Battle of the Sexes – Part A

Editor’s note: The phrase ‘Battle of the sexes’ refers to the conflict between male and female gender roles. In a way, it is a struggle for power in any given situation while taking into account the social, physical, and biological differences innate to the two sexes. Read Manaswini’s 2-part series to know how this battle extends to the animal kingdom and plays a huge part in evolution.

Evolution is a gradual process that has taken billions of years for life to exist in its present form. It acts by selecting organisms who are better adapted to their environments, be it constant or changing at any given time scale. It does so by using spontaneous mutations, among other evolutionary forces, to its advantage either to flourish or wipe out populations, and this continuously changes life forms. Survival and reproduction are essential for all living organisms. Therefore, natural selection acting on these traits dictates the evolutionary trajectories a given population takes, over time. Naturally, traits that aid survival are very important, but these get valued further when individuals reproduce and pass on heritable genes. 

For sexually reproducing organisms, mating with the opposite sex is as crucial as finding food and shelter for survival.

Given the significance of mating, nature provides us a wealth of visuals to appreciate the extent to which sexually reproducing organisms invest in securing a partner. Be it the shiny and gorgeous plumage of birds, flashy colors of fishes, singing and dancing in many avian species or fighting between males in various insects and mammals, gorgeous flower colors to attract pollinators, one thing that stands out is the selection for characters that enhance the ‘visibility’ of a particular sex to attract potential partners thereby increasing the probability to mate and reproduce. Sexual selection^1-3(mostly occurring in non-monogamous mating systems) is not merely a special case of natural selection⁴; it acts on variation in reproductive success among individuals of any given species whereas natural selection acts on variance in all other fitness components. So, what is sexual conflict⁵? As the term ‘conflict’ suggests it is a sex battle that arises when the two opposite sexes have conflicting reproductive strategies. Simply  put, their reproductive goals are different. While mating is mostly cooperative, it involves plenty of competition within and between sexes. The competition between sexes is relatively straightforward to understand and that’s because although both males and females are hardwired to try and reproduce offspring that can survive and further reproduce – a common goal – but they do this in totally different ways! For most males, it is all about quantity, the more the partners they get to court and mate, the better it is for them to attain a greater reproductive fitness. Sperms being physiologically easy to mass produce relative to eggs help the males attain their higher fitness. Additionally, most males in the animal kingdom spend little or no time and energy in parenting. Exceptions, such as jawfish⁶ (where the males guard the eggs in their mouth until they hatch) and seahorses⁷(where the males get ‘pregnant’ i.e., they carry the eggs in a brood pouch until they hatch) are the gentlemen of the animal kingdom!

https://www.youtube.com/watch?v=0auqYct-K9Q?start=243

Watch the video above to see Adelie penguins^8-10 serve as fine examples of each parent going to extraordinary lengths to raise their offspring in one of the harshest climates of the planet – the Antarctic. It begins with the males traveling thousands of miles to the nesting area in order to prepare the nest for their mate, where they pile stones high up to make sure the eggs do not freeze. When the female mates arrive, both father and mother take turns to keep the egg warm and once the eggs hatch, both parents take turns to go fishing. There is always one parent beside the offspring to guard! 

https://youtu.be/FOrvbE06Mps??start=133

Another interesting Avian species is Namaqua Sandgrouse^11,12 found in arid regions of the Kalahari desert in Southern Africa. Click on the link above to see the father birds take responsibility of keeping the newborns hydrated. Once they find a water source in the desert, they absorb the water in their stomach feathers, fly back to the offspring and let them drink all the water as much and as long as the young can.

Turning to amphibians again, watch the video above to see African bullfrog fathers ^13,14 guarding their eggs and protecting the tadpoles from predators. They also dig canals for the tadpoles to move from drying up ponds to water-filled ones.

The Darwin’s frogs ^15,16 are also another fascinating example exhibiting parental care where the males guard the eggs during development, followed by ingesting the eggs and holding them in his vocal sac. Still paternal care appears restricted to a few species while considering the entire animal kingdom; it’s mostly a single mother bringing up her offspring. 

For males the best strategy is to mate in bulk: the more the number of mating events, the greater the chances of passing on the genes to the next generation. Females, in contrast, have a limited supply of eggs in addition to the huge time and energy investment that goes into parenting. Being in an energy intensive and expensive process, females strategize mating and want the most ‘fit’ males. While some choose to mate with the biggest, flashiest or strongest, the rest choose to court with multiple males and then choose a winner!

Notably, the choosier females are, the harder it is for the males to get her to mate and reproduce,  and this is where sexual selection comes to play.

In short, the more promiscuous males are, the choosier females get, and the more intense the sexual selection arms race. Males have evolved ways to restrict female discretion and in turn females have evolved to resist them. It is, therefore, a battle of the sexes and indeed a very fascinating biology to study ^17-20. Extensive research and thorough studies over decades have laid down some strong grounds of sexual selection in general and as a potent driver of population divergence and speciation. Keep watching this space for the next part to learn more about the fundamentals of sexual selection. 

Author

Illustrator

Bhrugu is passionate about communicating the science in a creative way. He loves to create simple but effective write-ups and illustrations that present the scientific story in a fun and impactful manner. Have a look at his SciComm portfolio on his website; Art BY Scientist. Currently, Bhrugu is working as a postdoctoral fellow at Emory Vaccine Centre, Atlanta, USA and trying to cure HIV/AIDS. He gets the energy to do all these different things by meditating in his soul laboratory where he attempts to bring amalgamation of science and spirituality. Would you like to connect with him? If so, you can find him on LinkedIn and Twitter.

Editor and Blog design

Dolonchapa is a Postdoctoral Fellow at NYU Langone working on Infectious disease with a focus on cell wall metabolism to identify new targets for therapeutic attacks by Pseudomonas aeruginosa, a common opportunistic human pathogen. She also serves as the Co-Chair of the National Postdoctoral Association’s Outreach Committee. She believes in the power of technical storytelling as an effective tool for scientific outreach and looks forward to practicing this art as an editor at Club SciWri. Follow her on Twitter.  

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References:

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