The development of a woman's titties, during puberty, is caused
by sex hormones, chiefly estrogen. This hormone has been demonstrated
to cause the development of woman-like, enlarged titties in men,
a condition called gynecomastia, and is sometimes used deliberately
for this effect in male-to-female sex reassignment surgery.
A vast number of medical conditions are known
to cause abnormal development of the titties during puberty. Virginal
tittie hypertrophy is a condition which involves excessive growth
of the titties during puberty, and in some cases the continued
growth beyond the usual pubescent age. tittie hypoplasia is a
condition where one or both titties fail to develop during puberty.
The orb-like shape of titties help limit heat
loss, as a fairly high temperature is required for the production