Why are men attracted to butts?

According to a study, men find spine curvature with a 45 degree angle very attractive. The study claims that when like butts, they are actually attracted to the spine curvature that makes the butt seem bigger. 

According to a study, women with 45 degree curvature in their spine can have much higher mobility while they are pregnant, while other women might have a problem doing that. The absence of such a curvature can put 800 times more pressure on the hips during pregnancy.

Women who have a spine with a 45 degree angle are  able to get pregnant more than once without inuring their backs. The shape of the butt can communicate this information to the male indirectly. Men thus think that they found the rear attractive, while in fact it's the spine that mattered.

The sight of a woman's butt can signal her readiness for pregnancy since it can indicate that a woman has enough fat. Men were designed to get attracted to women who have a higher chance of pregnancy.

Just like breasts, a woman's butt can signal her youth. The shape of the butt can indicate whether a woman is young enough, and as a result, make her seem more attractive. 

A woman with an hour glass body shape appears more attractive to men than other women because this body shape signals fertility. According to psychologists, men find fertile women more attractive than other women.

Women with balanced body shapes and round butts are healthier than women with unfit body shapes. According to psychologists, men find healthy women more attractive than other women.

Men get attracted to each feature that makes a woman look more feminine. This is one reason why men like long hair. (See Why Do Men Find Women With Big Eyes Attractive?)

Since intercourse from behind is popular, many men find the sight of a woman's butt sexually attractive, as it reminds them of sex. (See Why Sex is Addictive?)

According to a research by Oxford University, a big butt can indicate that a woman is intelligent. If this information can be communicated to the man indirectly through the shape of the butt, the man might find the woman more attractive.