Too attractive to succeed

Sexual selection is an evolutionary mechanism that favors the appearance of ornaments and elaborate structures, such as the colorful plumage of birds or the menacing antlers of deer. It operates mainly among males that compete for females to reproduce, so it would be expected that the most ornate or attractive individuals in a population would be preferred by females and achieve greater reproductive success.

However, in a recent study based on reproductive data taken since 1984 in pied flycatchers, small migratory birds, we have observed that being the most attractive may not always be the most beneficial.

The origin of sexual selection

It was Charles Darwin, in his book The origin of man and selection in relation to sex (1871), who originally suggested the theory of sexual selection. After enunciating the theory of natural selection – the one that indicates that organisms better adapted to their environment enjoy higher survival rates than those less adapted – he observed that many living beings had ornaments or elaborate weapons, such as colorful plumage or heavy antlers. , which far from helping, seemed to reduce their survival.

Darwin suggested that such traits could evolve if they are favored in mating – either by preference for the opposite sex or by greater access to it – and thus increase the reproductive success of their carriers.

Today we know that individuals with attractive characteristics would carry resources or high-quality genes that would have a positive impact on the survival or reproductive success of both the offspring and the couple. Thus, they allow the genes that determine the selected traits to be transmitted from generation to generation and evolve in the population. In short, if natural selection is about survival of the fittest, we might think of sexual selection as survival of the prettiest.

In general, it is assumed that it is the females who choose and the males who are chosen. Females of most species spend, on average, more time and resources than males in the development and rearing of offspring, including the production of female gametes—relatively more expensive compared to sperm–, gestation and parental care. That is why females generally make a rigorous selection before choosing their sexual partner, while males compete with each other to try to mate with as many females as possible.

However, there is a great diversity of examples in nature that question this generality. The clearest is that of the species where the roles are reversed; that is, the upbringing of the offspring is the total responsibility of the males, while the females are the ones who wear ornaments and compete for the males.

Colored outfits to attract females

The display of conspicuous traits is a signal that provides important information for potential mates, since they usually indicate the health status and genetic quality of an individual.

In the case of terrestrial vertebrates, including birds, melanins are one of the most frequent pigments in their plumage and ornaments. The combination of the different melanic pigments is responsible for a wide range of colors ranging from black to reddish brown.

These colorations are a good study model in the field of sexual selection. Its expression is related to behaviors that confer an advantage during the competition for access to females, such as greater territoriality or aggressiveness.

Young male pied flycatcher (Ficedula hypoleuca).
Aelwyn / Wikimedia Commons, CC BY-SA

In particular, in pied flycatchers, back coloration, a sign of attractiveness and social status, can range from pale brown to the compelling bay black of dominant males. In this species, individuals with darker plumage tend to arrive earlier at the breeding grounds after spring migration, enjoy the best territories and are more successful at mating. Given this scenario, one would expect that being tremendously attractive would be a direct passport to success.

The (high) price of being attractive

The problem with ornaments is that they can be extremely expensive to produce and maintain and individuals must establish what functions they spend their energy resources on. The costs act as a guarantee for potential partners who must trust the signal. Only those individuals of really high quality will be able to afford the costs associated with displaying and maintaining an elaborate ornament.

In the case of birds, including flycatchers, the melanin pigments in their plumage require energy to produce. But, in addition, they must be worn throughout the reproductive season and kept until the next moult several months later. And that is not an easy task.

To understand this, think of reproduction as a chain of events in which costs accumulate. To obtain a good quality territory, attractive males will need to migrate earlier or faster, quite possibly experiencing the adverse weather conditions typical of early spring.

Then, they must defend their territory against other competitors, whose usurpation attempts will accumulate in the good quality territories that were occupied first.

Finally, it will be time to feed the offspring and protect them, not to mention protecting yourself from striking plumage visible to predators.

At the midpoint is virtue

Faced with such a level of demand, bluffing your own abilities or finding yourself in an environment that was not what you expected can be very expensive. An intense cold snap in early spring or an unusual number of competitors or predators can cause the cumulative costs of having conspicuous plumage to skyrocket.

When it comes to feeding and protecting offspring, the most attractive individuals will be exhausted, reducing care for their offspring and compromising their survival. This is what appears to be happening in the study population of pied flycatchers.

The result is that evolution rewards males of intermediate attractiveness, who will obtain greater reproductive success in terms of offspring, thus preventing the selected traits from evolving out of control. And it is that, as Aristotle proclaimed, “virtue is at the midpoint”.

The slow pace of science

We live in a world dominated by speed, where immediacy and replacement occupy a privileged place. Science, as part of society, has been infected by the cult of speed when it comes to generating and applying knowledge. The problem is that many of the most important questions in ecology and evolution that affect individuals, populations, and species take years or decades, rather than hours or weeks, to unravel.

This is where the importance of long-term studies lies –especially in species under natural conditions–, those where data collection lasts for years or even decades in the case of the longest-lived species and which usually generate results long after from its start. These types of studies are the basis of much of our knowledge on evolutionary aspects, such as senescence, resistance to diseases, adaptation to the environment or the costs of reproduction. But they have also made a decisive contribution to better understanding the negative impact of climate change, the loss of habitats or the overexploitation of natural resources.

In the case of flycatchers, our long-term study has allowed us to see evolution in action and understand how sexual selection has shaped the characteristics of individuals according to environmental conditions. This lays the groundwork for understanding how species will adapt in the face of future environmental changes.

Quite possibly, most of the above questions were not in the imagination of the scientists who began the studies. That is, perhaps, the great value of long-term studies and so-called basic science: equipping us with tools and information to solve problems that we cannot yet predict or imagine.

In the spring of this year, after 38 years of continuous research, the flycatchers will return to the forests of La Hiruela (Madrid) and we will be there waiting for them. The continuity of our work will undoubtedly lead to new and exciting scientific discoveries.

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