females more fertile.

A recent study on Trinidadian guppies has shown that females who mate with multiple males produce more grand-offspring than females who have mated with only a single partner.

This is an intriguing result from an evolutionary perspective. It’s obvious why having more mates helps males leave behind more children, but it’s surprising for females because one male will usually suffice to fertilise all her eggs.

So what’s the evolutionary benefit for a promiscuous female?

For some time, we’ve known that females can ensure they’ll receive high-quality sperm to fertilise their eggs if they mate with multiple partners. Either that or they might directly benefit by receiving gifts of food from their suitors (as in the case of charming gift-giving spiders).

The new study – about to be published in the journal BMC Evolutionary Biology by Miguel Barbosa of the University of St. Andrews and colleagues – shows an unusual avenue to increased evolutionary prospects: boys. Multiply-mated females produce more sons, which in turn, give their mothers more grandchildren.

Monogamy vs multiple men

In their study Barbosa and colleagues allowed some females to mate with multiple males, and others to mate with only one, and then examined the offspring the females of both groups produced.

They found no difference in common measures of offspring fitness, such as size at birth or growth rate, but they did find that multiply-mated females had a higher proportion of male offspring that survived to adulthood. While the numbers of daughters the two groups produced was the same, promiscuous females produced 83% more sons than their monogamous contemporaries.

The study went further and measured the offspring produced in the second generation. The researchers found females who mated with more than one partner produced significantly more grand-offspring, which the authors attribute to their having more sons in the first generation.

Interestingly, the sons of multiply-mated females did not themselves produce more offspring – the difference in the number of grand-offspring came about solely because there were more sons available to become fathers.

In the context of the experiment, where all sons were given access to a virgin female with whom they could mate, having more sons is obviously beneficial. But in the real world, such a strategy can quickly unravel, and having more sons is not always a good thing.

The problem with boys

While the social constructivists may cry foul, there is an undeniable biological difference between males and females when it comes to the number of descendants they can leave behind.

Pioneering work by an English geneticist Angus John Bateman showed us, as far back as 1948, that there are big differences in the benefit of having extra sexual partners for males and females.

That’s because all females will typically have similar numbers of offspring regardless of the number of matings they have (past a minimum threshold at least), and the upper limit is bounded by physiological constraints.

Contrast this with males, some of whom can have hundreds or thousands of offspring, while others die without ever having reproduced. This difference can be summarised by saying that males have a higher potential reproductive rate, meaning that under some circumstances males can leave behind many more offspring than is possible for a single female.

A great example of this is everyone’s favourite warlord Genghis Khan.

While these principles were established and developed with experiments on flies and guppies, they apply to every animal that has one mother and one father per offspring produced (i.e. pretty much all of them). This is known as the Fisherian condition, and applies equally to humans as to other animals.

Missing out

Just because males have a higher potential reproductive rate, doesn’t mean all males will leave behind more offspring. And here lies the root of the problem with overproducing sons as an evolutionary strategy. When there are a lot of extra boys kicking around in a population, it means there are a lot of males who will not mate at all.

Adding to this already inflated number of males means your sons will have a lower number of children, on average, than daughters would. As a general biological rule, the best option is to produce the sex that is rarer in the population (so they’re in demand). This process is known as frequency dependent selection and it’s why animal populations (including humans) so often have equal sex ratios.

Male-biased sex ratios are problematic for other reasons as well, and can be a major cause of social trouble in human societies.

When “spare” males accumulate, they have little hope of finding a mate and this often leads to conflict and violent behaviour. This phenomenon is sadly common in humans, in nations where sons are strongly preferred over daughters.

The lost boys of the Mormon faith are also victims of the fact few males can fertilise many females, but not the other way around. In the animal world, too many males can be a bad thing for females as well, increasing the sexual harassment of females to such a degree it can sometimes lead to death by copulation.

Solid sex strategy?

So is mating with lots of males, and thereby producing more sons, a golden ticket to evolutionary success for females?

Certainly not – as a general rule – I’m afraid. Even if we take away the problems the society at large suffers from an oversupply of boys, for the mothers themselves, be they human or guppy, it makes no sense to produce offspring that have a lower than average prospect of mating (in this case, males).

The findings of Barbosa and colleagues suggest an interesting effect of multiple mating on offspring sex ratios (or sex-specific survivorship at least).

But unfortunately for the hedonists out there, mating with multiple partners doesn’t make for a great long-term evolutionary strategy.