Why do mammals produce milk? What makes them produce milk?
Mammals include all those animals that fall under the class Mammalia of the Animal Kingdom. They are known for their characteristics of breastfeeing their young ones.
Now, why do mammals produce milk? What’s the reason?
The most important reason why mammals produce milk is to nourish their young ones from birth to a period of three to five years of age. The milk helps to keep the baby healthy and develop very fast.
The milk is the first goodness that the baby will feed upon. It’s a natural behavior of the mammalian babies to go and lick the mother’s teats to drink milk out of it as soon as they are born.
The milk supplies all the necessary nutrients that are required for the baby to grow in the proper proportions.
The milk also has the ability to provide immunity and protect the baby against allergies, sickness, diseases, diseases, and obesity.
When the baby is a fetus that is when it is developing in the mother’s womb, then the placenta help the fetus to get all the important nutrition needed for development inside the mother’s body.
But, when the baby is born it still needs to be developed more in order to be a part of the highest and advanced class of animals called mammals. So, it feeds on mother’s milk that will help it grow better and healthier.
So, what makes mammals produce milk? Both male and female mammals have mammary glands, but only the females with the mammary glands can produce milk by secreting it through their nipples connected to the alveoli cells where the milk is produced.
Mammary glands are the glands that have evolved from modified sweat glands to produce milk for offspring.
In simple words, the mammary glands can secrete milk and store it in the alveoli of the breasts.
Next when the baby sucks the milk the muscles around the alveoli of the breasts squeeze the milk out through the milk ducts and nipples.
Do all mammals produce milk?
Yes, all species of mammals produce milk and so have the milk-secreting mammary glands.
Except in the case of a few monotremes like Platypus and Echidna that do produce milk but don’t have nipples to suck it out like other mammals.
Unlike the other mammals, monotreme mammals like Platypus and Echidna don’t have teats/nipples, instead, they concentrate milk on their belly and feed their young by sweating it out.
Mammals got their name and classification due to the presence of mammary glands. These glands are the part of the breasts with special ducts which channel the milk from the glands into the teats.
So, in simple words, yes all mammals can produce milk and in those of Platypus and Echidna, the milk is secreted onto the surface of the skin like sweat and licked off the body hairs by the young ones as they don’t have teats.
And, it is also to be noted that only the female mammals produce milk. Milk production and secretion is actually a beneficial adaptation that requires an investment of time and energy to care for the newborn offspring by the mothers.
Male mammals also have mammary glands but the nerves that signal the glands from the brain to produce and secrete milk are blocked, opposed, and lost due to the presence of male sex hormones like testosterone.
The male sex hormones control the gender-specific absence or presence of mammary gland nerves that sense the amount of milk available in breast milk ducts.
What actually is the Mammary Gland?
Mammary glands are the modified sweat glands that are present in the mammalian body. These are a type of exocrine glands because these glands can secrete substances onto an epithelial surface by way of a duct.
Two or many teats of mammary glands can be present in both sexes. Just like in humans, we have a pair of mammary glands, and in the case of mammals like cows there are 4 teats, and in cats there are 8 teats, and the number of teats varies between the species.
The mammary glands of females are only able to produce milk and those of the males don’t.
They become functional at puberty in response to estrogen in females and normally remain underdeveloped in males.
After the pregnancy and childbirth, the mammary glands reach full development and function in excessive milk production than normal after the delivery of the infant.
As being already said, the mammary glands are the modified sweat glands present in each of the breasts with a nipple attached to it for sucking out the milk.
In mammals like in humans, the nipples are surrounded by a circular pigmented area of skin called areola.
The nipples can get erected when its time to secrete milk due to the contraction of the sebaceous glands around the aerola. Note that, there are no muscles in the breasts.
The mammary glands of each of the breasts are actually the exocrine glandular tissue that also does contain a variable amount of fats.
This glandular tissue of each mammary gland of the breast contains 10 to 20 mammary lobes containing cluster of cells called alveoli.
The function of alveoli is to secrete milk, which remain stored in the alveolar cavities. The alveoli open into the mammary tubules and the tubules of each lobe join to form a mammary duct.
Several mammary ducts join to form a wider mammary ampulla which is connected to the lactiferous duct through which the milk is being sucked out.
Kindly Note: The basic components of the mammary gland are the alveoli cells which are the hollow cavities like cells which are a few millimeters large that are being lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells.
How do female mammals produce milk?
Milk in the female mammal’s breasts are produced by the biophysical action of the mammary glands along with the ducts, and also due to the biochemical actions taking place in the body as well.
The biophysical action explains how the milk is secreted by the nipples of the breasts due to contraction, erection, and relaxation of the breast and nipple tissues.
The biochemical action explains how the various hormones like prolactin and oxytocin works when present in the blood by taking the various nutrients from the blood and make milk out of it in the mammary glands.
The mammary glands are made up of 10 to 20 mammary lobes. The lobes are separated by connective tissue that attached all the lobes together in one breast.
The adipose tissue in each of the lobes secrete fats, and also align the size and shape of the breasts.
The glandular tissue of mammary glands contains the alveoli cells that produce and secrete the milk. The milk gets stored in the alveolar cavities.
