While in humans, females have a more attractive physical appearance than males; it is the opposite in birds. Males have a more appealing physical appearance than females. Further, the attractiveness increases with age for women but decreases with age for female birds? The best example is the attractiveness of the peacocks which far outclasses that of peahens.
Some of the distinctions in the physiques of males and females are designed to suit each sex for the role it plays in reproduction, while others exist to help aid in male – female mutual attraction.
Women are better groomed. They are blond and fair-skinned, while men are often dark-skinned. Women have fully buoyant bosoms (breasts) while men have flat chests. Women have less deeper voices because they have smaller voice boxes. The bigger voice boxes of men makes the cartilage surrounding their voice boxes to protrude more, which gives them neck lumps called Adam’s apple. Women have wide faces, fuller lips and high eyebrows; while men mostly have stronger brows, cheekbones and jawlines. Women have less body hair than men, especially no facial hair and no chest hair.
In chicken, roosters often are better groomed and have showier plumage than hens. Their colors are often bright and vibrant. Many roosters have iridescent feathers that capture the light and show off beautiful blue and green highlights. Roosters tend to have redder combs and wattles.
Hens, especially those which have been laying will have feathers that are possibly broken, ruffled, dirty and ragged looking, probably due to playing in the dirt, being followed by roosters and also due to molting. Initially, hens have waxy, bright red and warm combs and wattles. Then, as they start laying, they divert the yellow color in their pigment from their vent, eye rings, legs, and beak to be deposited in their yolks. These body parts, therefore, become white, pink, or bluish-white in color. The loss of color is also called bleaching.
The above describes “Sexual dimorphism”, which is the scientific term for physical differences between males and females of a species.
The opposite morphological features between males and females of humans and chicken species is as a result opposite orientation of their sex chromosomes. I will explain this in a bit. But first let me explain what sex chromosomes are.
All animals and plants are made up of cells. In the centre of each cell is an area called the nucleus. Inside the nucleus of each cell, there are these thread-like structures, called chromosomes, that hold genes. Genes are the ones that carry hereditary information that determines the traits of an animal or plant, such as blood type. Chromosomes occur in pairs, and there are a different number in different species of animals and plants. Humans have 23 pairs in each cell (a total of 46 chromosomes), elephants have 28 pairs, cats have 19 pairs, the carrot plant has 9 pairs and monkeys, chimpanzees and apes have 24 pairs of chromosomes (a total of 48 chromosomes).
Each cell of the human body carries 22 pairs of chromosomes (also called autosomes) and one pair of sex chromosomes. You inherit half of your chromosomes from your mother (22 chromosomes and one sex chromosome), and the other half from your biological father (22 chromosomes and one sex chromosome . The sex chromosomes decide whether you will be born male or female.
Most men have an X chromosome from their mother and a Y chromosome from their father. Most women receive an X chromosome from their mother and an X chromosome from their father. The X chromosome carries genes that are not present in the Y chromosome.
In birds, however, the story is the opposite. A male carries two X chromosomes in each cell, while the female has one X chromosome and one Y chromosome. The male passes on one of his X chromosomes and the female either her X or her Y chromosomes to their chick. If the chick ends up with XX then it will be a male, if it ends up with a Y it will be a female. In birds, the female decides the sex of the baby, the opposite of humans, where males are XY and females XX. As an example, chicken have 78 chromosomes. That is they get 39 chromosomes from each parent, and one of these is a sex chromosome.
Scientists have proven that the genes on the X chromosome are the ones that solely determine the physical appearance (phenotype) of an animal or plant.
And this may explain the opposite presentation of physical features between females of human and birds, and those between males of humans and birds.