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Only mammals have mammaries and mammary glands.
What evolutionary factors determines the number of mammaries (nipples/teats/breasts) a species has?
Is it always an even number?
The number of mammary glands a species has is related to litter size. The relationship generally follows the "one-half rule," which states that the average litter size is equal to half the number of mammaries. The number of mammaries also tends to put an upper limit on litter size. It's not necessarily a hard limit, but survival tends to drop noticeably when number of offspring being nursed exceeds number of mammaries.
The number of mammaries is almost always even, but a notable exception is the Virginia possum, which has thirteen.
Gilbert, A.N. 1986. Mammary number and litter size in Rodentia: The "one-half rule". Proc. Natl. Acad. Sci. USA. Vol. 83, pp. 4828-4830.
As an addition to the excellent post by Erin.
As a name-giving key feature,mammals have mammary glands.
"Animals that are bilaterally symmetric have mirror symmetry in the sagittal plane", which includes Chordates such as mammals.
Some have seen this iconic picture, showing the bilateral symmetry in chordates:
It is easy to see why most commonly even number of mammary glands are found in mammals, given that they form in pairs, distal from the notochord.
Evolutionary, an uneven number of mammaries may be explained through a subjectively-mild mutation which may be retained as a vestigial-like feature, somewhat-functional or not.
On a molecular level, unilateral mammary glands likely involve key developmental pathways such as hedgehog and likely has underlying polygeneic root causes, - for the termination of gland formation on one side.
See this intersting paper:
Kohei Amakasu, Katsushi Suzuki, Hiroetsu Suzuki* "The Unilateral Urogenital Anomalies (UUA) Rat", Comp Med. 2009 June; 59(3): 249-256.
We established an inbred rat strain with unilateral urogenital anomalies from an incidentally identified male rat with unilateral renal agenesis and an undescended left testis. These rats were characterized by unilateral renal agenesis in both sexes, undescended testes with agenesis and hypoplasia of the accessory sex organs in male rats, and complete and partial agenesis of the uterine horn in female rats. All of these urogenital anomalies were unilateral and restricted to the left side; we named this phenotype unilateral urogenital anomalies (UUA). Breeding tests showed that these abnormalities were inherited as polygenic traits
It is noteworthy that the mammary glands in the male human population didn't decrease fitness to such a degree where evolutionary pressure would have lead to cease gland formation altogether, by adding extra information to the sex-chromosome(s). As such human males retain mammary glands, albeit typically never reaching a functional state.
The factors weighting against such additional genes are, that the default formation in most mammals is the female body plan, which conserves functional mammary glands to potentially feed their offspring, chipping away at the chance of genes involved in the cessation of mammary gland formation.
Secondly, selection pressure for information retained on the sex-chromosomes is higher, as opposed to autosomal (non-sex) genes. (See this nice concise post by Danielle Venton, The Degenerating Y Chromosome,Genome Biol Evol (2011) 3 1338-1339. )
Concluding, naturally, it would be easy for an asymmetric sponge to have an uneven number of protrusions or orifices, less so for humans, unless the orifice is located along an axis of symmetry.
Ad biological symmetry:
The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant.   The milk can flow through the nipple passively or it can be ejected by smooth muscle contractions that occur along the ductal system. The nipple is surrounded by the areola, which is often a darker color than the surrounding skin.  A nipple is often called a teat when referring to non-humans. Nipple or teat can also be used to describe the flexible mouthpiece of a baby bottle. In humans, nipples of both males and females can be stimulated as part of sexual arousal. In many cultures, human female nipples are sexualized,  or "regarded as sex objects and evaluated in terms of their physical characteristics and sexiness." 
7 Celebrities Who Have Freed the Nipple on Social Media
In 2012, filmmaker Lina Escoꂾgan working on Free The Nipple,ਊ film about a group of women determined to end the censorship of women&aposs breasts. Though activists had already launched a movement to normalize female breasts in public spaces, Esco&aposs film made it an official and more widespread campaign.
Around the time the movie came out in 2014, celebs like Miley Cyrus,ꃊra Delevingne, and Lena Dunham took to social media to show their support for the movement𠅊nd they&aposre still behind it to this day.
