email - January 2002

Sweating Milk

Did sweat glands evolve into mammary glands? At least one evolutionist thinks so.

Andy wrote this reaction to our September 2000 parody, “Stone Age Mutant Mammal Turtles.”

Subject: Evolution
Date: Thu, 13 Sep 2001 12:57:35 +0100
From: Andy

I have just read your article on the webpage below and a few things came to mind. I wondered if you would have the time to comment?

[anti-Christian hate speech deleted]

You also wonder how breasts/udders come from nothing. In fact, these are specialised sweat glands, and it is easy to imagine new born proto-mammals licking their mothers for moisture. Later the species mutates/evolves and one by one a new nutient is added to the sweat/milk.

[more non-scientific material deleted]



The tone of Andy’s email certainly seems to indicate an emotional, rather than rational, look at the evidence. The numerous spelling (or typing) errors would suggest a passionate, hastily-written letter, rather than a calm, reasoned analysis.

The only part of his letter that addresses any scientific issue at all is his assertion that milk is just nutritious sweat. (If I knew where Andy buys his milk, I would make it a point NOT to patronize that dairy! smiley face )

We wrote back saying, “The key word in your letter is ‘imagine’. We think medical doctors would find it more difficult to imagine than you do.”

Andy was able to find support for his position in a medical book. He wrote,

I have now remembered where I read that breasts were specialised sweat glands, and ironically it was a medical textbook.

Human Anatomy and Physiology, by Van Wynsberghe, Noback and Carok, published by McGraw-Hill. I have the third edition (ISBN 0-07-113540-5), and on p141 it says:

"Sudorific glands are also known as sweat glands. Two types of suderic glands exist: eccrine and apocrine... The female breast contains apocrine glands that have become adapted to secrete and release milk instead of sweat."

An interesting point to note is the word "adapted".



Yes, we know that “adapted” is a code word for “evolved.”

The book clearly says that sweat glands evolved into mammary glands. The question is, “Is the book correct?” Biology books are full of assertions that something evolved into something else without any facts to back them up. Is there any medical evidence that sweat glands evolved into mammary glands?

According to the fable, reptiles evolved into mammals. I have lived in the desert for more than 30 years and have seen lizards (reptiles) on a daily basis from March through October every year. After all these years of observation I have come to the conclusion that if lizards have a motto it is, “Never let ‘em see you sweat!” Even when it is over 110 degrees F, as it often is here, I’ve never seen a lizard with a single drop of sweat on it. That’s because reptiles don’t have sweat glands.

Mammals are named for the mammary glands used by all female members of this class to suckle their young (Fig. 24-39c). In addition to these unique milk-producing glands, the mammalian body is arrayed with sweat, scent, and sebaceous (oil-producing) glands, none of which are found in reptiles. 1 [emphasis supplied]

We have just criticized Andy for believing stuff he reads in a textbook. Why do we believe what is written in this textbook? It is a fair question. There is a good answer.

Everything stated in that quotation can be experimentally verified. One can verify that all female mammals suckle their young. Of course, that is really a tautology because they are classified as mammals because they suckle their young. The second sentence is more interesting. Countless biology/medical students have dissected mammals and reptiles. They have found (and perhaps even removed) those glands in mammals, and never found any such glands in the reptiles they have dissected. There is experimental evidence that mammals have sweat glands and reptiles don't.

There is no experimental verification that sweat glands turn into mammary glands. Reptilian sweat glands could not possibly have evolved into mammary glands because reptiles don’t have sweat glands.

Reptiles are cold-blooded animals. They regulate their body temperatures primarily by moving in and out of the sun. They can get away with this because they can survive with a larger body temperature range than warm-blooded mammals. Mammals need sweat glands to cool them off, so they don’t overheat. Reptiles don’t need this capability. So, the first question one might ponder is, “Why would reptiles evolve a cooling mechanism they don’t need as a step toward evolving milk glands?”

One might argue that they evolved a warm-blooded metabolism first (no small feat in itself), and consequently needed sweat glands to cool themselves off. But both of these miracles would have to evolve at the same time because a reptile that evolved warm-bloodedness before evolving sweat glands would overheat and die.

Suppose, however, that some reptiles evolved high metabolism and sweat glands at the same time. Then, millions of years later, some of these reptiles “adapted” their sweat glands to secrete milk instead. Isn’t it reasonable to think that some of the reptiles that didn’t evolve mammary glands might still be alive today? Why aren’t there some reptiles that have evolved sweat glands but not mammary glands?

But, suppose that there were some lizards with sweat glands that are now extinct. Sweat glands produce sweat in response to temperature. Mammary glands produce milk in response to hormonal changes that occur at the end of pregnancy. What would make a gland produce a different kind of fluid in response to a different kind of stimulus?

Furthermore, we are puzzled why a gland would evolve that eliminates nutrients from the body. In a previous essay we estimated the maximum amount of milk a cow could give, based on the number of calories in a quart of milk and the number of calories in a cow’s diet. Clearly a cow cannot produce more calories than she ingests. If she could, she would violate the law of conservation of energy. In our calculations we treated the cow as a machine with a particular efficiency, where the calories not turned into milk were considered as “waste.” But from the cow’s point of view, these calories aren’t wasted at all. They are what she lives on! The more calories she gives to the farmer, the fewer she has for herself.

In light of this, what is the evolutionary advantage to a mother who wastes calories by secreting them as milk? It robs her own system of needed calories.

There aren’t any good answers to these questions. There is no plausible explanation of how sweat glands could have evolved into mammary glands, nor any evidence that it actually did occur.

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1 Audesirk & Audesirk, 1996, Biology, page 489 (Ev)