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The other day Aaron Feuk, in discussing the relationships of monotremes
to eumammals asked about Mourning Dove that "produce milk through the
tops of their heads".
Well, that is not quite precise. They do produce a milky susbstance called
crop milk. It is regurgated from the crop which is part of the digestive
system. It providesurishment for the nestling. It is very different in origin
from the milk of mammals and in composition. At best, it is a case of
convergence. The tissue is different, the product is chemically different.
There is one interesting similarity which, howerver, says absolutely
nothing about the relationship of birds & mammals. That is, both types
of product are produced under the influence of the same hormone:prolactin.
Comparative physiology is full of similar examples. The functions of hormones
Brush@Uconnvm.uconn.edu Physiology & Neurobiology
Mystic, CT University of Connecticut