The #FreeTheNipple hashtag now has over 4 million posts on Instagram, and every so often, a celeb will share a new picture proudly showing off their nipple. Why are people fighting to free the nipple? Some want it to be just as acceptable for women to expose their nipples in public as it is for men others are protesting laws that prevent women, but not men, from going topless. It&aposs also about ensuring that women have ownership over how they choose to present their bodies to the world.
Here, seven times celebs showed that they&aposre all for nipple freedom on social media.
Male Nipple Abnormalities
There are characteristics associated with the female breast and nipple that can abnormally occur in the male breast and nipple as well. Some are the result of the dysregulation of hormones, while others may be triggered by genetics.
While men will not lactate under usual circumstances, the male breast can produce milk if under the influence of the hormone prolactin.
The condition, known as male galactorrhea, often occurs as a result of a medication or medical condition that triggers a drop in male hormones (primarily testosterone) and an associative increase in female hormones.
One such example is the drug Motilium (domperidone), which not only treats lactation problems in women but can be used in men to treat nausea, vomiting, gastroparesis, and Parkinson's disease.
Other causes include malnutrition, pituitary gland disorders, hypothyroidism (low thyroid function), and frequent nipple stimulation.
Gynecomastia is the enlargement of male breasts that commonly occurs in older men as testosterone levels progressively drop with age. In addition to the general swelling of breast tissue, gynecomastia can trigger the enlargement of the nipples and surrounding areolas.
Gynecomastia can also affect boys and younger men for any number of reasons. In some cases, the condition will be temporary, particularly in adolescent boys undergoing puberty.
Other common causes include:
- Adrenal or pituitary gland tumor
- Cancer chemotherapy
- Kidney failure
- Prostate medications like Propecia (finasteride) and Aldactone (spironolactone)
- Tricyclic antidepressants
Gynecomastia is associated with an increased risk of breast cancer in men. While rare, breast cancer in men is most commonly detected by the formation of a hardened lump under the nipple and areola.
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You can find more tips in our guide, How to Find a Disease Specialist. We also encourage you to explore the rest of this page to find resources that can help you find specialists.
- To find a medical professional who specializes in genetics, you can ask your doctor for a referral or you can search for one yourself. Online directories are provided by the American College of Medical Genetics and the National Society of Genetic Counselors. If you need additional help, contact a GARD Information Specialist. You can also learn more about genetic consultations from MedlinePlus Genetics.
Monotremes and Their Non-Existent Feeding Organs Like Mammals
There are only five species of monotremes in the world, and these five species are the only species of mammals that are not viviparous. The entire living subclass is made up of four echidna species and the platypus. Like reptiles, a monotreme lays eggs. Like marsupials, monotremes store these eggs in an external pouch where the eggs develop. Finally, like every mammal, monotremes rear their young with milk from their mammary glands.
However, monotremes do not have both, so how in the world do they feed their young? Why, through their mammary hair of course! A monotreme&aposs mammary glands produce milk that is released through the pores. It eventually makes its way to patches of fur that the young monotremes suck on.
- ↑ For the purposes of this article, non-mammals that appear to have breasts on their chest will have those features conjecturally referred to as "breasts" based on visual resemblance, even though they are not identified as such in the sources cited.
- ↑ 2.02.12.22.18.104.22.168.72.82.9The New Essential Guide to Alien Species
- ↑ 3.03.13.23.22.214.171.124Alien Encounters
- ↑ 4.04.14.24.34.4Star Wars: Episode VI Return of the Jedi
- ↑ 5.05.15.2 "Skin Deep: The Fat Dancer's Tale"—Tales from Jabba's Palace
- ↑ "Out of the Closet: The Assassin's Tale"—Tales from Jabba's Palace
- ↑Epilogue: Whatever Became Of.
- ↑ "Goatgrass: The Tale of Ree-Yees"—Tales from Jabba's Palace
- ↑Strongholds of Resistance
- ↑ 10.010.1The Official Star Wars Fact File12(WOO1, Wookiees)
- ↑ 11.011.111.211.3The Essential Guide to Alien Species
- ↑ 12.012.1 "Murder in Slushtime"—Star Wars Adventure Journal 14
- ↑ 13.013.1Children of the Jedi
- ↑ 14.014.1The Essential Guide to Planets and Moons
- ↑The Courtship of Princess Leia
- ↑The Last Command
- ↑The New Jedi Order: Star by Star
- ↑Dark Nest II: The Unseen Queen
- ↑ 19.019.1The Essential Guide to Droids
- ↑Star Wars: Episode III Revenge of the Sith
- ↑Timeline 4: The Empire Changes Strategy
- ↑Star Wars 53: The Last Gift From Alderaan!
