Sexual Dimorphism in the Animal Kingdom
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When you look at a human male and female, there are obvious differences that allow you to tell them apart. But this isn’t limited to people and is something that’s seen frequently in the animal kingdom. This is known as sexual dimorphism.
What is Sexual Dimorphism?
Sexual dimorphism is a term used to describe the physical differences between the males and females members of any given species. For example, male lions have a mane, whereas females don’t. However, these differences can encompass various aspects, including size, shape, and color of the animal. In some cases, there can even be differences in behavior traits between the two.
This phenomenon isn’t limited to animals alone; many plants in the flora kingdom also exhibit obvious differences between males and females.
Where animals are concerned, males are usually larger as they tend to be the dominant sex. But this isn’t always the case. Female hyenas, as an example, are much larger and more aggressive than their male counterparts. This is largely because groups are matriarchal and led by a dominant female.
The seahorse is another example of where traditional sexual dimorphism goes out of the window. Male seahorses are famous for their reproductive abilities in that it is them that carries the young, rather than the females getting pregnant. Even after birth, the male is the one to nurture the babies, which isn’t something we often see in animal species.
Why Does Sexual Dimorphism Occur?
The purpose of sexual dimorphism all boils down to reproduction. Females have to produce eggs, while males have to produce sperm in order to reproduce. The cost of producing these cells is typically larger for the female and Darwin proposed that this very phenomenon, along with the pressure of carrying and giving birth to young, is what initially sent each gender down its own evolutionary path.
What’s more, it is theorized that the differences between males and females are what attracts them to one another, in a phenomenon called sexual selection. By having different traits, forced by sexual dimorphism, males and females are attracted to one another, therefore increasing reproductive success.
Moreover, studies have shown that these differences help individuals to fight off the competition of same-sex competitors, known as intra-sexual dimorphism.
Intrasexual Competition
When I talk about intrasexual competition, I’m not referring to members of the same gender trying to mate with one another. I’m talking about the competition that happens, often between male animals, who fight for the right to breed with any given female.
This is why they often have physical characteristics like a larger size, horns or antlers, and increased aggression in males. All of these things allow the males a greater chance when competing.
Intersexual Selection
When it comes to females, they often get the pick of the bunch which gives them a reproductive advantage as they can share their genes with males that are fitter, stronger, and healthier.
One of the reasons that males tend to be showier is that this serves as a point of attraction to the females. Of course, when she sees him fighting off competition, this also displays his strength which encourages her to want to mate with him.
When the female chooses a mate with strong physical and genetic traits, this increases the chance of those characteristics being passed onto the young, who will then be more likely to thrive and survive.
Causes of Sexual Dimorphism in Animals
It’s interesting that sexual dimorphism can be triggered in a variety of ways. While it’s commonly accepted that this is something that happened along the evolutionary path of many animals, there are different things that cause it. This could be down to the animal’s natural environment, its hormones, and various genetic factors.
Hormonal Influences
In female animals, the dominant reproductive hormone is estrogen while in males, more testosterone is present. These hormones are essential in determining the physical traits of an animal, even during pregnancy. As a result of this, secondary characteristics like the presence of antlers or a mane will develop.
Testosterone is responsible for these male traits, and it also causes the males of most species to become physically larger and stronger with increased muscle growth, for example. On the other hand, the presence of more estrogen in females causes a greater amount of body fat, which is essential in supporting her pregnant body and when she is lactating.
Even more interesting is that estrogen can stimulate maternal behavior, and studies have shown that female mammals and rodents will increase their nurturing behavior by the very presence of this hormone.
However, there are cases where females display more masculine traits, but this only happens when it would be advantageous. It’s something we see in birds of prey where the females typically have a larger physical size owing to the need to incubate her eggs. The bigger she is, the more cover she can provide for the eggs.
Female hyenas have large clitorises that look like a penis, and their overall body size is larger. This is because of increased testosterone that causes them to appear male as a way of protecting their young from predation, even from male hyenas who display cannibalistic behaviors.
By the same token, there are some species whose male members have decreased testosterone, which causes them to display less aggressive behavior and not as many secondary characteristics.