When it’s time for the milk to be sucked out, the nipples get erected and get raised in response to stimulation to lick or touch.
This contracts the mammary glands and the alveoli cells start to secrete milk in the alveolar cavities.
Those stored milk come out through the mammary tubes and then through the minute mammary duct.
There are several mammary ducts that join to form a wider ampulla which is all connected to a single lactiferous duct.
This lactiferous duct opens out through the nipple through which the milk comes out when the breasts are pressed or when the nipples are sucked.
Now, how the milk comes out was a part of the biophysical process. Now, what’s the biochemical process that influences milk secretion? Let’s Know!
Mammary gland function is regulated by hormones. First of all, at the age of puberty, increasing levels of estrogen hormone in the female blood stimulate the development of mammary glands in the female breasts.
It’s due to the estrogen hormone you will see female breasts growing bigger as she gets old and mature starting from puberty. Estrogen also causes the breast to increase in size through the accumulation of adipose tissue.
There’s another hormone called Progesterone that stimulates the development of the duct system that includes the lactiferous duct, and mammary ducts.
The ovaries and the adrenal glands secrete eastrogen in the blood. Adrenal glands only make small amounts of this hormone.
Although the corpus luteum in the ovaries is the major site of progesterone production in humans. Progesterone is also produced in smaller quantities by the ovaries themselves, the adrenal glands, and during pregnancy by the placenta.
Another two hormones that are also secreted signal and influence milk production. These are prolactin and oxytocin.
Prolactin from the anterior pituitary stimulates the production of milk within the glandular tissue, and oxytocin produced in the hypothalamus causes the ejection of milk from the glands.
All of the hormones viz. estrogen, progesterone, prolactin, and oxytocin are secreted from their sources directly to the blood stream.
So, prolactin reaches the target mammary glands and stimulates the absorption of various fats and nutrients from the blood by the cells while causing the production of milk.
And, during breastfeeding, oxytocin promotes the movement of milk into the breast, allowing it to be excreted by the nipple.
How the evolution of milk production and breastfeeding have occurred in mammals?
The evolution of milk production and breastfeeding have occurred in mammals years before mammals have set foot on earth.
The ancestors of mammals called synapsids had some of the traits of mammals by 275 million years ago. These traits also included the presence of mammary glands and milk production from it.
At present, mammals are the only one that has mammary glands and it’s because the ancestors they have evolved from were the synapsids. The synapsids are now extinct.
So, it would be much better to say that milk originated as a glandular skin secretion in synapsids, perhaps as early as the Pennsylvanian period, that is, approximately 310 million years ago.
It is also to be noted that the early synapsids had apocrine-like glands that had helped them secrete various glandular skin secretions for moisture over their laid eggs.
Evolution of the apocrine-like glands into partial mammary glands have already occured in those ancestral synapsids.
So, as per the evolutionary hypothesis are considered, the comparative analyses of the evolutionary origin of milk constituents support a scenario in which these secretions evolved into nutrient-rich milk long before mammals arose.
The mammary glands of the present-day mammals have probably much more advanced and have better evolved from the all-around modifications of the apocrine-like glands.
So, the evolution of mammary glands can be better traced and tracked if we study how the modifications may have occurred from the initial stages.
So, we can conclude that all the primary milk constituents evolved before the appearance of mammals, and some constituents may have origins that can be linked to the evolutionary splitting of the synapsids from sauropsids (the lineage leading to reptiles and birds).
Do any male mammals produce milk?
In general, males don’t lactate, as the male sex hormones like testosterone create various changes in their body and in their mammary gland system by opposing the production and secretion of milk.
Also, the other hormones like prolactin, oxytocin, estrogen, progesterone are also secreted in very low or ignorable amounts in males. Thus, overall causing the presence of underdeveloped mammary glands with no milk production at all.
However, with some rare exceptions, male lactation (milk production) has been observed in a few domesticated animals like cats, goats, and guinea pigs on rare occasions.
And, male lactation seems to be common only in Dayak fruit bats, though it’s not yet clear why males of this species have the biology to spontaneously produce milk.
In normal times, non-pregnant females can cause a medium concentration of milk production. However, during or after a few months of pregnancy there can be a large spike in the concentration of milk production.
It’s because the concentration of prolactin in the blood increases or just doubles during or after a few months of pregnancy that causes a large spike in the concentration of milk production.
Man has really a very low or ignorable amount of prolactin in the blood that doesn’t influence the development and growth of mammary glands and the milk production in it.
Research has shown that male prolactin levels can also increase, if after suffered months of food starvation the male gets adequate nutrition, resulting in hormonal spikes that can also cause lactation.
Similarly, a condition called Liver cirrhosis can cause lactation by disrupting the organ’s normal, hormone-metabolizing function.
Also, in some rare cases, if any health issues affect the pituitary gland or the hypothalamus, by normally inhibiting the release of prolactin, can also cause male milk production.
So, in very simple words, male mammals don’t produce milk at all and that’s the rule with the 99% male mammals.
However, with the rarest of the rarest exceptions, the remaining 1% of the males can also produce milk.