- ↑Star Wars: The Force Unleashed video game
- ↑The Essential Reader's Companion
- ↑Star Wars 2: Prelude to Rebellion, Part 2
- ↑Dark Times 7: Parallels, Part 2
- ↑ "A Boy and His Monster: The Rancor Keeper's Tale"—Tales from Jabba's Palace
- ↑ 28.028.128.228.3Ultimate Alien Anthology
- ↑ 29.029.129.2Star Wars 74: The Iskalon Effect
- ↑X-Wing: Isard's Revenge
- ↑Agent of the Empire—Iron Eclipse, Part 2
- ↑ "Alien Encounters"—Star Wars Adventure Journal 13
- ↑Legacy 34: Storms, Part 1
- ↑Crimson Empire II: Council of Blood 2
- ↑Wretched Hives of Scum & Villainy
- ↑Star Wars 102: School Spirit
- ↑ Mon Calamari in the Encyclopedia(content now obsolete backup link)
- ↑Star Wars: Clone Wars – "Chapter 25"
- ↑Star Wars: Clone Wars – "Chapter 5"
- ↑ 40.040.1Star Wars: Episode VI Return of the Jedi(Special Edition onward)
- ↑Scum and Villainy
- ↑Star Wars: Episode I The Phantom Menace
- ↑Star Wars: The Clone Wars – "Hunt for Ziro"
- ↑Star Wars: The Clone Wars – "Lethal Trackdown"
- ↑Legacy 14: Claws of the Dragon, Part 1
- ↑Legacy 43: Monster, Part 1
- ↑Star Wars 34: Darkness, Part 3
- ↑Death Star
- ↑Planets of the Galaxy, Volume Two
- ↑The New Essential Guide to Alien Species. The book classifies Rodians as reptilian, yet says, "In addition, females are physically distinguished by their mammary glands." It is unclear if this is intended to mean Rodian females have milk-producing mammary glands, or simply the appearance of the type of mammary glands called breasts in primates.
- ↑Star Wars 64: Serphidian Eyes
- ↑Star Wars 40: The Devaronian Version, Part 1
- ↑Star Wars: Episode II Attack of the Clones
- ↑ "Unknown Soldier: The Story of General Grievous"—Star Wars Insider 86
- ↑The Old Republic: Annihilation
- ↑Republic 56: The Battle of Jabiim, Part 2
- ↑Shadows of the Empire: Evolution 1
- ↑Slave Ship
- ↑Purge: The Tyrant's Fist, Part 1
- ↑Star Wars: Episode I Racer: Prima's Official Strategy Guide
- ↑Star Wars: The Old Republic
- ↑ 62.062.1The New Essential Guide to Droids
- ↑Star Wars: The Clone Wars – "Evil Plans"
- ↑Agent of the Empire—Iron Eclipse, Part 2
- ↑The Official Star Wars Fact File94(TDL1-2, TDL Nanny Droid)
- ↑ 66.066.1The Star Wars Holiday Special
- ↑ 67.067.1 "The Toys That Never Were"—Star Wars Insider 92
- ↑ Clarke, Gerald. "Show Business: I've Got to Get My life Back Again". Time. Vol. 121 #21, Time Inc., 1983-05-23. ISSN 0040-781X. "Chewbacca is a favorite of Lucas', and he can go on and on about the Wookie [sic] tribe. They come from a damp jungle planet where they reside in tree houses and live to be 350 years old. The six-breasted females deliver their offspring in litters." (web archive)
- ↑Young Jedi Knights: Diversity Alliance
- ↑Star Wars 91: Wookiee World
- ↑Chewbacca 1
- ↑ "Star Wars Style: Return of the Jedi, Part 6"—Star Wars Insider 158
- ↑ Star Wars Mysteries: Who is Wiebba-Wiebba? on StarWars.com (article) (backup link)
- ↑ Star Wars: The Legacy Collection (Pack: Yarna D'al' Gargan) (backup link)
- ↑From Pencil to Pixel: The Art of Star Wars Galaxies
- ↑The Art of Star Wars Episode III: Revenge of the Sith
- ↑Star Wars: The Clone Wars – "Water War"
The basic components of a mature mammary gland are the alveoli (hollow cavities, a few millimeters large), which are lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join to form groups known as lobules. Each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells contract under the stimulation of oxytocin, excreting the milk secreted by alveolar units into the lobule lumen toward the nipple. As the infant begins to suck, the oxytocin-mediated "let down reflex" ensues, and the mother's milk is secreted — not sucked — from the gland into the baby's mouth. 