Genetic Factors
An animal’s genes are determined by its two parents, and go even further back than that. Traits are passed down between generations, but there are some that only pass between father and son or mother and daughter. For example, the presence of baldness in humans is a genetic trait. This is caused by a mutation of the KITLG gene and the same happens in animals. When a gene mutates, this can cause exaggerated traits based on gender.
Interestingly, the genes that cause male-only traits, such as those responsible for antlers, are found in both sexes. However, they’re only activated in males. You might compare this to the presence of genes for the growth of body hair in humans. It’s present in both genders but (for the most part) only shows in males. Although, there are cases where this gene is more active in females, causing more masculine traits.
In females, specific genes, like the glucocorticoid receptor gene, not only stimulate the growth of mammary glands but also stimulate lactation.
Some of the traits we see in males of certain species may be attractive to females but may not be compatible with survival. This brings us to a theory called the runaway selection hypothesis. Let’s take the peafowl as an example.
The males of this species have brightly colored, showy tails that attract the attention of females but could also make the individual more obvious to a predator. So why would males evolve this trait, and why would a female choose to mate with the most brightly colored male? If she does then this means that her male offspring will display similar, potentially risky physical traits.
Well evolution is to blame for this, unfortunately. It is theorized that initially, female birds would have chosen a male based on him having a longer tail. This trait would enable him to fly better and more quickly escape predators; a trait she’d surely want to pass onto the offspring.
However, over time, this desire for a longer tail started to runaway and, in the case of the peafowl, the tail became so large that it simply got in the way. Where their longer tail feathers once helped them to evade predation, they soon became a hindrance. Still, females will choose males with larger, more brightly colored tails so they’re more likely to result in reproductive success, and those traits will continue to be passed on.
Mammals
Many mammal species, including humans, have very obvious sexual dimorphism. Typically males are much larger and have features like horns and antlers that allow them to fight. They’re also much more aggressive and territorial than females.
African Lion
The African lion is perhaps one of the most obvious examples of sexual dimorphism, with a clear difference between males and females. Males grow a mane, whereas females don’t, and the purpose of this is twofold.
For starters, the mane serves as a way of attracting a female. Interestingly, the size and color of the mane can tell the female a lot about the health of the male, allowing her to make the most advantageous reproductive choices. But manes also show dominance and will even protect the lion’s neck as he goes into battle with competitors for the right to mate.
Female lions lack a mane, largely because they’re the ones that do the ‘lion’s share’ of the hunting. Without a mane, the females are more streamlined, allowing them to move faster. However, there have been five reported cases of female lions with increased testosterone growing a mane.
Orangutan
Male orangutans are often much larger than the females, which is a common trait in many species. That said, during their early years, females grow much more quickly than males.
However, one of the most obvious differences between the two genders is the facial plates which, in males, have heavily padded cheeks. It’s thought that this serves as an attraction to females but isn’t something seen in all males.
A portion of male orangutans have these cheek pads, along which pouches in the throat that allow them to make vocalizations used in courtship and for competing with other males. However, this is usually only seen in dominant males, whereas the subordinates have a very similar appearance to females.
In studies, it has been observed that males with padded cheeks have much greater reproductive success.
Mandrill
The mandrill is a species of old world monkey, and there are clear differences between the males and the females.
Males are typically much larger and have longer, more prominent canine teeth as well as brightly colored, showy markings on the face. While males may weigh up to 82 lbs (37 kg), the females can weigh less than half of this, at around 33 lbs (15 kg).
These physical differences allow the male to display his dominance when competing for a mate as well as serving as an attraction to females. However, studies have shown that females aren’t necessarily attracted to the most dominant male but the one with the brightest coloration.
So different are the genders that the mandrill is considered to be the most sexually dimorphic species of primate in the animal kingdom.
Elephant Seal
The elephant seal is the largest species of seal on the planet. While females are big, males can be between three and seven times larger, weighing up to 7,000 lbs (3,175 kg)! In order to maintain their enormous size and impress females, male elephant seals will engorge themselves with food, putting reproduction as a priority over their very survival.
