Why males and females of the same species look different

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Sexual dimorphism is a phenotypic difference between males and females of the same species. This means that there are obvious differences between the male and female. The major difference is the genitalia proving if it is a male or female, but in a lot of species there are often secondary sex characteristics, such as size deference, ornamentation and behavior.

When talking about size difference most examples are with males that are larger than the females. In a lot of species e.g. seals, lions and elephants males are larger than females. There are also species where the females are larger than the males, e.g. spiders, Spotted Hyenas and Blue whales. In the ocean, the most interesting one is the Angling fish (Triplewart Seadevil). They live in the deep ocean and females can grow up to 30cm while the male reach 1.05cm. When the male finds a female he will attach to her to not lose his chance to reproduce. The female can have more than one male attached to her and when attached they look like small fins.

A reason females are larger than males is because it takes more energy to create eggs then to create sperm, so a larger female can produce more eggs than smaller females. “Why are some males larger than the females?” Larger males can protect more than one female. In a cichlid fish species (Lamprologus callipterus) the males are 60 times larger than the females, males will collect empty snail shells in which his females can breed. The larger the male the more shells he can collect and protect. If the female would grow too big for her shell she can’t lay eggs where a male can protect them.


A common type of dimorphism is the ornamentation. This means that the main difference between the males and females is the coloration or attribute that are normally used to attract a partner. This is seen in a lot of birds and reptiles. Peacocks are a good example of this. They have a large plumage that attracts the peahens. Peahens are brown and can blend in to the bush to hide herself and her chicks. The strong color and size of the peacock’s tail hinders him from hiding and flying well, a big tail shows the peahen that he is a survivor. Other examples of this are the Birds of Paradise, Argus Pheasants and the Guppies. Ornamentation in females is rarely seen. It’s only used by males since the females are making the choice on which male have the most attractive feature and therefore the right genes. The theory is that “sperm is easier/cheaper for the body to produce then eggs” so the male can put more energy into ornaments. The female also take care of the fertilized eggs and need to hide and protect the young ones. A big ornament is both in the way and can be seen by the predators.


The last Sexual dimorphism is behavioral. Males or females will be doing something to attract the other sex. One great example of this is song birds and humpback whales where the males create a complex song to attract the females that don’t produce the song. Other behavioral differences that are used to attract the opposite gender is to build nests, caring and collecting food for the offspring’s.


Sexual dimorphism means different ways to choose the perfect partner to give their offspring the best chance of survival.




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