There are many things I see beauty in: a cheerful choreography, a striking song or an intriguing face. But I can't always consciously explain why I consider them beautiful.
The spiders of the genre Maratus they are like that too. Males have a beautiful colored abdomen that they exhibit when doing a complicated dance. All this to be able to conquer a female.
These spiders and I are not alone - when it comes to finding a mate, many animals seem to have a pretty clear sense of what they consider beautiful.
These preferences are apparently arbitrary. It's hard to understand how a spider Maratus A female can benefit from choosing the most beautiful male in the group as a partner. But in reality, that kind of choice may have had a profound effect on the way we evolved.
Peacock mating is the best example of selection for beauty
The idea that animals have "beautiful" traits to lure their opposites into mating was first put forward by Charles Darwin. For him, one sex - usually the male - vies for the other's attention.
Darwin called this "sexual selection." By the same theory, the sex being courted - usually the female - prefers the partner with the most desirable qualities.
This dispute does not kill anyone. Instead, the unsuccessful suitor ends up having fewer puppies. This is quite different from what Darwin called natural selection, or survival of the fittest: animals with poorer genes, which make them more vulnerable to disease or predators, tend to die earlier, so only the best genes are. transmitted to the next generations.
Sexual selection and natural selection led the animals to evolve in different ways in a kind of tug of war.
Darwin set out many examples of extreme and beautiful traits that evolved through sexual selection: the beautiful feathers of so-called birds of paradise, the great stems of deer, the wonderful colors of some insects, and the singing of birds.
These qualities can sometimes be harmful. A colorful plumage can attract more predators, for example. But it can be offset by the ability to find the best partner possible and produce several healthy puppies.
Darwin, however, could never explain how these preferences arose in animals. "You can't just assume that beings have a sense of aesthetics and that it led to the process of sexual selection," says Adam Jones of Texas A&M University.
A key explanation was given in the 1970s by biologist Robert Trivers, who realized that the key to everything was the effort that many animals put in caring for their young.
According to him, species that invest time and strength in rearing their young tend to be more selective when choosing their mates compared to animals whose young need less attention. At this time, beauty is indicative of the healthiest partners.
The elaborate peacock tail is perhaps the best known example. The longer a male's tail, the harder it is for him to escape predators. However, females prefer precisely those with the largest number of prints on their tail.
For Jones, the female's decision, however, is not always conscious. "The attraction to something beautiful can simply be a physiological response," he says.
The Olive Fly Female (Bactrocera oleae), for example, prefers males that can flutter their wings quickly. It is a sexual selection, but not necessarily a conscious decision.
The response to beauty can also be instinctive, which is certainly the case with humans. Men tend to prefer women with an ideal waist-to-hip ratio, while they prefer partners with a lower voice and square jaws. Like the peacock's tail, these traits are indicators of health and resistance to parasites, and are difficult to artificially breed.
They also show our fertility. Attractive traits in men indicate higher testosterone, and in women more estrogen - two hormones involved in conception.
But did our hominid ancestors also have similar preferences to ours? Looking at other primates it is possible to gather promising evidence.
A 2006 study showed that monkeys Rhesus, as well as humans, are also attracted to more symmetrical faces as indicators of higher quality partners.
Orangutan females prefer males with wider cheeks. This suggests that man and his relatives have been using their faces to advertise their genetic qualities for a long time.
"It's also natural to choose younger partners with no sign of disease," says Glenn Sheyd of New Southeastern University in Florida. This combination "activates the desire to reproduce rather with one individual than with another."
Then another question arises: How do our specific preferences evolve? A study of small birds called mandarin (Taeniopygia guttata) can give a clue.
The discovery by Nancy Burley, a professor at the University of California at Irvine, happened by chance in 1982. When her laboratory received new specimens of mandarins, they were given small colored bands so the scientist could identify them.
To Burley's surprise, birds that wore certain colors were more successful in finding a mate and even took better care of their young. Females preferred males with the red stripe, while they preferred those with pink or black strips.
Mandarins have developed a whole new code of sexual preferences in the lab. Apparently these birds have a natural inclination to value certain signals. But the study shows that there is something almost random about the traits that animals consider beautiful.
Like man, Burley's mandarins could be made more attractive just by manipulating his appearance. According to her, this suggests that some preferences are well implanted in the brain.
In the future, these random changes in mandarin DNA may produce new beauty traits that will be chosen by future partners. Without this innate response to beauty and the competition that results from it, life can be quite different.
Common drosophila, for example, are usually quite promiscuous. In 2001, a study found that when males are forced into monogamy, they evolve into smaller bodies and produce less sperm. Similarly, when females are genetically engineered to be monogamous, they also become less fertile.
That is, if there were no sexual selection, sex itself would cease to exist. While most of us will never get a chance to watch the spider and bird of paradise dances, we are surrounded by beautiful animals that have been partly shaped by sexual selection.
Much of the diversity and glory of life is due to animals' appreciation of beauty. "If you have to compete for partners and you need to be beautiful for it, the dispute adds a new dimension to that organisation's evolution," says Jones.
In a way, it doesn't matter if I don't find out why I find certain landscapes or certain beautiful people. The important thing is that I have these preferences. Without them, our evolutionary history could have been quite different.