All the milk-secreting tissue leading to a single lactiferous duct is collectively called a "simple mammary gland" in a "complex mammary gland", all the simple mammary glands serve one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia.
Maintaining the correct polarized morphology of the lactiferous duct tree requires another essential component – mammary epithelial cells extracellular matrix (ECM) which, together with adipocytes, fibroblast, inflammatory cells, and others, constitute mammary stroma.  Mammary epithelial ECM mainly contains myoepithelial basement membrane and the connective tissue. They not only help to support mammary basic structure, but also serve as a communicating bridge between mammary epithelia and their local and global environment throughout this organ's development.  
A mammary gland is a specific type of apocrine gland specialized for manufacture of colostrum when giving birth. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Many sources assert that mammary glands are modified sweat glands.    Some authors dispute that and argue instead that they are sebaceous glands. 
Mammary glands develop during different growth cycles. They exist in both sexes during embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development depends on systemic (and maternal) hormones,  but is also under the (local) regulation of paracrine communication between neighboring epithelial and mesenchymal cells by parathyroid hormone-related protein (PTHrP).  This locally secreted factor gives rise to a series of outside-in and inside-out positive feedback between these two types of cells, so that mammary bud epithelial cells can proliferate and sprout down into the mesenchymal layer until they reach the fat pad to begin the first round of branching.  At the same time, the embryonic mesenchymal cells around the epithelial bud receive secreting factors activated by PTHrP, such as BMP4. These mesenchymal cells can transform into a dense, mammary-specific mesenchyme, which later develop into connective tissue with fibrous threads, forming blood vessels and the lymph system.  A basement membrane, mainly containing laminin and collagen, formed afterward by differentiated myoepithelial cells, keeps the polarity of this primary duct tree. These components of the extracellular matrix are strong determinants of duct morphogenesis. 
Estrogen and growth hormone (GH) are essential for the ductal component of mammary gland development, and act synergistically to mediate it.      Neither estrogen nor GH are capable of inducing ductal development without the other.     The role of GH in ductal development has been found to be mostly mediated by its induction of the secretion of insulin-like growth factor 1 (IGF-1), which occurs both systemically (mainly originating from the liver) and locally in the mammary fat pad through activation of the growth hormone receptor (GHR).      However, GH itself also acts independently of IGF-1 to stimulate ductal development by upregulating estrogen receptor (ER) expression in mammary gland tissue, which is a downstream effect of mammary gland GHR activation.  In any case, unlike IGF-1, GH itself is not essential for mammary gland development, and IGF-1 in conjunction with estrogen can induce normal mammary gland development without the presence of GH.  In addition to IGF-1, other paracrine growth factors such as epidermal growth factor (EGF), transforming growth factor beta (TGF-β),  amphiregulin,  fibroblast growth factor (FGF), and hepatocyte growth factor (HGF)  are involved in breast development as mediators downstream to sex hormones and GH/IGF-1.   
During embryonic development, IGF-1 levels are low, and gradually increase from birth to puberty.  At puberty, the levels of GH and IGF-1 reach their highest levels in life and estrogen begins to be secreted in high amounts in females, which is when ductal development mostly takes place.  Under the influence of estrogen, stromal and fat tissue surrounding the ductal system in the mammary glands also grows.  After puberty, GH and IGF-1 levels progressively decrease, which limits further development until pregnancy, if it occurs.  During pregnancy, progesterone and prolactin are essential for mediating lobuloalveolar development in estrogen-primed mammary gland tissue, which occurs in preparation of lactation and nursing.  
Androgens such as testosterone inhibit estrogen-mediated mammary gland development (e.g., by reducing local ER expression) through activation of androgen receptors expressed in mammary gland tissue,   and in conjunction with relatively low estrogen levels, are the cause of the lack of developed mammary glands in males. 