Another obvious difference between males and females is the presence of a large proboscis which resembles a trunk; no prizes for guessing where this species got its name.
This trunk-like characteristic enables the males to make extremely loud vocalizations during the breeding season and assert their dominance.
Deer
There are 43 species of deer, and the males always have a set of antlers, which females lack. The only species not known to possess antlers is the water deer. In all other species, males shed their antlers annually before growing a new set.
Antlers are used to attract a female, and the size of the antlers tells her a lot about the health of a male and whether she wants to use his genes to reproduce.
What’s more, male deers use their antlers to fight when it’s time to establish dominance for the right to breed with a female.
Female reindeer have antlers, but they are much smaller than the males. While this is not a common trait in female deer, there are some reports of hormone imbalances that cause females of other deer species to grow antlers unexpectedly.
Reptiles
We often see sexual dimorphism in reptiles and, like mammals, the males are often bigger and stronger. Males also have additional features that act as a showpiece for attracting a female.
Eastern Box Turtle
The eastern box turtle takes its name from the fact that it can completely close off its shell and hide inside, like one might in a box.
In terms of sexual dimorphism, there are two key differences between the genders. Males tend to be much larger, but they also have a different-shaped shell.
Where females’ shells are dome-shaped, that of the male is concave on the underside, which makes it easier for him to mount the female during copulation.
Green Anole
The green anole is a species of lizard found in the southeastern United States. While both males and females may have some similarities, there is one obvious trait that sets them apart.
Males have dewlap which is a fold of fleshy skin around the throat and, in this species, is bright red in color. When trying to attract a mate, the male will inflate his dewlap in the hopes of impressing her. This is also used to warn other males and defend territory. Some males also have a crest on their back which is used for the same purpose.
Other differences between male and female include size, with males being slightly larger. Although this isn’t the most reliable way to sex them. However, the size and shape of the head is a more reliable method, with males having a larger head and longer snout.
Frilled Lizard
As the name suggests, the frilled lizard has a frill around its neck but this is typically larger and more brightly colored in males. That’s because they use it to attract females as well as telling other males to back off and appear intimidating.
At the same time as displaying their frills, males will also bob their heads, wave their limbs, and lash their tails.
Male frilled lizards are also larger than their female counterparts, and can grow up to and above 36 inches (91 cm) in length.
Gharial
The gharial is a species of crocodile, sometimes called the fish-eating crocodile that has a very interesting sexually dimorphic trait.
Not only can males grow to almost double the size of females, up to 19 feet (5.8 meters), they are also equipped with a bulbous growth at the end of the snout. This is called a ghara and is made from soft tissue on top of a bony support. It is not present in females.
The purpose of the ghara is primarily to enhance the gharial’s communicative hisses, which echo through the ghara and can be heard from as far as a kilometer away. What’s more, while there isn’t a lot known about the ghara, scientists do believe that it acts as an attractive feature to females.
Birds
Sexual dimorphism can be very complex in bird species, with males often being much more brightly colored compared to their dull female counterparts. Males may also have longer, showier feathers and may also have a more beautiful song, which they use to woo a potential mate.
Peafowl
If there’s one species that everyone is familiar with in terms of sexual dimorphism, it’s the peafowl. While the hens have a dull brown coloration, the males are much brighter in shades of bright blue and gold. What’s more, they possess a brightly colored, fanning tail called a train, which interestingly is shed each year but quickly grows back.
It’s this vibrant tail plumage that these birds are most well-known for, and males are known to use it in courtship displays to impress a female.
The story goes that Darwin’s theory of evolution was thrown into disarray because of the peacock. He simply couldn’t see the evolutionary value of this huge, elaborate tail which is how he arrived at his theory of sexual selection.
Mallard
As a child, the mallard was one of the first examples of sexual dimorphism I noticed. It took my parents to explain that the ducks I saw swimming together were actually the same species; they just looked different.
With mallards, the coloration of the male is far more vibrant, with a white ring around the neck and glossy, green plumage on the head. He also has a brightly colored beak and black bordered feathers that really make him stand out.
