You may have heard of Darwin’s theory of evolution and natural selection; heritable variations among individuals of a given species lead to differential reproductive success. The most adaptive heritable traits then spread out through the population over time, and the less adaptive ones disappear. Survival of the fittest. Yes, ok. But can this explain the existence of all traits? For instance, could you tell me why peacocks have developed the admirable ornament of their tail feathers? They can surely compete for a spot among the most elegant tails on the planet, but can we truly describe these feathers as “fit”, in the Darwinian sense? In 1859, Darwin published “The Origins of Species, by Means of Natural Selection”, where he describes mostly his former theory. But even then, he knew a piece of the puzzle was missing. He wondered how unfavorable traits - whether morphological or behavioral - like the peacock's tail, could not be eliminated by selection. He came up with an explanation in 1871 in “The Descent of Man, and Selection in Relation to Sex”.

Parental investment and paternity uncertainty

Before diving into it, let us navigate through some fascinating concepts. An almost universal feature that differentiates males and females rests in the size of their gametes, the reproductive cells (egg cell and sperm). Female gametes are typically larger than male gametes, an observation called “anisogamy”. This seemingly futile difference does matter, as it involves that for the same amount of resources (time and energy/metabolism, which are both limited), males can produce more gametes than females can, rendering female gametes rarer. Thus, the cost of parental investment for a descendant, which is the ultimate goal in the lens of evolution, is cheaper for males than for females.

In addition, another crucial parameter entering the parental investment equation is the time devoted to caring for the baby. For instance, in seahorses, females deposit their eggs into a specialized brood pouch on the male's body, which will then provide protection, oxygenation, and nourishment to the developing embryos, until they give birth. This example of male parental investment is not the only one in the animal kingdom, but is nevertheless exceptional; in the vast majority of species, females are responsible for the pregnancy and giving birth to their offspring, which is also often followed by a disproportion in the care provided after birth. This asymmetry in parental investment is, in addition to the anisogamy previously described, partly due to a phenomenon called paternity uncertainty. What does that mean? In species with internal fertilization, males can never be 100% sure that their offspring is theirs, while females can, since they carry their descendants in their bodies and give birth to them. Therefore, from the male standpoint, providing time and attention to a baby that is potentially not your own instead of trying to spread their genes elsewhere is an evolutionary catastrophe. Thus, every time males took care of descendent that were actually not theirs, their genes for “paternal behavior” have not been transmitted to the next generation, hence rendering it rarer over time throughout the population. On the other side, taking care of a baby they know is theirs has been a good strategy for females, thus the genes for “maternal behavior” have been favored through evolution.

Intra-, inter-, and Sexual selection

Knowing how anisogamy and paternal uncertainty affect parental investment, we can now understand that, for most mammal species, females invest more resources than males in their offspring, namely in the form of embryonic in-utero development, breastfeeding, and caregiving. This investment takes time, which can therefore not be dedicated to other activities, such as reproducing again, whereas males are basically always ready to dance. This leads to an interesting situation; being busier than males, females become a scarce and limited resource for mating. That means that, statistically, for each female ready to reproduce, more than one male is waiting on the dancefloor.

Now let’s step back for a minute and do a thought experiment. Imagine you are trying to sell something, but you don’t have any buyers. In this case, you would probably agree to sell your product for a price cheaper than initially hoped. Now let’s suppose that you found the right formula and that plenty of people desire what you offer; you do not need to be a phenomenal businessman to realize that you can demand more from your potential customer. They will compete among themselves to offer more than what others can afford, and eventually, you will choose the one that provides the best offer.

This situation is not so different from what we observe in the animal kingdom. When members of one sex compete for the other, a limited resource, and this other sex is in the position to choose, the conditions are fulfilled for what Darwin called sexual selection. As stated in the description, the theory contains two components: on one hand, the competition among individuals of the same sex, also called intra-sexual selection. On the other hand, the choice made by individuals of the other sex over the former, referred to as inter-sexual selection. For the reasons previously discussed, males are often the ones competing and females the ones choosing.* Nonetheless, in species like seahorses where the total parental investment is reversed and males are “busier” taking care of their descendants, they become the scarce resource, and the pattern reverses; they become the ones choosing, and the females compete.

The competition between individuals of the same sex can take many different forms, such as fights, ornaments, or even constructions. The underlying goal is to impress the other sex in order to be chosen. In the case of fights, it seems pretty self-explanatory that the one who wins signals that he is stronger and can provide “stronger genes” for his descendants than the male he just beat. In most cases, however, it is more subtle. Ornaments signal an implicit message associated with better genes for the descendant. As these ornaments can be unfavorable regarding predation, and thus appear in contradiction with the theory of natural selection at first, they must bring another advantage in reproduction, since the most ornated males are preferred by females. The higher reproduction rate compensates for the disadvantage of the ornaments in survival. Let’s have a look at some species in the natural world.

Swallows and peacocks

Male and female swallows are similar in many aspects, but males have on average a longer tail. In a study done in 1989, the experimenters compared 4 groups: male swallows with shortened tails, some with elongated tails, and two control groups with normal tails. The results show that males with shortened tails take more time (14 days on average) to find a female than control animals (10 days on average), while males with lengthened tails take very little time (3 days on average). We can therefore deduce that females prefer males with longer tails. But why? The experimenters went further and showed that the offspring whose biological fathers have long tails were more resistant to mites and parasites than those whose fathers have shorter tails. The exact reason is difficult to determine, but from this study, it seems that there is a positive correlation between feather length and genetic resistance to parasites as feather growth has a metabolic cost that is better insured in a bird with few parasites. This study illustrates one potential reason underlying the reproductive benefit conferred by ornaments.

And what’s up with the peacocks then? The long feathers constituting their tail are certainly not an attribute favoring their survival as it makes them more visible to predators and comes with a higher energetical cost. However, it has been demonstrated in an experimental study in which the researchers manipulated the number of “eyes” visible in the peacocks’ tails that the reproductive success, measured in matings, of mutilated males was reduced compared to the controls, even though this variable was not the only one influencing the females choice. The reason why females prefer males with longer tails is not clearly established, but it is thought that the flourished train indicates a healthier state and thus better genes for the offspring.

Conclusion

To sum up, Darwin’s theory of natural selection is not limited to the survival of the fittest, but also encompasses sexual selection; in both, there must indeed be heritable variations in individuals, causing differences in the number of offspring surviving to reproductive age. However, sexual selection specifically targets the differences between individuals due to competitors (of the same sex), and potential partners (of the opposite sex) who choose them or not. As a result, sexual selection enables us to understand why some characteristics such as ornaments or complex mating behavior, which can seem useless or even detrimental, have been conserved through evolution, and to account for the important role of sociosexual selective forces within each species.

 * It has to be noted, however, that in certain species organized with social structures where the competition among males is particularly violent, such as in Sea Lions, Deer, or many of our ape cousins, the situation is different. The most dominant male, often call the alpha male, has a huge influence on the group and has more opportunities to mate with females, whether they choose him or not, thereby contradicting the theory.

Author: Pablo de Chambrier

References

Pape Møller, A. (1989). Viability costs of male tail ornaments in a swallow. Nature, 339(6220), 132-135.

Dakin, R., & Montgomerie, R. (2011). Peahens prefer peacocks displaying more eyespots, but rarely. Animal Behaviour, 82, 21-28