Before birth Edit
Mammary gland development is characterized by the unique process by which the epithelium invades the stroma. The development of the mammary gland occurs mainly after birth. During puberty, tubule formation is coupled with branching morphogenesis which establishes the basic arboreal network of ducts emanating from the nipple. 
Developmentally, mammary gland epithelium is constantly produced and maintained by rare epithelial cells, dubbed as mammary progenitors which are ultimately thought to be derived from tissue-resident stem cells. 
Embryonic mammary gland development can be divided into a series of specific stages. Initially, the formation of the milk lines that run between the fore and hind limbs bilaterally on each side of the midline occurs around embryonic day 10.5 (E10.5). The second stage occurs at E11.5 when placode formation begins along the mammary milk line. This will eventually give rise to the nipple. Lastly, the third stage occurs at E12.5 and involves the invagination of cells within the placode into the mesenchyme, leading to a mammary anlage (biology). 
The primitive (stem) cells are detected in embryo and their numbers increase steadily during development 
Postnatally, the mammary ducts elongate into the mammary fat pad. Then, starting around four weeks of age, mammary ductal growth increases significantly with the ducts invading towards the lymph node. Terminal end buds, the highly proliferative structures found at the tips of the invading ducts, expand and increase greatly during this stage. This developmental period is characterized by the emergence of the terminal end buds and lasts until an age of about 7–8 weeks.
By the pubertal stage, the mammary ducts have invaded to the end of the mammary fat pad. At this point, the terminal end buds become less proliferative and decrease in size. Side branches form from the primary ducts and begin to fill the mammary fat pad. Ductal development decreases with the arrival of sexual maturity and undergoes estrous cycles (proestrus, estrus, metestrus, and diestrus). As a result of estrous cycling, the mammary gland undergoes dynamic changes where cells proliferate and then regress in an ordered fashion. 
During pregnancy, the ductal systems undergo rapid proliferation and form alveolar structures within the branches to be used for milk production. After delivery, lactation occurs within the mammary gland lactation involves the secretion of milk by the luminal cells in the alveoli. Contraction of the myoepithelial cells surrounding the alveoli will cause the milk to be ejected through the ducts and into the nipple for the nursing infant. Upon weaning of the infant, lactation stops and the mammary gland turns in on itself, a process called involution. This process involves the controlled collapse of mammary epithelial cells where cells begin apoptosis in a controlled manner, reverting the mammary gland back to a pubertal state.
During postmenopause, due to much lower levels of estrogen, and due to lower levels of GH and IGF-1, which decrease with age, mammary gland tissue atrophies and the mammary glands become smaller.
Hormonal control Edit
Lactiferous duct development occurs in females in response to circulating hormones. First development is frequently seen during pre- and postnatal stages, and later during puberty. Estrogen promotes branching differentiation,  whereas in males testosterone inhibits it. A mature duct tree reaching the limit of the fat pad of the mammary gland comes into being by bifurcation of duct terminal end buds (TEB), secondary branches sprouting from primary ducts   and proper duct lumen formation. These processes are tightly modulated by components of mammary epithelial ECM interacting with systemic hormones and local secreting factors. However, for each mechanism the epithelial cells' "niche" can be delicately unique with different membrane receptor profiles and basement membrane thickness from specific branching area to area, so as to regulate cell growth or differentiation sub-locally.  Important players include beta-1 integrin, epidermal growth factor receptor (EGFR), laminin-1/5, collagen-IV, matrix metalloproteinase (MMPs), heparan sulfate proteoglycans, and others. Elevated circulating level of growth hormone and estrogen get to multipotent cap cells on TEB tips through a thin, leaky layer of basement membrane. These hormones promote specific gene expression. Hence cap cells can differentiate into myoepithelial and luminal (duct) epithelial cells, and the increased amount of activated MMPs can degrade surrounding ECM helping duct buds to reach further in the fat pads.   On the other hand, basement membrane along the mature mammary ducts is thicker, with strong adhesion to epithelial cells via binding to integrin and non-integrin receptors. When side branches develop, it is a much more "pushing-forward" working process including extending through myoepithelial cells, degrading basement membrane and then invading into a periductal layer of fibrous stromal tissue.  Degraded basement membrane fragments (laminin-5) roles to lead the way of mammary epithelial cells migration.  Whereas, laminin-1 interacts with non-integrin receptor dystroglycan negatively regulates this side branching process in case of cancer.  These complex "Yin-yang" balancing crosstalks between mammary ECM and epithelial cells "instruct" healthy mammary gland development until adult.
