Written By: Amaan Danish
Email: ammandaud@gmail.com
Introduction
Imagine you're trying to find the coolest outfit to wear to a party. You want to look your best to impress your friends, right? Well, animals do something similar when they're looking for a mate! This is called sexual selection.
Consider the majestic and elaborate tailpiece of a peacock. (fig.1) This trait hinders the escaping chances of peacocks by conspicuously drawing attention of predators and significantly decreasing a peacock’s chances of survival. It only exists for the sole reason of aiding a peacock in reproduction. Females choose to mate with the peacocks with the most impressive tails!
Sexual selection is when animals develop special traits or behaviours to attract a mate. It's like a competition to see who can be the most impressive!
Why Do Animals Do This?
Animals want to pass on their genes to their offspring. By choosing the best mate, they increase the chances of having strong, healthy babies. That's why they go to such great lengths to impress each other!
Figure 1. Rather than aiding in survival, the peacock’s tail aids in his reproduction.
Mechanisms of Selection
Sexual selection takes two major forms: intersexual selection (also known as ‘mate choice’ or ‘female choice’) in which males compete with each other to be chosen by females; and intrasexual selection (also known as ‘male–male competition’) in which members of the less limited sex (typically males) compete aggressively among themselves for access to the
limiting sex. The limiting sex is the sex which has the higher parental investment (typically females), which therefore faces the most pressure to make a good mate decision.
The Concept of Sexual Selection and its Role in Animal Behaviour
The success of reproduction is not solely determined by the number of offspring but by their ability to survive and reproduce themselves.Sea turtles lay hundreds of eggs, but only a few hatchlings survive. Those that do grow up and lay their own eggs are considered successful. This concept highlights the importance of offspring quality over quantity.
Parental investment is another crucial factor. Males and females often contribute differently to offspring care. For instance, in some species, males provide significant parental care, while in others, females bear the primary responsibility. This disparity in investment can influence mate choice and competition among males.
Bateman's principle suggests that females typically invest more energy in reproduction than males, making them choosier in selecting mates. This differential investment creates competition among males for access to females.
Figure 4.The two graphs produced by Bateman in his two series of experiments. Dashed lines represent females while solid lines represent males.
In conclusion, sexual selection is a complex interplay of factors including sexual ornaments, reproductive success, parental investment, and mate choice. These elements collectively influence the evolution of species and their behaviors.(YouTube video titled "What is Sexual Selection? | Theory of Sexual Selection in Animal Behaviour | CSIR NET Life Science)
Sexual Dimorphism
Sexual dimorphism refers to the noticeable differences between males and females of the same species beyond their reproductive organs. These differences can range from size and coloration to behaviour and physiology. Sexual dimorphism is often driven by sexual selection, the process where individuals compete for mates and reproduce successfully.
Figure 5. Visualisation of some aspects of sexual dimorphism
Sexual dimorphism, shaped by sexual selection, significantly influences animal behaviour:
Mating Systems: Differences between males and females often lead to distinct mating systems, such as polygamy (one male with multiple females) or monogamy (one male and one female).
Parental Care: Sexual dimorphism can influence parental roles. For example, in some species, males provide most of the parental care while females focus on reproduction.
Social Structure: Differences in size and behaviour between sexes can impact social hierarchies and group dynamics.
In conclusion, sexual dimorphism is a fascinating aspect of the animal world, intricately linked to sexual selection and behaviour. It highlights the diversity of strategies animals employ to increase their reproductive success.
The Modern Era of Sexual Selection Research
The concept of sexual selection was first introduced by Charles Darwin in his groundbreaking book, The Descent of Man, and Selection in Relation to Sex (1871). While his primary focus was on natural selection, he recognized that certain traits, like the elaborate plumage of peacocks, seemed to contradict the principles of survival. He proposed that these traits evolved due to a separate process, which he termed sexual selection.
In recent decades, research on sexual selection has exploded. Scientists have used a variety of methods, including observation, experimentation, and genetic analysis, to study this fascinating phenomenon.
Ethology: The study of animal behaviour has provided rich insights into the courtship displays and mating systems of various species.
Experimental Evolution: Researchers have conducted experiments to manipulate mating conditions and observe the resulting changes in traits.
Genetics: Advances in genetics have allowed scientists to identify the genes involved in the development of sexual ornaments and behaviours.
Key Findings:
The importance of female choice: Researchers have shown that female preferences can drive the evolution of extravagant male traits.
The role of sensory biases: Some female preferences might be based on pre-existing sensory biases, such as a preference for bright colours.
The costs and benefits of sexual ornaments: Elaborate ornaments can be costly to produce and maintain, but they can also signal male quality.
While the concept of sexual selection is over a century old, it continues to be a vibrant area of research, with new discoveries made all the time.
History of Sexual Selection Research
The history of sexual selection research shows prevalent male precedence which has, in particular, delayed knowledge not only about sexual selection in females, but also more broadly about reproduction and evolution in both females and males. This prevalent male precedence can partly be explained by the biological features that are more obvious and more amenable to study, but is also partly due to cultural gender bias. History also shows repeatedly that abandoning stereotypic assumptions about passive females has led to progress in the field.
The quantification of publications in sexual selection shows that studies have historically and continue to focus much more on sexual selection in males than in females. This result in part reflects that sexual selection is often stronger in males, but also that a lot of what females do in terms of competition for reproduction is not included in the prevailing narrow-sense definition of sexual selection—such as female-female reproductive competition for resources other than mates, breeding opportunities to conceive or rear young and laying eggs in other females’ nests. The focus on narrow-sense sexual selection has perpetuated ignorance about the selection of females. Thus, sexual selection research is still heavily focused on sexual selection in males while sexual selection in females is understudied. Awareness of this bias and increased efforts to counteract it would result in a more comprehensive understanding of evolution. The history of sexual selection illuminates how we can learn to recognize biases and identify knowledge gaps, suggesting strategies for alleviating biases in this field and beyond.( Ah-King, M. The history of sexual selection research provides insights as to why females are still understudied. Nat Commun 13, 6976 (2022). https://doi.org/10.1038/s41467-022-34770-z
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Reprinted from Bateman AJ (1948) Intra-sexual selection in Drosophila. Heredity 2: 349–368, with permission from Macmillan Publishers Ltd. Bateman's Principles: Original Experiment and Modern Data For and Against
The Nilesh Soni. “What Is Sexual Selection? | Theory of Sexual Selection in Animal Behaviour | CSIR NET Life Science.” YouTube, 10 Apr. 2024, youtu.be/AdSdAy2Okm0?si=hYSt4KsBcFLmFXcz. Accessed 18 July 2024.
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Ah-King, Malin. “The History of Sexual Selection Research Provides Insights as to Why Females Are Still Understudied.” Nature Communications, vol. 13, no. 1, Nov. 2022, p. 6976, https://doi.org/10.1038/s41467-022-34770-z.
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