Sexual dimorphism means that there is a significant difference in appearance between males and females of the same species (animal). This includes differences in body size, body shape, and body color, but can also refer to body hair, skin, teeth, and wings. Often, when we think of sexual dimorphism, we think of males with a striking appearance, but sometimes it is the female that has a more notable appearance compared to the male.
Red Deer
In red deer, the difference in appearance between the male and female is quite evident. The male is much larger, but what is especially notable is the antlers. The female, also called a hind, does not have antlers. A calf also does not have antlers, and young males have only small antlers, just like old males. A male deer in its prime has the largest antlers, averaging about 70 centimeters in size, but can grow up to 90 centimeters. The antlers serve for self-defense, but even more as a means to fight with other males to win over the females. In young males, the antlers fall off in March or April, in old males often during the last winter months. After the antlers have fallen off, a new set immediately begins to grow. There is, however, a downside for the male.
During the mating season, the males exert themselves greatly to win over the females. In addition, they consume little food, resulting in a loss of up to 30 percent of their body weight. This often makes them an easy prey for wolves or bears. In the Netherlands, red deer are found in two places: the Veluwe and the Oostvaardersplassen.
Peacocks
We mostly know peacocks from petting zoos. They were brought to Europe from Mesopotamia over 4,000 years ago for the same reason as today: their beauty. Later, peacocks were also brought over for their meat, and their feathers were used for decoration. Only the male has a beautiful appearance. In the green peafowl, the male and female are less distinguishable, but in the blue peafowl, the difference is quite clear.
The male is beautifully blue-colored and has a long tail that is green/blue-colored. Additionally, this tail also has the so-called peacock eyes. This beautiful appearance serves to scare off predators and to stand out. During mating season, the male tries to win over the female by displaying his beautiful feathers, but also by crowing, which is somewhat of a strange sound.
The female is more brownish-gray in color, serving as camouflage so she can lay and hatch her eggs unnoticed. The blue peafowl that we are most familiar with is found in the wild in India. The green peafowl lives in Malaysia, Indochina, and Java.
Spiders
In most animals, the male is larger, but in most insects, birds of prey, toads, and spiders, the female is larger. In the wasp spider, for example, the difference is quite noticeable. The male is much smaller than the female, making it seem as if both spiders are from different species. The male is usually cocooned and later eaten by the female after mating. A second mating is thus usually prevented. A male can only mate at most two times, as one of his two genitalia remains in the female during mating, to prevent other males from successfully mating with the female.
Males can avoid an unsuccessful fertilization by choosing a virgin female. Such a female emits a specific pheromone that is picked up by the males. Should a male be fortunate enough to survive mating, he still is not granted a long life. A male lives significantly shorter than a female and only a few days after reaching adulthood. The female, like insects and toads, is larger as she can lay more eggs. The wasp spider is also present in the Netherlands, albeit in limited numbers.
Elephant Seals
In many mammals and birds (except most birds of prey), the males are larger. A striking example is the elephant seal. Males are much larger than females. Males, also called bulls, can reach about six meters in length and weigh around 3000 kilograms. Females can reach three meters in length and weigh about 650 kilograms.
Another important difference is that the males have a small trunk (hence the name elephant seal) which is absent in females. With this trunk, the males can produce their roaring sounds. Elephant seals are diving champions: females can stay underwater for about 20 minutes, males for about an hour. A southern elephant seal is known to have stayed underwater for about two hours, while a northern elephant seal is known to have dived about 1.5 kilometers deep.
The males are aggressive. During mating season, the males fight with each other to win over the females. These fights are often fierce, resulting in scars and sometimes even death of one of the males. The winning male often has a harem of about twenty females. Females that do not want to mate are forced by the male, who lays on the female with his heavy body weight. If the female resists, she is bitten hard. Young are often crushed to death by the males. The northern elephant seal is found along the west coast of North America, the southern elephant seal in the waters of the Southern Ocean. Both species were on the brink of extinction due to hunting for their thick layer of fat. There were probably only about twenty animals left of the northern elephant seal at the end of the 19th century. In 1992, this number had risen to about 125,000. The number of southern elephant seals is about 600,000.
Walruses
In walruses, the male is larger and heavier than the female, but the biggest difference is the tusks. Females also have tusks, but they are much smaller. The males have tusks about 50 centimeters long, but there are also cases of a meter known. Males are about 320 centimeters long and weigh around 1270 kilograms, females average 270 centimeters in length and weigh around 850 kilograms. Outside the mating season, the herds are separated by gender.
Within a herd, there is a fixed hierarchy determined by the size of the tusks and body size. Especially bulls can, even outside the mating season, engage in fights that are usually caused by a struggle for a resting place. If threatening is not sufficient, then the fight is engaged, where the tusks are also used, often leading to bloody wounds. Just like the elephant seals, the males fight fiercely, and the winning male has a harem of about twenty females.
