Question - Why do cows have four mammary
glands?
Consider this for a few minutes before viewing
these further thoughts. The answer is not as
straight forward as you may initially think.
Thought A
Cows do not necessarily have only four teats. In all mammalian species, the mammary
glands form on the ventral or underside of the animal. The number of glands that an
animal has is determined at a very early stage of mammary development when the
animal is a developing embryo (see the resources section on Mammary Development).
In the developing embryo, the early developmental structures of the glands form along
a line on each side that is lateral to the ventral midline, a little to each side of the
navel. This means that the number of glands that an animal will have is determined
when it is an early fetus. She will not grow more teats or glands during pregnancy if
she is carrying more than the normal number of offspring.
Where the glands and nipples form on an animal is determined by the genetic
programming of the species. For example, in humans and elephants they form in the
chest region, in guinea pigs and cows in the inguinal (posterior) region, in mice in both
the thoracic and inguinal regions, and in pigs and dogs in the thoracic, abdominal and
inguinal regions.
Thought B
As many as 50% of the dairy calves are born with more than two teats on a side, but
these extra teats, called supernumerary teats, always form on the same line with the
two primary teats on each side. If the supernumerary teats are allowed to develop
further then they can hinder the milking process when the cow is mature. Even though
they usually are not very productive in making milk, they can add to the cow's risk of
the udder becoming infected with bacteria. With those risks in mind, the extra teats
usually are removed when the calf is only a few days old. Today we select for dairy
cattle that have four teats of standard size, placed squarely under the udder, because
this enhances ease of milking with a machine. This in turn makes the milking process
more efficient and minimizes the potential for problems with the udder being exposed
to bacteria and causing infection.
In fact, many mammals develop more than the typical number of nipples that we
normally associate with that species. Even humans sometimes develop with
supernumerary nipples. For James Bond fans, a fictional reference to this is the
distinguishing feature of the villain in The Man With The Golden Gun, who had an extra
nipple. Pigs usually have five to seven teats on each side, but I have seen them with
an eighth teat as far posterior as on the inner thigh of the rear leg.
Sheep normally have two teats in the inguinal region, one on each side. Sheep
typically can produce twins or triplets or sometimes quadruplets, so there is
competition for the milk when there are only two teats but three or more lambs trying
to nurse. The inventor, Alexander Graham Bell, tried to enhance prolificacy of sheep by
establishing a strain that had multiple teats, up to six or more. While he was able to
get multiple teats selected for in the sheep, most of the extra mammary glands were
not very effective at producing milk and so the idea was not ultimately successful.
I am not sure of what the selective or evolutionary advantage of having four
mammary glands would have been to the ancient Auroch prior to domestication and
selection for milk production. I also am not aware of conditions where calves are born
with less than four glands.
Thought C
We might expect that there is a relationship between the number of teats that an
animal has and the number of offspring she bares in a reproductive cycle. This is a
reasonable expectation, however, this relationship is imperfect. Cattle typically have
four functional glands, but normally only give birth to single offspring, sometimes
twins, and rarely triplets. Goats and sheep have only two glands, but more often give
birth to twins, triples or occassionally quadruplicates.
Furthermore, we might expect that litter-bearing species (that have multiple offspring)
would have glands all along the ventral surface, while species baring only singles or
twins would have glands located in only one area of the underside. This is generally
the case. Below are several examples of species that reflect the range of anatomical
location and number of glands.
Thoracic
Region
Abdominal
Region
Inguinal
Region
Cattle
-
-
4
Goat, Sheep
-
-
2
Horse
-
-
2
Pig
6
6
4
Cat
4
2
2
Dog
4
4
2
Rat
6
2
4
Mouse
6
-
4
Guinea pig
-
-
2
Human
2
-
-
Species
Thought D
Although an animal is born with a set number of glands and teats, the total amount of
mammary tissue that develops when she is pregnant may vary with the number of
fetuses she is carrying, at least in some species (see the resources section on
Mammary Development). All of the glands present on a female develop in an
equivalent manner while she is pregnant. Once offspring start suckling, the number of
glands that remain functional will be determined by the number of offspring suckling,
and by how rigid the teat-order (teat preference) is of the species. Non-suckled glands
will undergo involution (see the Mammary Development Resources section on
Involution).
Thought E
Just as sometimes there is a teat or nipple present on an animal that is part of a very
small or no gland, there are some cases where a gland has no nipple. These often are
called inverted nipples. They are common in swine, present in humans, but do not
occur very often in dairy cows. Mistakes in the developmental pattern of the teat
during the early stages of fetal mammary development may lead to the absence of a
functional nipple, but the rest of the gland develops normally. The mammary tissue
will develop normally during pregnancy, but because there is not a teat to allow for
removal of milk, the gland undergoes involution soon after parturition similar to a
gland that is not suckled.
What do you conclude as to why cows have 4 mammary glands?