The reproductive phase is characterized by
repeated cycles of growth. The cycle may be
entrained on an annual rhythm as in seasonally
breeding species or operated independently
of season as is mainly the case in
pigs. The variations in relative size of
organs and parts is not restricted to the
organs directly associated with reproduction.
The mammalian female is characterized
by the storage of energy in adipose
depots during pregnancy and the progressive
release of this energy in the lipid of the
milk during lactation. Dairy cows gradually
lose condition (or fat) during lactation as
also do sows, particularly if the litter is
large and the lactation extended. Some seagoing
mammals are extreme examples of
this phenomenon. In seals (Pinnipedia),
which come ashore to breed, the whole of
the lactation occurs on land but the dam
does not feed at all during this period. The
lactation is thus supported only by body
reserves of fat and protein. Another extreme
is the stag (the male of the Scottish red deer,
Cervus scotticus) which usually does not
feed during the whole of the rut (breeding
season). The breeding life of both male and
female is thus marked by considerable fluctuations
in body weight and in the amount
of fat reserves in the body.