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Bird Milk,
Reptile Milk, Fish Milk
Crop Milk
In the middle is a discus-fish parent infested with
its beloved fry.
It’s moderately well known that
several families of birds (pigeons and doves, flamingos, and penguins)
secrete a kind of milk (called “crop milk”, though penguins don’t have
crops) for their young. This is real milk, secreted by a special gland –
not regurgitated, half-digested food from the parent’s stomach.
(Regurgitators put their bills into the chick’s mouth to deliver food,
whereas in milk-providing species the chicks put their bills inside the
parent’s mouth to nurse. ) The three families evolved this capacity
independently, for three different reasons. With penguins it seems to be
an emergency food substitute in case the feeding parent (usually the
female) doesn’t return to the nesting parent in time. With flamingos it’s
apparently because the chicks aren’t able to handle normal food, and
perhaps because the flamingo’s bill makes the regurgitation method
awkward. In the case of pigeons, it allows them to raise several
small broods per season (only one or two eggs each), since their well-fed
squabs grow extraordinarily quickly. (This is probably also why squabs are
so fat, juicy and tasty.)
It's less well known that at least
one species of fish, the discusfish, feeds its young with a secretion
that might as well be called milk. (Mammary glands are skin glands
too, in the big picture of things). The tiny fry live off their egg-sacs
for several days, and then migrate and attach themselves to one of the
parents, where they feed off the fish-milk.
The capacity to produce milk is
thought to trace back to the therapsids -- Triassic links between reptiles and the mammals and birds. (The discus-fish are different,
unrelated story.) But regardless of everything, mammals will continue to be called
mammals, and pigeons ain't mammals. Don't let your biology teacher fool
you with a trick question.
The most interesting thing that
came out of my quest for exotic milk is that the hormone
prolactin seems to be associated with parenting activities by either sex
-- not only
in mammals and birds, but even in fish (and by conjecture and
extrapolation, even in therapsids). I'm less narrow-minded than most, and
I don't have any difficulty with the idea that human, mammalian, and avian
parents all feel similiar emotions. Proto-mammals, maybe. But fish?
Cold-blooded things? The mind boggles..
Discus-fish fry, now old enough to separate from
parents
Pigeons and miscellaneous:
“Thus three very different groups
of birds have evolved the capacity to produce milk as solutions to very
different problems: the need for protein and fat in the pigeons, which
feed very little animal material to the squabs; the need for liquid food
consumption during the development of the specialized feeding apparatus of
the flamingos (which would make any other form of food difficult for the
chicks to ingest); and the need for a convenient food supplement when
breeding on the barren Antarctic ice shelf favored by penguins.”
http://www.stanfordalumni.org/birdsite/text/essays/Bird_Milk.html
Crop-milk production allows the
rapid production of multiple broods in a nesting season, but only one or
two young per brood:
http://elibrary.unm.edu/sora/Wilson/v101n01/p0011-p0025.pdf
Like mammalian colostrum, crop
milk gives resistance to disease:
http://www.albertaclassic.com/adenovirus.php
Recipe for fake crop milk:
http://www.internationaldovesociety.com/Recipes/macmilk.htm
Everything you want to know about the intestinal tract of birds:
http://www.nhahonline.com/bird_digestion.htm
Bird digestion in great detail:
http://www.biology.eku.edu/RITCHISO/birddigestion.html
Many PubMed articles on crop milk
(pay site):
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=PubMed&term=+%22crop+milk%22+&tool=QuerySuggestion
Flamingos
Quick flamingo sketch:
http://www.sandiegozoo.org/animalbytes/t-flamingo.html
Flamingo traits in detail:
http://spot.colorado.edu/~humphrey/fact%20sheets/flamingo_caribbean/flamingo.htm
Flamingo chicks are unable to
filter-feed until they are three months old…. Flamingo crop milk is bright
red due to the presence of canthaxanthin, and feeding parents can lose the
red color in their feathers…. Parents can recognize their own young by its
call, and will not feed others in the flock:
http://www.durrellwildlife.org/upload/MainSite/Documents/pdfs/chilean%20flamingo.pdf
Fake flamingo mommy:
http://www.bronxzoo.com/426208/185480
Flamingo males and females share child-raising approximately equally, but
tend to specialize in particular tasks. Sometime a second female helps
out:
http://www.zoofederation.org.uk/uploads/Vol3No4.pdf
Discusfish
Everything you want to know about
discusfish (fantastic site)
http://park10.wakwak.com/~discus/Ev/e-breeding1.html
Discusfish breeding cycle with
lots of photos:
http://www.zestweb.com/events/1stbabies/babies080801.html
More detail on the discusfish
breeding cycle:
http://members.fortunecity.com/dempseydiscus3/e-breeding3.htm
A Romanian point of view on
discusfish:
http://discus.1hwy.com/
Therapsids
“Lactation appears to be an
ancient reproductive trait that predates the origin of mammals. …..
Mammary patch secretions were co-opted to provide nutrients to hatchlings,
but some constituents including lactose may have been secreted by
ancestral apocrine-like glands in early synapsids. Advanced Triassic
therapsids, such as cynodonts, almost certainly secreted complex,
nutrient-rich milk, allowing a progressive
decline in egg size and an increasingly altricial state of the young at
hatching. This is indicated by the very small body size, presence of
epipubic bones, and limited tooth replacement in advanced cynodonts and
early mammaliaforms. Nipples that arose from the mammary patch rendered
mammary hairs obsolete, while placental structures have allowed lactation
to be truncated in living eutherians.”
www.kluweronline.com/article.asp?PIPS=460568&PDF=1
“The
secretion of nutrient rich milk probably began in therapsids, such as
cynodonts”:
http://www.ijdb.ehu.es/ijdb200448023/ft249.pdf
Therapsids as a missing link between reptiles and mammals:
http://www.geocities.com/CapeCanaveral/Hangar/2437/therapsd.htm
Prolactin
Non-parental helping behavior in
woodpecker nesting is associated with presence of prolactin and is
apparently adaptive.
http://scholar.lib.vt.edu/theses/available/etd-103199-194729/unrestricted/mzkhandiss.pdf
Below studies of a dozen or more
bird, fish, and mammal species show that prolactin is associated with, and
possibly a cause of, paternal behavior by males. (It was already known to
be associated with maternal behavior by females). Three versions of about
the same paper, two of them pdfs.
http://physiologyonline.physiology.org/cgi/content/full/14/6/223
http://ecc.pima.edu/~rnyberg/202/202%20Articles/Endocrine%20system/223a.pdf
http://www.allmanlab.caltech.edu/classes/cns217/Schradin_prolactin.pdf
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I am emersonj at gmail dot com.
Original materials copyright John J
Emerson
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