Litter can’t exceed teats!

Mammals are vertebrates with hair and mammary glands, which produce milk for nursing their young. Mammals may be divided into two groups Prototheria and Theria. Prototherian are oviparous and Therian are viviparous. Theria is a subclass of mammals. It includes Eutherians (placental mammals) and metatherians (marsupial mammals). Mammals have a wide variety of skin glands, but all seem to be variations of two major glands, Sudoriferous (sweat) and Sebaceous (oil) glands. Mammary glands appear to be modified sebaceous glands. Mammary glands arise in both sexes from a pair of ribbons of ectoderm called milk lines which extend along the ventro lateral body wall of the fetus from the axilla to the groin.

A teat is the projection from the breast or dug or udder or mammary glands of mammals from which milk is ejected for the purpose of nursing young. The number and positioning of mammary glands and teats varies widely among 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. Teats may further divided into true teats (man etc), in which all ducts open at the tip of the nipple as a result of the elevation of the duct bearing area during development and false teats (Ungulates), in which the skin around the duct openings becomes elevated and all ducts empty into cistern. Suckling young are not concerned about the terminology as long as the teats are provident.

A litter is the live birth of multiple offspring at one time in animals from the same mother and usually from one set of parents. The word is most often used for the offspring of mammals, but can be used for any animal that gives birth to multiple young. In comparison, a group of bird’s eggs and the offspring that hatch from them are frequently called a clutch, while young birds are often called a brood. Animals from the same litter are referred to as litter-mates. Animals frequently display grouping behavior in herds, swarms, flocks or colonies, and these multiple births derive similar advantages. A litter offers some protection from predation, not particularly to the individual young but to the parent’s investment in breeding. With multiple young, predators could eat several and others could still survive to reach maturity, but with only one offspring, its loss could mean a wasted breeding season. The other significant advantage is the chance for the healthiest young animals to be favored from a group. Rather than it being a conscious decision on the part of the parents, the fittest and strongest baby competes most successfully for food and space, leaving the weakest young, or runts, to die through lack of care. In the wild, only a small percentage of the litter may survive to maturity, whereas for domesticated animals and those in captivity with human care the whole litter almost always survives. 

In Prototheria (Monotremes) do not develop typical nipples. Instead in both sexes modified sweat glands produce nutritious secretion, which is lapped off a convenient tuft of hairs by the young. Teats would probably be useless in the Monotremes, since it appears doubtful whether the young hindered by horny beaks and lacking muscular cheeks and lips that could nurse. Through natural selection, there is an optimal litter size for a species, population or individual of a particular age or condition and this optimal number is reflected in the modal or mean litter size. Number of teats effectively limits litter size in a marsupial species if young outnumber functional teats at birth such that supernumerary ones die soon after. This condition is typical of polyprotodont marsupials for example in Virginia opossum (Didelphis virginiana), which generally have 13 teats and in Tasmanian devil (Sarcophilus harrisii), have four teats and a modal litter size of four. In contrast medium to large sized species of marsupials, such as Kangaroos, Wallabies and larger Possums typically have four teats but a single young per litter. In those species number of eggs shed at ovulation controls litter size. Mammalian neonates can be divided into two categories, one is altricial (those born essentially naked, with eyes and ears closed, with poorly developed nervous and thermoregulatory systems) and Precocial (born well furred, with eyes and ears open and well developed nervous and thermoregulatory systems). In mammals with an obligate teat attachment phase of development, such as marsupials, litter size obviously cannot exceed the number of mammae.

During pregnancy a battery of hormones causes the formation of sac like secreting terminal (alveoli) at the ends of the duct system. The distribution and number of mammary glands and nipples vary with the species. Nipples may be axillary, thoracic, abdominal and inguinal. A single pair of thoracic nipples occurs in apes and man. Bats also have thoracic nipples. Insectivores and some lemurs have one pair of thoracic and one pair of inguinal nipples. Flying Lemurs and Marmosets have a single pair in the armpit (axillary nipples). In Cetacea, nipples occur near the groin (inguinal nipples). In Pigs, Dogs, Edentates and many other mammals, a series of axillary, thoracic, abdominal and inguinal nipples is scattered all along the milk line. Supernumerary nipples may occur in any mammal including man. In general there are sufficient nipples for the number of young in a litter and they are in locations appropriate to the habits of the species.

Human females have prominent breasts even when they are not lactating, due to fat deposits around the actual mammary glands. Some scientists proposed that they evolved for the purpose of sexual attraction. Some scientists believe that breasts are actually an honest signal of reproductive potential. Some think breasts mimic honest signals. A theory also suggests that breasts evolved in response to the shape of the baby’s face. Polythelia, the condition of having extra nipples and Polymastia, the condition of having extra breast are harmless and redundant in humans. Accessory nipples, if presents look much like moles or brown dots in humans.

If we see its evolutionary perspective, we can say that both traits (number of teats and litter size) show co-evolution. It is clear that teat feeding is extreme case of maternal care and emotional attachment. Emotional attachment requires highly developed conscience and nervous system with well developed sensory organs which are present in mammals. Here the evolutionary dilemma arise that number of teats drives litter size or litter size drives number of teats during evolution. Here both traits drive each other during co-evolution. It means litter size drives number of teats and number of teats also drives litter size. It is an interdependent evolution. Not only teats size and shape but shape and size of lips, mouth and cheek muscles are also shaped during evolution for milk sucking.

It is clear that if litter size exceeds the number of teats, young ones will face hunger. But here we must also consider the physiology of milk production and how much milk is producing from mammary glands of females as it completely depends on its metabolism. If milk is producing at faster rate, then there is no difficulty in exceeding litter size to number of teats because young ones can alternate feeding. It also depends on Precocial or altricial state of young’s. Precocial young do not require much milk and they can start eating solid food readily. If number of teats exceeds the litter size there it must be redundant but however if rate of milk production is slow then young ones can feed on more than one teats. Positioning of teats is also important because teats must positioned so that all young can feed properly and simultaneously. Simultaneous feeding is not necessary but young ones generally mimic each other.

Oviparous animals produce hundreds and thousands of eggs because of lower rate of survival due to predators and environmental risks. With evolution of viviparity in mammals, they started to release and keep limited number of eggs in their womb to reduce the risk of predators and environmental risks. Litter size and number of teats in Eutherian mammals is limited from 1 to 20. It is evident from above study that there is strong positive correlation between optimal number of teats and optimal litter sizes due to co-evolution in these traits. It is clear that both traits showing synchrony and divergent evolution in influence of different environments. But it is also true that we cannot generalized “one-half rule” to all mammals because it is affected by many other factors such as their wild or domestic status, their altricial or Precocial state, metabolism, physiology of lactation, weight and age of mother, behavior of sucking, degree of maternal care, hormones, diet, size of new born, size of uterus etc. It means both traits are continuously evolving, dynamic and interdependent.