There is preliminary evidence that soybean intake mildly stimulates the breast glands in pre- and postmenopausal women. 
Secretory alveoli develop mainly in pregnancy, when rising levels of prolactin, estrogen, and progesterone cause further branching, together with an increase in adipose tissue and a richer blood flow. In gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells' branching.   When the lactiferous duct tree is almost ready, "leaves" alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates further alveologenesis, milk protein production, and regulates osmotic balance and tight junction function. Laminin and collagen in myoepithelial basement membrane interacting with beta-1 integrin on epithelial surface again, is essential in this process.   Their binding ensures correct placement of prolactin receptors on the basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts.   Suckling of the baby causes release of the hormone oxytocin, which stimulates contraction of the myoepithelial cells. In this combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.
During weaning, decreased prolactin, missing mechanical stimulation (baby suckling), and changes in osmotic balance caused by milk stasis and leaking of tight junctions cause cessation of milk production. It is the (passive) process of a child or animal ceasing to be dependent on the mother for nourishment. In some species there is complete or partial involution of alveolar structures after weaning, in humans there is only partial involution and the level of involution in humans appears to be highly individual. The glands in the breast do secrete fluid also in nonlactating women.  In some other species (such as cows), all alveoli and secretory duct structures collapse by programmed cell death (apoptosis) and autophagy for lack of growth promoting factors either from the ECM or circulating hormones.   At the same time, apoptosis of blood capillary endothelial cells speeds up the regression of lactation ductal beds. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin and other growth inhibiting hormones and local factors.  This major structural change leads loose fat tissue to fill the empty space afterward. But a functional lactiferous duct tree can be formed again when a female is pregnant again.
Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components,  or from a mechanical change in the tension of mammary stroma.  Under either of the two circumstances, mammary epithelial cells would grow out of control and eventually result in cancer. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.
The breasts of the adult human female vary from most other mammals that tend to have less conspicuous mammary glands. The number and positioning of mammary glands varies widely in different mammals. The protruding teats and accompanying glands can be located anywhere along the two milk lines. In general most mammals develop mammary glands in pairs along these lines, with a number approximating the number of young typically birthed at a time. The number of teats varies from 2 (in most primates) to 18 (in pigs). The Virginia opossum has 13, one of the few mammals with an odd number.   The following table lists the number and position of teats and glands found in a range of mammals:
|Species ||Anterior |
|Goat, sheep, horse |
|Dog ||4||2||2 or 4||8 or 10|
|Virginia oppossum  ||0||0||13||13|
|Southern red-sided opossum ||0||0||25 to 27||25 to 27|
Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice do not have nipples,  male marsupials do not have mammary glands,  and male horses lack nipples and mammary glands. [ citation needed ] The male Dayak fruit bat has lactating mammary glands.  Male lactation occurs infrequently in some species. 
Mammary glands are true protein factories,  and several labs have constructed transgenic animals, mainly goats and cows, to produce proteins for pharmaceutical use.  Complex glycoproteins such as monoclonal antibodies or antithrombin cannot be produced by genetically engineered bacteria, and the production in live mammals is much cheaper than the use of mammalian cell cultures.
There are many theories on how mammary glands evolved. For example, it is thought that the mammary gland is a transformed sweat gland, more closely related to apocrine sweat glands.  Because mammary glands do not fossilize well, supporting such theories with fossil evidence is difficult. Many of the current theories are based on comparisons between lines of living mammals—monotremes, marsupials, and eutherians. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist   and free from infection   (monotremes still lay eggs). Other theories suggest that early secretions were used directly by hatched young,  or that the secretions were used by young to help them orient to their mothers. 
Lactation is thought to have developed long before the evolution of the mammary gland and mammals see evolution of lactation.
The female breast is a complex organ. Understanding its anatomy and which parts perform certain functions is helpful if you choose to breastfeed, as you will be able to recognize sensations (such as the let-down reflex) and how milk production works. It is also important to become familiar with your breasts so you can determine what's normal for you and what's not.
If you are concerned by any changes in how your breasts look or feel, get in touch with your doctor for an evaluation.