Oestrogenic activity in urine from males was detected as early as 1927. A few years later mass excretion of oestrogens was detected in the stallion and other equines, and in the boar.
The following naturally occurring oestrogens have been chemically identified in urine from males: oestrone (stallion, bull, boar, man, rat), oestradiol-17β (stallion, boar, man, rat), oestradiol-17α (stallion, rat), oestriol (man).
The levels of oestrogens in male urine is subject to very marked species differences, the stallion showing extremely high levels, followed by other equines and by the domestic boar. During recent years a large number of wild animals has been screened for oestrogens in urine. Generally the values are very low, a few μg/100 ml of urine, while for stallion's urine the values recorded are in the mg range.
The testicular origin of the major part of urinary oestrogens was early anticipated and has been confirmed by recent biochemical investigations. A minor part is probably of adrenal origin.
The biological significance of oestrogenic hormones in the male is obscure. Mass secretion of the hormones in males of some species still represents an enigma.