Lactation - Evolution

Evolution

Further information: Evolution of mammals#Milk production (lactation) Further information: Mammary gland#Evolution

Darwin correctly recognised that mammary glands developed from cutaneous glands and hypothesized that they evolved from glands in brood pouches of fish where they provided nourishment for eggs. The later aspect of his hypothesis has not been confirmed, but recently the same mechanism has been postulated for early synapsids. Instead the discus fish (Symphysodon aequifasciata) became known for (biparentally) feeding their offspring by epidermal mucus secretion. A closer look reveals that similar to most mammals the secretion of the nourishing fluid may be controlled by prolactin.

Later therapsids such as cynodonts appear to have secreted complex, nutrient-rich milk. This brought them evolutionary advantage by allowing a decline in egg size. However, Sinocodon, a small early mammal or mammaliomorphan appear to have had teeth from the very earliest age, and probably did not suckle. This may indicate the therapsids used their milk for moistening their eggs only.

During early evolution of lactation the secretion was through pilosebaceous glands, and mammary hairs transported the nourishing fluids to the eggs or youngs. Later the development of the mammary patch rendered mammary hairs obsolete.

Another well known example of nourishing young with secretions of glands is the crop milk of pigeons. Like in mammals and disc fish this also appears closely related to prolactin. Other birds such as flamingos and penguins are utilizing similar feeding techniques.

Lactation is also the hallmark of adenotrophic viviparity - a breeding mechanism developed by some insects, most notably tsetse flies. The single egg of the tse-tse develops into a larva inside the uterus where it is fed by a milky substance secreted by a milk gland inside the uterus. At least one cockroach species is also known to feed their offspring by milky secretions.