For many years, it has been been thought that living beings were basically organisms that are born, grow, reproduce and die, or that they were organisms that fulfilled their physiologic functions: nutrition, relation and reproduction. Such statements are, with some nuances, valid definitions of a “living being” and they all share a common denominator: they mention the reproductive function as a characteristic. This gives us a hint of the importance of such function for individuals. In this article, we are going to discover why sexual selection is so essential.
Most individuals — if not all — of every animal species engage in some type of sexual behaviour throughout their lifetime. The objective of such behaviour in individuals is to pass on their genes to the next generation, ensuring their viability. However, their success will depend on the different sexual strategies of each species and gender, as well as on each mating system:
Reproductive strategies in males and females
The reproductive strategies of males and females are related to the characteristics of their respective gametes: while sperm cells are abundant, “cheap” and easy to replace, ovules are scarce and more costly to obtain; this leads males and females to carry out different reproductive strategies. Nonetheless, the role of each gender during reproduction is determined by environmental factors (for example, by the distribution of resources) and vital cycles (if oestrus is synchronised or not).
Generally, males try to copulate as many times as possible and compete over females, while females procure the best care for their offspring and choose the most fit males. To do so, they analyse the parental care that the male can provide and they favour males with attributes that indicate genetic quality.
There is, therefore, a conflict between males and females, as the former try to mate indiscriminately, whereas females act more intentionally. For certain species, the conflict is even more severe: in some insects, for example, the males’ semen contains substances that affect the females’ brain, making them behave in a way that increases their chances to reproduce successfully, but this substance also shortens the female’s life.
For this conflict between males and females not to be insurmountable, there are several mating options:
A system through which a couple establishes an exclusive sexual relationship, particularly during reproductive cycles, that can last an entire lifetime. This system is predominant amongst birds (over 90%), although some intercourse with other mates is also common. Monogamy occurs when:
- Males are unable to monopolise females or resources, or when mating with several females has no reproductive benefits.
- The likelihood of successful breeding is higher with the help of both parents, and males can contribute to the raising of the offspring.
A system in which a male monopolises a harem of females and mates with them on a regular basis. The females have an exclusive sexual relationship with said male. This is the most common system amongst mammals (over 80%) and uncommon amongst birds (barely 2%). It occurs when males are able to monopolise females (no oestrus synchronisation) and/or resources.
In this case, it is the female who establishes exclusive mating relationships with several males. Polyandry is a common mating system amongst fish, as well as amongst some males and birds. It occurs when:
- Food is scarce, or in areas with an abundance of food, but that have been affected by a natural disasters.
- Males assume the care of their offspring, or when the male ratio is a lot higher.
Both sexes mate randomly with different individuals. This system occurs in many pelagic fish, and when there are large amphibian populations in small ponds. It also occurs amongst some birds, but to a lesser extent (barely 6% of bird species).
Female mate choice
For mating to take place, individuals have to pair up first. Females take several factors into account in order to choose a mate:
- Hunt for natural resources. The likes for a male to be chosen by a female are higher when it defends a territory with abundant resources (water, vegetation, etc.). This type of choice occurs, for example, in frogs.
- Good genes. Females tend to choose males that are easy to spot. This is why males use bright colours, ornaments and extrinsic elements during courtship. The flame bowerbird (Sericulus aureus) decorates the nest it builds during courtship with a variety of gifts (fruits, feathers, flowers…), increasing its likelihood of being selected.
Consequences of sexual selection
Due to the different strategies and factors explained in this article, sexual selection has a series of consequences for species and individuals:
- Sexual dimorphism, meaning variations in external physiognomy amongst males and females of the same species. It tends to be more pronounced in polygamous species.
- (Apparent) conflict with natural selection. The high reproductive success of males with exaggerated features compensates for other costs: for example, having bright colours may be an advantage in sexual selection, but it can also cause a higher predation rate.
- Conflict between growing and maturing. Specifically amongst polygynyc males, there is intrasexual competition (individuals of the same sex compete over the opposite sex), which leads to a later sexual maturity that females, as well as a shorter reproductive period.
- Differences in reproductive potential. Such differences are very significant amongst polygynyc males, and can considerably increase if their behaviour entails high risk. For example, a deer can increase its reproductive potential if it fights and competes with other males over a harem of females, but this also increases their mortality rate.
Translated by Carlos Heras