Bitch: On the Female of the Species

by Lucy Cooke

Graves looked at how the platypus Y was different to the human Y and calculated how much genetic material had been lost in the time since our species diverged. This enabled her to estimate how long it might be before the human Y chromosome disappeared completely.

‘It turned out the human Y was losing about ten genes per million years, and there’s only forty-five genes left. So it doesn’t take Einstein to figure out at that rate we’re going to lose the e...

Graves is confident her gloomy prophecy won’t spell the end of mankind. She’s certain that the human male would simply evolve a fresh genetic trigger for his gonads. Other mammals have already done so.

In South America there are nine species of vole from the genus Akodon in which a quarter of females are XY, not XX. Their Y chromosome is complete with SRY, yet they still develop ovaries and produce viable eggs, suggesting they must have an entirely new master switch gene that can suppress the bossy SRY.

Sex is a master at reinventing itself. It has to be. It is essential in order for sexually reproducing species to persist, after all. This anarchy of common genes may have once been more logical and linear, hundreds of millions of years ago at the start of sex. But eons of evolutionary time have left their mark, creating an extraordinary array of apparently nonsensical, yet somehow functional, botched systems in this ever-evolving sex-defining chaos.

‘You have to get over the idea that this was meant to be. Nothing is meant to be. We’re all being buffeted by the forces of evolution all the time.’

The confusion of sex chromosomes seen in mammals is just the tip of the iceberg when it comes to the bewildering diversity of systems that exist in nature.

Heat is just one of several known external sex-determination stimuli. Exposure to sunlight, parasitic infections, pH levels, salinity, water quality, nutrition, oxygen pressure, population density and social circumstance – how many of the opposite sex are in your neighbourhood – can all influence an animal’s sexual fate.

the master switch for these frogs to develop testes rather than ovaries is sometimes genetic, sometimes environmental, sometimes a bit of both. It all depends on where the frogs are from.

The frogs I caught as a kid were in the southern range and their sex is a little more fluid. All tadpoles are XX and develop as females. But as they emerge from the pond, around half of these genetic females reverse their sexual development. Their ovaries transform into testes and they become XX males.

In the laboratory, frogs have been encouraged to change sex from male to female by exposure to chemicals that mimic oestrogen. These are found in herbicides like Atrazine, popular with lawn-growers in the USA whose liberal use of them forces male frogs to switch sex and become female.

It would be easy to dismiss this sexual mishmash as the glitches of an imperfect, less-evolved system of sex determination. Many scientists have. But that’s a primitive, mammal-centric viewpoint. This extraordinary plasticity is now understood to persist in a range of reptiles, fish and amphibians. It’s persevered for hundreds of millions of years across diverse species, which suggests there must be some evolutionary benefit.

If during development a clutch of ZZ male eggs gets baked by too much Australian sun, the high temperature overrides their chromosomal sex and ZZ males switch sex to female. These sex-changed ZZ females have a unique constellation of male-like and female-like physical and personality traits. They lay twice as many eggs, yet their behaviour is more in line with male dragons – they’re bolder, more active and their temperature is higher. This novel variation allows genetic or sex-reversed female dragons to respond differently to a more diverse range of environmental pressures, giving them an evolutionary advantage.

Rather than being seen as ‘an aberration’, this hotchpotch of sex-determination systems and the resulting sex-reversed females could, in fact, be a powerful driver of evolutionary change.

One side of the bird’s body was covered in striking scarlet feathers and topped off with a dramatic red crest, while the other side was a dowdy buff brown. It looked as if two half birds had been glued together down the middle, and, in a way, they had. The bird was a gynandromorph – an exceptional intersex that’s split straight down the centre line. The showy red side was a male cardinal bird complete with solitary internal testicle, whereas the brown side had an ovary instead. This bilateral condition is rare but has been documented in a number of birds, butterflies, insects and crustaceans – animals all with the ZW sex-determination system.

These ‘half-siders’ offer a unique opportunity to test the authority of gonadal sex hormones in shaping brains and behaviour. Gynandromorphs may be made up of two sexes, but they share just one bloodstream, which means they’re bathed in the same hormonal milieu. Is the solitary testis and its brawny androgens the supreme driver of sexual fate for the chimera’s entire brain, as the Organizational Concept predicts, or could the ‘passive’ feminine side somehow prevail?

Zebra finch are songbirds, but only the males sing, so their neural circuitry is more developed than females’.

