SEXUAL DIMORPHISM: WHAT IS IT, IN VERTEBRATES AND INVERTEBRATES - BIOLOGY - 2025

The sexual dimorphism is observed phenotypic differences between males and females of the same species. These distinctions are not limited to the field of morphology (such as body size, color, among others), they also include characters at a physiological and ethological level. In contrast, when the individuals of both sexes in the same species are similar or identical in appearance, the opposite term is used: monomorphic species.

These characteristics that allow differentiating between the sexes are usually more prominent in males than in females - although there are important exceptions - and are considered adaptive. It is proposed that these traits increase the biological fitness or fitness of each sex, maximizing reproductive success.

Pair of mandarin ducks (Aix galericulata), in England. The male is on the left, and the female is on the right.

Source © Francis C. Franklin / CC-BY-SA-3.0

These characteristics increase biological fitness in two ways: by being more attractive to the opposite sex (such as the colorful colorations of birds, such as peacocks and birds of paradise) or by serving as weapons to fight in encounters with individuals of the same sex (like horns) and the reward is access to the opposite sex.

Although the term has been used mostly in zoology, the phenomenon has also been reported in plants.

What is sexual dimorphism?

The word dimorphism means "two forms." Therefore, sexual dimorphism refers to the differences between individuals of both sexes in the same species.

Sexual dimorphism begins to be more noticeable as the organism grows and develops. Generally, in the premature stages of the organism, the appearance between the sexes varies insignificantly.

Sexual characteristics that appear after the age of sexual maturity are called "secondary sexual characteristics." The primary sexual characteristics, for their part, are directly related to the reproductive process: the sexual organs.

Differences between males and females can occur at different levels: morphological characters (anatomy, size, coloration), physiology, ecology, behaviors, among others.

For example, in some species the males are large and colorful, and the females are small and cryptic in color. Similarly, there are behaviors that only occur in one sex and that we never see in the other.

Evolution of sexual dimorphism

Why are certain traits unique to one sex, in one species? Why are there species with sexual dimorphism, while in other phylogenetically close groups they are monomorphic species?

These questions have stimulated the curiosity of evolutionary biologists, who have proposed various hypotheses to explain them. As we shall see below, the mechanisms of natural selection and sexual selection seem to satisfactorily explain this widely distributed phenomenon in the natural world.

Role of sexual selection

Understanding the mechanisms that led to the emergence of sexual dimorphism has fascinated evolutionary biologists for decades.

Already in Victorian times, the famous British naturalist Charles Darwin began to hypothesize about it. Darwin believed that sexual dimorphism could be explained through sexual selection. In this context, evolutionary forces act differentially in the sexes.

These differential characteristics give an individual advantages over his companions of the same sex and of the same species, in terms of the opportunities to find a mate and to copulate. Although there are some questions related to sexual selection, it is a crucial component of evolutionary theory.

Why are males usually flashy and females not?

Sexual selection is capable of acting with considerable force and bringing physical characteristics to a naturally selected optimum.

As the variation in reproductive success in males is typically greater than in females (this occurs predominantly in mammals), sexually dimorphic traits often appear in males.

An example of this are the striking colorations in the fur, ornaments and weapons to face their male opponents and attract females.

Exceptions

Although it is the most common, exaggerated and colorful features in males are not ubiquitous in all living organisms. Competition for reproduction between females has been reported in several species.

Therefore, it is the females who exhibit arbitrary exaggerated traits related to the increase in success in finding a partner and being able to reproduce.

The best known example of this phenomenon occurs in the bird species Actophilornis africanus. The females have larger bodies than the males, and they fight in intense battles to get a mating opportunity.

Role of natural selection

Other traits seem to be better explained using the mechanism of natural selection than sexual selection itself.

For example, in the Galapagos Islands live a variety of finches belonging to the genus Geospiza. In each species, the morphology of the beak varies between male and female members. This fact is explained by the different eating habits that characterize each sex in particular.

Likewise, natural selection is able to explain differences in animal sizes - females generally have larger body sizes and masses.

In this case, the larger size energetically favors the gestation and lactation processes, in addition to allowing the birth of larger individuals.

In conclusion, the characters that differentiate individuals of both sexes could arise both by natural selection and by sexual selection. However, the boundary between the two is difficult to establish.

Today, it is considered that the degree of sexual dimorphism existing in certain species is the result of the existing differences in terms of the sum of all the selective pressures that affect males and females differentially.

Ecological causes

An alternative view tries to explain the appearance of sexual dimorphism in nature. It focuses on the ecological causes of the process and how different sexes are adapted to different ecological niches.

This idea was also featured in Darwin's writings, where the naturalist doubted whether or not gender-specific ecological adaptations were common in nature. This hypothesis, related to the ecological niche, has been tested mainly in birds.

Differences in phenotypic traits translate into niche separation. This fact leads to a decrease in intraspecific competition (within the same species).

In animals

In the animal kingdom, the phenomenon of sexual dimorphism is quite common, both in vertebrates and invertebrates. We will describe the most relevant examples of each lineage.

In vertebrates

In vertebrates, sexual dimorphism is present at the physiological, morphological and ethological levels.

Fishes

In some species of fish, males have bright colorations that are related to the courtship of the opposite sex.

Some fish have struggles between males to gain access to females. There is no general pattern of size between the sexes; in some species the male is larger, while in others the female has the largest body size. There is an extreme case where the male is 60 times larger than the female.

Amphibians and non-avian reptiles

In amphibians and reptiles, the level of sexual dimorphism varies widely depending on the lineage studied. In this group, the differences are usually present in the size, shape and coloration of certain structures. In anurans (frogs), males display melodic night songs to attract potential mates.

Birds

In birds, sexual dimorphism is manifested in plumage coloration, body size, and behavior. In most cases, the males are larger than the females, although there are marked exceptions.

The vast majority of males exhibit bright coloration and a significant variety of ornaments, while females are cryptic opaque colors. As we mentioned earlier, these differences are mainly due to unequal contributions during the reproductive process.

Males often do complex courtship displays (such as dances, for example) to find a mate.

Such a marked coloration and the presence of protruding structures are believed to indicate to the female the physiological state of the male - since a dull coloration is related to the presence of pathogens and poor health.

In species where the contribution to reproduction and parental care are similarly distributed in both sexes, the dimorphism is less pronounced.

Mammals

In mammals, the male is usually larger than the female, and this difference is attributed to the mechanisms of sexual selection. The differences found between both sexes are dependent on the species studied, so it is not possible to establish a general pattern.

In invertebrates

The same pattern that vertebrates exhibit, we observe it in invertebrates. Different species vary in terms of body size, ornaments, and colors.

In this lineage, competitions between males are also observed. In some butterflies, the males have striking iridescent colorations and the females are white.

In some species of arachnids, females are significantly larger than males and exhibit cannibalistic behaviors.

In plants

The term sexual selection has been widely used by zoologists. However, it can be extrapolated to botany. The differences are quite marked in terms of general characteristics and not very significant when we focus on secondary sexual characteristics.

While it is true that most flowering plants are hermaphrodites, sexual dimorphism has evolved in different lineages with separate sexes.

References

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4. Geber, MA, & Dawson, TE (1999). Gender and sexual dimorphism in flowering plants. Springer Science & Business Media.
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6. Kelley, DB (1988). Sexually dimorphic behaviors. Annual review of neuroscience, 11 (1), 225-251.
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