The Ins and Outs of Sex: answering a few fun questions – STIAS Public lecture by Michael Jennions

14 August 2023

Is it better to produce sons or daughters? Is there any evidence that smarter males are sexier? Is there more variation among males than among females in behaviour?

These are three questions that Michael Jennions set out to tackle in his very amusing, at times tongue-in-cheek, public lecture at STIAS (the first of the second semester and fourth of the year).

“Many of us are fascinated by sex and differences between the sexes,” he said. “In this talk I will introduce some of the tools and logic that evolutionary biologists use to predict how animals will invest in different traits – be these physiological, morphological or behavioural. I will illustrate this approach using three (hopefully) engaging examples from my own lab’s research associated with male-female interactions.”

STIAS Fellow Michael Jennions during his public lecture on 8 August 2023

Jennions is an evolutionary biologist and ecologist who is particularly interested in understanding aspects of evolution linked to sex, how natural selection works in the natural world, and what common patterns we find across species.

He is a Professor at the Research School of Biology at the Australian National University. He completed his BSc and MSc at the University of the Witwatersrand, followed by a PhD at Oxford University. He then spent five years at the Smithsonian Tropical Research Institute in Panama. He has worked on frogs, zebra finches, damselflies, fiddler crabs, plants, cichlid fish, crickets, beetles and live-bearing fish. “We tend to ask a question and then pick a study animal that can answer it,” he said.

Aside from evolutionary biology, with a special focus on sexual selection, he is interested in the dangers posed by a scientific system with reward incentives that encourage biased presentation of research findings and even fraud.

He is also interested in communicating science, with one of his many claims to fame a YouTube video that has garnered 2.2 million views titled ‘Does size matter?’ which he describes as his “more frivolous research”.

More sons or daughters?

“So, is there an optimal ratio of sons to daughters to produce?  Do we want to make sons or daughters, or both, or equal numbers of each?” he asked.

Jennions explained that although we might think either that more sons are better because each can have many children leading to lots of descendants or that daughters, by contrast, limit population growth, hence the number of descendants, the right answer is actually 50/50.

“The only point at which natural selection does not favour a shift in the sex ratio is when there are equal numbers of sons and daughters produced,” he said. “Equal numbers means that individuals of each sex have the same reproductive value at conception. Reproductive value is the expected lifetime reproductive success of an individual at that stage of life (e.g. at birth). Fisher’s principle (from British mathematician, statistician, biologist and geneticist Ronald Fisher) – which respects the truism that in most species every individual has one mother and one father – underpins the argument that the evolutionary stable sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females.”

Jennions then pointed to unusual natural histories that affect the offspring sex ratio. For example, he mentioned the impact of sex-specific mortality (like that caused by sending males to fight wars or by encouraging women to terminate pregnancies if the foetus is a girl), or seasonal differences in sex-specific mortality when sexual partners are inter-generational that change the value of each sex.

He and his colleagues have shown seasonal shifts in mate availability in Eastern Mosquitofish (Gambusia holbrooki), allowing them to predict optimal offspring sex ratios that deviate from 50/50 given the argument from the Fisher condition for equal investment in sons and daughters. Mosquitofish are the group’s most common animal model because they are easy to sex and “like to mate a lot” added Jennions.

“In the mosquitofish, the sexes only have the same reproductive value at birth if the ratio is male-biased in spring and female-biased in autumn. Evolution by natural selection therefore favours the seasonal changes in sex ratios that we observed,” he continued.

Is being clever a smart idea?

Is there any evidence that smarter males are sexier? “Some of you will be hoping the answer is a resounding yes,” said Jennions.

But is it true that cognition plays a role in sexual selection and therefore evolutionary success?

Jennions again pointed to his group’s work with Mosquitofish in what he described as the “CSI fishpond” where the male fish underwent a battery of cognition tests which included measuring their inhibitory control, spatial and associative learning with the reward being getting to spend time closer to female fish.

The study found that the main predictors of paternity were those males with strong inhibitory control and better spatial learning. “Interestingly, body size didn’t matter,” said Jennions, “so smart is sexy, the academics won.”

Similarly, studies in Australian Magpies have shown a positive association between cognitive performance and reproductive success in females, but also point to the importance of sociality in shaping cognitive development.

Are males more variable than females?

Turning to his third question, in nature is there more variation among males than among females in behaviour? Jennions explained: “The answer has potential implications for popular arguments you will see in the media to explain why more men than women are described as geniuses. But is there a biological basis for the idea that you get both more geniuses and more idiots in males?”

In this regard, he pointed to a meta-analysis of over 200 studies which looked at sex differences in variability in five personality-like behaviours across species, including birds, reptiles, amphibians, fish, insects and mammals.

“The work has shown no conclusive evidence for greater male variation. This undermines evolutionary arguments for the greater success of men, and suggests that social factors are probably a more important reason why fewer females have won Nobel and equivalent prizes,” he said.

Beautiful nature

The title of Jennion’s STIAS project is ‘Nature is beautiful’ which he highlighted with stunning photographic examples. “Nature is sometimes frighteningly beautiful,” he said. “Nature can inspire awe. It makes us feel connected to something bigger than ourselves – an unbroken thread of ancestry, linking us to all life on the planet.”

“We have to ask ‘Who is the artist? Who made it and why?’,” he continued. “The answer is that the available facts point to a blind process of evolution. Evolution by natural selection is the common principle across species and allows us to generalise and predict. Sex is about reproduction and replication – we either get laid or die trying.”

“As Darwin proposed, individuals vary in traits, some traits predict reproductive success and some of these traits are heritable. So, natural selection mechanically results in the spread of heritable traits (genes) that increase our number of descendants. Basic evolution in three-part harmony.”

“But, overall, we have to remember evolution is ruthless – if you are not successful as a male or female, the genes are eliminated.  Humans are a manifestation of evolution. But evolution is relentless, it won’t stop, all that happens is that the traits that are favoured change,” he concluded.


Michelle Galloway: Part-time media officer at STIAS
Photograph: Ignus Dreyer

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