‘It blew me away,’ Arnold told Scientific American at the time. The gynandromorph’s semi-female brain cast doubt on the omnipotence of gonadal steroids to differentiate sexual dimorphism in birds. In other words, this bilateral intersex bird kicked the Organizational Concept in the nuts. Here was evidence that androgens were not the exclusive force sculpting the sexuality of a bird’s body, brain and behaviour. Instead the sex chromosomes, exerting their identity inside neural cells, must be playing a crucial role.

So, with birds at least, the genetic sexual identity of individual cells plays a significant role in driving sexual dimorphisms in the body and brain.

According to Crews there are five types of sex: chromosomal, gonadal, hormonal, morphological and behavioural. They don’t necessarily all agree with one another or even remain fixed for life. They are cumulative and emergent in nature, and can be influenced by genes or hormones, as well as the environment or even an animal’s life experience. This plasticity allows for the huge variety in sex and sexual expression that we see both within and between species.

If you don’t have variation you can’t have an evolving system. So it’s important that we have variation in sexual characteristics.’

Crews blames the Organizational Concept for promoting a rigid deterministic view of sex, which focuses on the differences between the sexes, reinforcing the binary concept and ignoring the glorious diversity of sexual characteristics found in nature.

The standard paradigm has, in his opinion, had its day. It is mammal-centric, overly simplistic and underplays the role of oestrogen as an organizing and activating sex hormone. ‘Females are just as differentiated [active] as males.

My conclusion has been that the female is the ancestral sex. I think there’s a lot of evidence for that.’

‘The earliest reproductive organism had to be able to lay eggs and that’s a female.’ Crews’ research estimates that 600–800 million years ago the only creatures in existence were these cloning egg-layers. Males did not arrive on the evolutionary scene until the dawn of sex, when gametes diverged in size, which Crews reckons was around 250–350 million years later.

‘Maleness evolved as an adaptation to femaleness,’ Crews continued. ‘When males came along what they did was to facilitate reproduction in the female. To stimulate and coordinate the neuroendocrinological processes that underlie the shedding of gametes. Males are behavioural facilitators.’

Thornton’s work on this, and other primitive animals like lampreys, has subsequently shown that the oestrogen receptor is the oldest transcription factor (a protein whose job is to turn genes on or off) in vertebrates – far older than previously thought, with an origin between 600 million and 1.2 billion years ago. The genes for androgen receptors did not evolve for a further 350 million years.

Crews has also reversed the sex of developing female lizards using oestrogen blockers. Oestrogen clearly plays a fundamental role organizing both female and male sexual development but also activating sexual behaviours later in life.

‘The “female” sex steroid has a critical role even in males, because males were originally females,’ Crews expounded. So, in the gospel according to Crews: Eve wasn’t created out of Adam’s rib, it was the other way round. In the beginning there was female, and she gave rise to male. From this alternative evolutionary perspective, the ultimate answer to what is a female is: she’s the ancestral sex. Relics of this primal egg-layer exist within all of us. Which puts a fresh spin on males getting in touch with their feminine side.

At the other end of the scale, many species of baleen whale have bigger females than males, including the blue whale. One female specimen taken off the coast of the island of South Georgia was almost 30 metres long and weighed in at 173 tonnes – three times the length of a double-decker bus and over thirteen times its weight. Which means that the largest animal to have ever lived was, in fact, female.

Known in zoological circles as leks, these events are essentially sage grouse discos; the battle for sex is played out using the medium of dance, with the males strutting about, providing their own unlikely soundtrack by, basically, beatboxing. Male greater sage grouse have a massively distended oesophagus which they can inflate by gulping down mouthfuls of air to create a large wobbly white-feathered throat balloon, which when fully swollen briefly exposes two bulbous patches of olive-green skin that pop forth from their feathers like a pair of nippleless shop-dummy breasts.

‘We’re basically pornithologists,’ Eric joked. ‘It’s our job to record bird sex.’ The avian orchestra began limbering up pre-dawn, teasing its audience with the eerie sound of echoing doinks as the sky turned from black to blue. By the time the rising sun had painted the surrounding snow-clad mountains pink, I was able to make out a bunch of black shuffling blobs in the distance. The show had begun and it would not disappoint.

About thirty male sage grouse were body-popping on a patch of ground roughly the size of two netball courts. The surrounding sunrise-tinted mountains provided a natural amphitheatre for their performance, enabling the sound to reverberate up to three kilometres, broadcasting their presence to the opposite sex.