Butterflies
There are some butterflies where the male can fly, but the female cannot. An example is the small winter moth. The male’s wings are about 12 to 16 millimeters long. The female has only wing stumps. The females mostly live on branches and tree trunks and crawl around until they are found and fertilized by the male. This mating can last 2 to 4 hours and sometimes the male takes the female with him during mating in flight. This butterfly is also found in our country in gardens, parks, orchards, forests, and other tree-rich areas. Other species of butterflies where only the male can fly can be found here.
Lions
In lions, the difference between the male and the female is clearly seen in the male’s mane. The females do not have these manes. In addition to the manes, the male is also larger than the female. The male is about 172 to 250 centimeters in size and weighs 150 to 280 kilograms. The female is about 140 to 192 centimeters in size and weighs about 100 to 182 kilograms. The manes, like the antlers in red deer, provide an imposing appearance. With these manes, the males try to impress the females. The bigger the mane, the more impressive the male is to the females and other males.
Lions live in groups with often one male who is the leader (but two males as leaders also occur), a number of females, and the cubs. Being the leader brings a number of benefits, such as the male (almost) never having to hunt and being the first to eat the caught prey. Additionally, he is the one who gets to mate as a male. However, the leader must defend his position against males within his own group or males from another group. Too young males are often not yet strong enough to take over leadership. Too old males are too weak to still carry the leadership or to become a leader. Males fight each other, and the winning male remains the leader or becomes the new leader. New leaders kill cubs, as females are fertile as long as they do not have young. Lions do not occur in the wild in the Netherlands, but can be seen in zoos.
Stag Beetles
The stag beetle is one of the largest beetles in Europe and can grow up to 9 centimeters in size. The males are significantly larger (3.4 centimeters to 8.6 centimeters) than the females (2.8 centimeters to 4.5 centimeters). However, the most notable difference between the male and the female is the male’s “antlers.” The female does not have antlers. These antlers of the male look impressive and are meant to be, but they are actually jaws. These jaws are not suitable for biting, so the males are harmless. The females have smaller jaws, but unlike the males, they can bite and cause injuries with their jaws. The males fight each other for the female, trying to overturn each other with their jaws. The winning male gets to mate with the female. During mating, the male keeps his jaws like a sort of fence in front of the female, so she cannot escape. The stag beetle also occurs in the Netherlands, albeit to a limited extent.
Pheasants
Pheasants are striking birds with beautiful colors, at least the males. The females are much less conspicuous. The pheasant originally did not live in Western Europe, but thanks to the Romans, the pheasant was spread to other parts of Europe. The Romans already appreciated the bird for its meat. The number of pheasants increased in the 18th century when the bird was bred and released for the purpose of pleasure hunting. Today, this is prohibited, but it still happens illegally.
Although the roosters are larger than the hens, the biggest difference between the two sexes is in the colors. The roosters are multicolored with a green-red head and a long tail. In this way, the male stands out and impresses the females and other males. The males fight with each other, and the winner gets the harem of females. During the mating season, you can hear the roosters crowing in the meadows with their characteristic sound. Often he jumps up and flaps his wings to be even more noticeable.
The female is less conspicuous and is brown-colored. This is for the same reason as peahens; the females need to be camouflaged while laying and incubating their eggs. Pheasants are also found in the Netherlands. Especially the roosters can often be heard crowing in the mating months (April and May), but they also make their presence known outside these months. Pheasants can be found in meadows and open forests. You will not find them in dense forests. Female pheasants are less often encountered, due to their hidden existence and their camouflage color.
Ruffs
We end with another bird. This bird is very special, not only do the males and females differ from each other, but the males also have different appearances. Thus, there is sexual dimorphism, but also dimorphism within the same sex. The male is about 30 centimeters long, and the female about 24 centimeters, much smaller. The male, in breeding plumage, has a colorful collar and crest, which the female lacks. This collar can take on different colors, from black, reddish-brown, orange to white. The males gather at a special place, called the “lek,” to perform mock fights. The females then come to watch and choose a male. There are three different types of males:
The largest group (84%) are the territorial males. These are the males with the colorful and large crest and collar. These males can mate most often with the females.
Another group (16%) are the satellite males. These males often have a white breeding plumage and less colorful and smaller crest and collar than the territorial males. These satellite males are less assertive but sometimes mate with the females.
The last group (less than 1%) was described in 2008, and these are the males that have the plumage of a female but the sexual organs of a male. These males are rare and have no collar or crest and rarely mate with a female.
Sexual dimorphism is thus particularly prevalent in bird species. However, there are many more examples of sexual dimorphism, such as in gorillas, hawks, black grouse, etc. In some animals, such as some fish or ducks, the male only gets striking colors in the mating and breeding season.
