Why do individuals of the same animal species have different morphologies according to their sex?
However, this is not so simple. Both the male and the female have to find the ideal candidate, in addition to making the individual of the other sex pay attention to them and want to reproduce with them.
In addition, it cannot be ignored that other males and females are also looking for the same goal, willing to compete to achieve success.
Sexual dimorphism refers to the differences that occur within a species between males and females , allowing them to be distinguished with the naked eye as well as influencing the way they behave both in breeding and in the search for a sexual partner.
In this article we are going to see this concept in more depth, in addition to relating it to some of the first evolutionary hypotheses around it, different types of dimorphism and what is special about the human species.
- It may interest you: “Lamarck’s theory: what is it and what does it say about biological evolution?”
What is sexual dimorphism?
Sexual dimorphism is the set of characters shown by individuals of a species that allows them to be differentiated based on their biological sex. This phenomenon occurs in many species of animals and plants, and allows males and females to be differentiated .
In the animal kingdom, this phenomenon occurs in multiple ways and it is possible to differentiate between the two sexes based on their size, colors, behavior and morphology.
There are very few animal species that reproduce without any type of interaction with other individuals. Some of these species are hermaphroditic or reproduce by budding and bipartition, creating daughter organisms that are practically clones of their parents.
They are the so-called asexual species and, due to the fact that there are no sexes or that a male and a female are not necessary for reproduction to take place, sexual dimorphism does not occur.
In contrast, in the case of most animal species, which are usually sexual animals, reproduction is carried out by two types of organisms: males and females. This type of organization is called dioecious and is present in 26 of the 30 known animal phyla , as well as being the exclusive reproductive strategy of at least 17 of those phyla.
Background: Charles Darwin
The phenomenon of sexual dimorphism was already described by Charles Darwin when he was developing the first proposals of his well-known evolutionary theory. The English biologist tried to explain sexual dimorphism.
Initially, he argued that the function of this phenomenon was to enable sexual selection, with dimorphism being an advantage in, for example, competitions between males to be able to mate with a female. Along with this explanation, Darwin indicated that it could also be an evolutionary mechanism to favor food competition.
It must be said that in many species, such as the peacock and other birds, sexual dimorphism can be a disadvantage to a certain degree, since it usually manifests itself in the form of very striking characters. The brightly colored male peacocks have difficulty camouflaging themselves, making them easy prey for predators.
It should be noted that the concept of sexual dimorphism focuses on the visibly perceptible differences between the sexes of a species. Other factors linked to sex, such as the production of certain hormones and reproductive cells, are not considered as elements that constitute sexual dimorphism by themselves.
Sexual dimorphism in the animal world
Among animals in which two sexes exist, there are many different types of sexual dimorphism.
Each sexual species has developed evolutionary mechanisms that manifest themselves during courtship , that is, when searching for a sexual partner, to guarantee reproductive success.
Whether to get the attention of the female or simply to prevent another individual from mating with her, males show their attributes to get a mate. In this section we are going to see some examples of sexual dimorphism.
1. Polygynous animals
Polygynous animal species are those that are organized in groups of individuals, where there are many females and few males. The male is responsible for fertilizing the females in order to have the maximum number of descendants .
If another male appears, the two males will have to compete with each other to gain control of the group of females. Therefore, in this type of situation, having a large size and greater strength are aspects that influence success in the reproductive process.
The males of polygynous species are larger than the females, in addition to having some type of organ that they use in competition with other males, such as horns.
Some examples of polygynous animals are deer, gorillas, chimpanzees, antelopes, and lions.
2. To differentiate from each other
There are species in which sexual dimorphism may not be so important in the competition for a sexual partner, but it does acquire it to be able to distinguish between males and females .
A case of this type of dimorphism is that of some birds, such as parakeets. The males of these birds have a blue and smooth beak, while the females have a brown and wrinkled one.
Experiments have been done in which if it colors the bill of the males as if it were a female one, other males will court it. In the opposite case, when painting the beak of the female with masculine colors, males will come to attack her.
3. Better sexual performance
The males of some species have characteristics that allow them to have a better grip during copulation, as is the case with amphibians such as toads and frogs. In these species, the arms are muscular , allowing the female to be grasped with greater fixation.
4. Larger females
In birds such as owls, eagles and hawks, it is the females that are larger than the males.
This is because the females of these birds are the ones that spend the most time taking care of the chicks. For this reason, they need a larger size to provide protection and warmth to the eggs and newborns.
There are also differences in their behavior. The females are more aggressive than the males, as this ensures that no intruder enters their nest.
In arthropods, that is, insects, spiders, crustaceans and myriapods, the female is also usually larger than the male, and the same is true in many species of reptiles and amphibians.
Being of considerable size, they can produce hundreds or even thousands of eggs.
Dimorphism in the human species
In the case of our species, men and women have anatomical and behavioral characteristics that clearly differentiate them. However, it must be said that human sexual dimorphism is not as striking as it is in species such as peacocks , praying mantises or gorillas.
In most cases, men, upon reaching puberty, show an increase in muscle, in addition to beginning to form hair in the armpits, face, pubis, legs and other parts of the body. The same happens in the case of women, but not in such quantity. Although, the most remarkable character that differentiates women from men in our species is the development of the mammary glands during puberty.
It must be said that in our species the individuals of one or the other sex can be more or less masculinized/feminized. That is why cultural differences and beauty canons can be important when it comes to socially defining what is meant by a prototypical man or woman, beyond the anatomical and chromosomal differences that may occur.
However, what is remarkable about the human species is its sexual dimorphism in terms of behavior . Although, as we have already said, there may be sociocultural factors that influence the way in which a woman and a man are interpreted, there are also biological factors that influence human behavior depending on whether one is male or female.
in the plant world
In plants there are few species that have sexually differentiated individuals. Most plants are hermaphroditic, with only 6% of plant species known to have a dioecious organization .
Even so, the flowers of males and females are usually very showy, especially in species that reproduce using insects and hummingbirds as instruments. This is because, for example, a butterfly will have to go through the male flower that produces pollen as well as the one that has the female organs to fertilize.
Bibliographic references
- Carranza, J. (1994). Mating systems and sexual selection. Ethology: An Introduction to Behavioral Science, 363-406.
- Fairbairn, D. J. (2013). Odd couples. Extraordinary differences between the sexes in the animal kingdom. Princeton, Estados Unidos. Princeton University Press.
- Hedrick, A. V., y Temeles, E. J. (1989). The evolution of sexual dimorphism in animals: hypotheses and tests. Trends in Ecology & Evolution, 4(5), 136-138.
- Hiraldo , F. , Alvarez , F. , & Valverde , JA (1974). Biometry and sexual dimorphism in the squid (Porphyrio porphyrio). Doñana Acta Vertebrata, 1, 103-108.
- Shine, R. (1989). Ecological causes for the evolution of sexual dimorphism: a review of the evidence. The Quarterly Review of Biology, 64(4), 419-461.
- TOC (2008). Sexual dimorphism and communication among mammals. Livestock World, 4(2), 72-75.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.