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The term abdominal hair refers to the hair that grows on the abdomen of humans and non-human mammals, in the region between the pubic area and the thorax (chest). The growth of abdominal hair follows the same pattern on nearly all mammals, vertically from the pubic area upwards and from the thorax downwards to the navel. The abdominal hair of non-human mammals is part of the pelage or fur.
Before puberty, the abdominal region of both males and females is covered with very fine vellus hair. In response to rising levels of androgens (mainly testosterone) during and after puberty, the skin of the abdomen begins to produce coarser, longer and more pigmented hair (terminal hair). This process affects primarily men. Initially hair grows in a vertical line from the pubic area up to the navel and from the thorax down to the navel. Slang terms for this line of hair include "happy line", "happy trail", "snail trail", "tiger line", or "treasure trail". Although the development of abdominal hair normally begins during puberty, it may also start later, between the ages of 20 and 30. With some men, the abdominal hair will stay within a clearly defined vertical line, but in others, terminal hair will appear laterally as well as vertically, particularly in the area around the navel. This lateral spreading can continue into middle age. The study noted below found that two in three young men have a (sagittal) "treasure trail" by the time they are seventeen, and nearly half have one by the time they are fifteen.
Some women may develop a small line of hair from the pubic area up to the navel. Excessive abdominal hair on women, following the male pattern, is a type of hirsutism.
Various studies of Caucasian subjects have documented four general patterns of pubic and abdominal hair including:
Richard Zickler performed a 1997 study (see citation below) of the above patterns and their occurrence in males and females, paying particular attention to the development of hair during puberty. In Zickler's study the horizontal pattern was most common in females with an incidence of about 80 percent. This pattern occurred in 6% of males, including 55% of 13–15 year olds and 28% of 16 and 17 year olds. The sagittal pattern was found in 44% of males under 16, 67% of males aged 16–17, 20% of males over 17, and 17% of females. The acuminate pattern occurred in about 55 percent of males and occasionally in females. The disperse pattern occurred in about 19 percent of the males studied.
The following journal articles include sketches of different abdominal hair patterns and observed percentages of men exhibiting each pattern.