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 umbilicus (navel). The abdominal hair of non-human mammals is part of the pelage or fur.
Human abdominal hair
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", 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. Abdominal hair (particularly the "treasure trail") often signifies young teen men stepping into adulthood, as it represents that their bodies are changing.
Some women may develop a small line of hair from the pubic area up to the navel. According to the female sex image in some cultures this is seen as desirable, and in others it is seen as unattractive. Excessive abdominal hair on women, following the male pattern, is called hirsutism.
Patterns of Abdominal Hair
Various studies of caucasian subjects have documented four general patterns of pubic and abdominal hair including:
- Horizontal - Characterized by upper surface of pubic hair terminating in a horizontal line with no hair extending to the abdomen.
- Sagittal - Resembles the first but with the addition of a narrow vertical band of hair extending from the pubic hair towards the navel.
- Acuminate - Characterized by a tapered, inverted 'V' pattern extending upward from the pubic hair. Upper limit may end below the navel, at the navel, above the navel or near the chest. (Example shown in accompanying photograph.)
- Disperse (or quadrangular) - Hair is distributed broadly over the abdomen without forming a discrete geometric pattern.
In Zickler's study (see citation below) the horizontal pattern was most common in females with an incidence of about 80 percent. This pattern was also common in males younger than 25 with an incidence of about 40 percent. The sagittal pattern occurred in about 17 percent of females and 6 percent of males. 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. 
- Setty, Laurel Raymond, 'Varieties of the Acuminate Abdominal Pattern of White Males' Journal of the National Medical Association, 1966 May;58(3):191-3 
- Setty, Laurel Raymond 'Varieties of the Quadrangular Abdominal Hair Pattern of White Males', Journal of the National Medical Association, 1967 Jan;59(1):45-7 
- Zickler, Rienhard A. "Sex Differences in Pubic Hair Distribution in White Population" 1997
The following journal articles include sketches of different abdominal hair patterns and observed percentages of men exhibiting each pattern.
- Varieties of the quadrangular abdominal hair pattern of white males. Journal of the National Medical Association. 1967 Jan;59(1):45-7
- Varieties of the acuminate abdominal hair pattern of white males. Journal of the National Medical Association. 1966 May;58(3):191-3
- The Regional Anatomy of the Human Integument with Special Reference to the Distribution of Hair Follicles, Sweat Glands and Melanocytes