As is the case with many species, this coloration serves as a way of attracting a female, whereas the drab coloration of the females allows them to camouflage when nesting. However, there are some reported cases of sexual in which damage to the reproductive organs can cause females to take on a more masculine appearance.
Northern Cardinal
The northern cardinal is one of the most attractive garden birds in North America but when you see that bright red coloration and black-masked face, you’re looking at a male. If you see a female, she will have a much more muted tone of dull reddish-olive.
The reason that males have a more vibrant appearance is primarily to attract a mate, and you’ll also notice that males are slightly larger than their female counterparts. However, this isn’t as obvious a size difference as we see in mammalian species. Moreover, males typically have a larger crest which again, serves as a way of attracting a female.
Pheasant
Like the mallard and the peafowl, female pheasants have a brown coloration that is perfect for camouflage. However, the males are much showier and have brightly colored plumage with a long tail, which they use to attract a mate.
Male plumage is bronze, and they have a white ring around the neck with a green face. Many males also have a red wattle, which is also used to attract a mate and is associated with individuals who have high testosterone levels.
The male pheasant is also slightly larger than the female.
Mandarin Duck
The male mandarin duck is a striking-looking bird with various colors, including white, brown, green, and orange. He has upturned wing feathers that some people say resemble sails on a boat and elaborate plumage around the neck. All of this is so that he can easily attract a female who has much less attractive plumage, typically in a gray to brown color.
Based on fossil evidence from more than 66 million years ago, scientists were able to determine that species with more pronounced sexual dimorphism are at greater risk of extinction. Fortunately, despite their obvious differences, the mandarin duck is listed as being of least concern on the IUCN Red List.
Fish
Under the water, many fish species also display sexual dimorphism which is usually seen as males being larger in size. While sexual dimorphism is seen in many fish species, it’s most common in ray-finned fish.
Triplewart Seadevil
The triplewart seadevil is a species of deep sea fish from the Ceratiidae family. This species boasts one of the most extreme cases of sexual dimorphism on the planet, with males being ten times smaller than females.
While a female may measure around 12 inches (30 cm), her male counterpart is just a miniscule 1.2 inches (3 cm). So small is he that he has to parasitize the female in order to mate. Males have specialized jaws that allow them to latch onto a female, where they’ll remain for life. Each year the fish will spawn and, hopefully, reproduce.
But it doesn’t end there, females are much better equipped for life in the deep ocean and possess traits like a bioluminescent lure to attract prey. It’s a good job then that, once the male latches on, he depends on the female for sustenance.
Seahorse
In the ocean, the seahorse’s reproductive behavior is probably one of the most talked about by humans. That’s because, in this case of sexual dimorphism, the traditional roles are reversed, and it’s the males that carry and raise the young.
In other species, the egg remains inside the female and is fertilized. However, female seahorses have an ovipositor that allows them to transfer eggs to the male’s brood pouch. This brood pouch is located on the abdomen at the base of the tail and is not seen in females.
It’s thought that this role reversal aids in faster reproduction. While the male is pregnant and raising the young, the female is busy preparing more eggs.
Guppy
Sexual dimorphism between guppies is most obvious in terms of shape and coloration. In order to attract a mate, males are much more vibrant and have beautiful patterns, whereas the females tend to have a uniform gray color.
What’s more, males typically have longer fins as well as a special type of anal fin known as a gonopodium which he uses during reproduction for transferring sperm.
Unlike many species, however, female guppies are usually larger than males. While females may grow to 2.5 inches (6.4 cm), the males may only reach 1.5 inches (3.8 cm).
Siamese Fighting Fish
Sometimes called a betta, the Siamese fighting fish is a popular aquarium pet, but it’s usually the males that are most sought after because of their beautiful appearance.
These fish have very obvious sexual dimorphism, with males having long, flowing fins and much brighter coloration than the dull females. This appearance is used for an advantage during breeding selection.
However, what’s interesting is that wild Siamese fighting fish don’t display anywhere near as bold colors as those that have been selectively bred in captivity.
