Across mammal species generally, there is a fairly consistent relationship between the average litter size and the typical number of teats possessed by the mother. As a rule, it can be said that there is one pair of teats for each offspring in the typical litter. However, suckling of the offspring is just one aspect of parental care in mammals.
Maternal care, which can include nest building, grooming of the offspring, and infant carriage, is found in all mammals. Paternal care is relatively rare and is usually restricted to grooming and/or carriage.
Predictably, paternal care in mammals is usually restricted to monogamous species in which there is a relatively high level of certainty of paternity. Once the effects of body size have been taken into account, it emerges that the pattern of maternal care for any mammal species is quite closely reflected in milk composition. Three principal components of mammalian milk are carbohydrates, fats, and proteins. As a crude approximation, it can be said that the carbohydrate content of milk reflects immediate energy needs of the offspring, while the fat content indicates energy needs over a longer term. The protein content of milk provides a fairly good indication of requirements for growth.
Milk composition also provides an indication of maternal behavior. Here, a major distinction can be drawn between mothers that feed on schedule and those that feed on demand. For offspring that are fed on schedule, it is the mother that determines the suckling frequency. Commonly, this applies to offspring that are left in a nest. These tend to be fast growing but relatively inactive altricial offspring that must maintain their body temperature unaided in the absence of the mother. As a result, the milk of such species tends to be high in protein and fat but relatively low in sugar. The most extreme example of suckling on schedule known for mammals is found in certain tree shrews species that keep their offspring in a separate nest and suckle them only once every 48 hours.
The milk of these tree shrews is extremely concentrated, containing more than 20% fat and 10% protein. By contrast, in mammals that feed on demand, it is the offspring (usually a singleton) that determines suckling frequency. Usually, this requires close proximity between the offspring and its mother, so that it can signal its intention to suckle. Suckling on demand is, for example, generally typical of primates, most of which show parental carriage of the infant. Because infants that suckle on demand are usually slow growing but quite active precocial singeletons, the milk tends to be low in protein and fat but relatively high in sugar. This is the case with human milk, which is evidently naturally adapted for suckling on demand.
Eventually, provision of milk by the mother comes to an end and the offspring are weaned. In altricial mammal species, there is usually a fairly constant lactation period, and weaning tends to occur within a few weeks after birth. In precocial mammals, the lactation period can be quite variable and it may last months or even years. In fact, there is some indication that for certain hoofed mammals and primates there is a feedback relationship between the frequency of suckling and the mother’s resumption of fertility, driven by the level of maternal nutrition. If food availability is low, the mother produces more dilute milk, which results in an increased suckling frequency. A higher frequency of suckling can suppress maternal fertility and also lead to an extension of the lactation period.
After weaning, the developing offspring must forage for food independently in order to meet its nutrient requirements for growth and maintenance. Eventually, it will attain sexual maturity and enter the breeding population. Here, too, altricial mammal species tend to have fairly standard ages for the attainment of sexual maturity, whereas precocial mammals can show more flexibility, according to prevailing environmental conditions. Despite such variability, the age of sexual maturity is an important milestone for comparisons among mammal species. It should be noted, incidentally, that sexual maturity may or may not coincide with the attainment of the adult condition in other respects, for example in body size and/or skeletal and dental maturity. In some cases, individuals may continue to grow for some time after achieving sexual maturity. It is noteworthy, however, that mammals (like birds) differ from reptiles, amphibians, and fish in showing a target body size. In each species, individuals tend to cease growing at a fairly standard size.