Descriptions of sage grouse leks have traditionally embodied androcentric typecasts. The birds made their dramatic debut on the ornithological scene with a male displaying his fully inflated sacs on the cover of Nature magazine in 1932. The author of the paper, R.Bruce Horsfall, took great delight in describing the males’ ‘queer antics’ but assumed their ‘rubbery plops’ must be directed at one another, and not the females.

‘That really was a reflection of this view that it’s got to be all about the guys,’ Gail explained. ‘All this communication was just about males threatening other males and it’s not driven by female choice.’

The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!’ Darwin’s queasiness concerned the unbridled frivolity of the peacock’s tail, which didn’t appear to benefit the bird’s overall quest of survival. In fact it seemed more likely to have a negative impact by impeding the peacock’s ability to hide or fly away from danger.

Darwin’s revolutionary proposition was that such ‘secondary sexual characteristics’ could be explained by two forces: male–male competition for females, which led to armaments like the outsized horns of a rhinoceros beetle; and female mate choice, which shaped ornaments like the peacock’s tail. ‘It is shown by various facts… the female, though comparatively passive, generally exerts some choice and accepts one male in preference to others.’

Darwin’s controversial claim was that females were not only sexually autonomous but had the wherewithal to make decisions that shaped male evolution.

The astonishing originality of Darwin’s new theory didn’t help its passage into cultural and scientific acceptance either.

the notion of sexual selection as an evolutionary force was without scientific precedent.

Darwin gave the impression that animals required a human-like sense of aesthetics for sexual selection to work. This gave the Victorian establishment a whip to beat Darwin’s new theory with. According to the thinking of the time, only the upper classes could appreciate art or music, so it seemed utterly absurd that a female, let alone a lowly peahen, would be endowed with an aesthetic faculty.

in 1881, Wallace took the bold move of censoring Darwin’s legacy. ‘In rejecting… female choice I insist on the greater efficacy of natural selection.

Wallace may have lost out on equal credit for the theory of evolution by natural selection, but he won the war when it came to modelling Darwinist thinking in the twentieth century. His trashing of female choice meant that Darwin’s second great theory of sexual selection became ‘the mad aunt in the evolutionary attic of Darwinian theory’, which, apart from a couple of exceptions, wasn’t allowed out to play for one hundred years.

Things have changed. The sexual revolution of the 1970s and the impact of feminism on evolutionary biology have helped stir Darwin’s daring idea from its century-long slumber. The idea that females – from birds to fishes to frogs to moths – are able to make sensory evaluations and exercise mate preference has been scientifically proven and accepted. Numerous studies have demonstrated how females in diverse species prefer brighter colours, louder calls, stronger odours and faster dances.

The sage grouse’s beatboxing strut looks as strenuous as it is bizarre. How much energy it costs the cocks is hard to say, but a recent study on great snipe, another lekking species, that courts females by drumming its wings, found they lose almost 7 per cent of their body mass after every daily courtship session.

To really get inside the hens’ heads and figure out what choices they are making, Gail created perhaps the only bird even more surreal than a beatboxing cock: a robot female sage grouse.

Thanks to Gail’s handicraft skills the resulting homespun robo-bird was actually incredibly realistic, apart from the wheels.2 These don’t seem to bother the males, however, who are an indiscriminate bunch. Gail has seen them attempting to mate with dried cowpats when there are no females around – the bar is set pretty low evidently.

Gail’s fembot has to be the ultimate tease. She must arouse the interest of a male, but duck out before things get too hot and heavy. ‘We have had males try to mate with the robot and that does not go well.’ Nevertheless, accidents do happen. Once a fembot lost her head mid-flirt, which was awkward, but fortunately the dumb cock wasn’t bothered. ‘The males don’t know how to react as they don’t recognize it as something, so I just drove her back headless.’

‘We see that the males are adjusting the rate of their display according to the proximity of the females. They’re actually responding and using their energy where it would matter most.Unsuccessful guys blast away at top level all the time and then when it comes to crunch they don’t have much left to put on a great show. And that’s probably a combination of social skills and their own underlying health.’

female choice could be responsible for shaping not only a male’s body and behaviour but also his brain. This idea isn’t new. Darwin himself proposed that sexual selection could, in fact, be responsible for the exceptional evolution of human cognition – especially the more ‘self-expressive’ aspects of human behaviour, such as art, morality, language and creativity.The idea that female choice might have polished the human brain into brilliance would have been the ultimate blow to the Victorian scientific patriarchy – hitting them right between the eyes, where it hurt most.