Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.
The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the teats seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.
Viviparous mammals are in the subclass Theria; those living today are in the Marsupialia and Placentalia infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an underdeveloped (altricial) newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. Some placentals, e.g. guinea pig, give birth to fully developed (precocial) young, usually after long gestation periods, while some others, e.g. mouse, give birth to underdeveloped young.
Maturity and reproductive age Sexual maturity and thus the earliest age at which mammals can reproduce varies dramatically across species. Members of the rodent family Cricetidae can reach sexual maturity in 1–2 months, e.g. the Norway lemming (Lemmus lemmus) in 39 days. Many dogs (family Canidae) and bovids (Bovidae) take about a year to reach maturity while primates (including humans) and dolphins (Delphinidae) require more than 10 years. Some whales take even longer, with the longest duration being recorded for the bowhead whale (Balaena mysticetus), which reaches maturity at an age of only about 23 years. Age at sexual maturity in mammals is influenced not only by body size but also by ecological and social factors, including resource availability, population density, and social structure.
Reproductive strategies Mammals exhibit a wide range of reproductive strategies that reflect trade-offs between offspring number, parental investment, and survival. These strategies include variation in mating systems, levels of parental care, and the degree to which reproduction is concentrated among particular individuals within a population. In some species, social structure strongly influences reproductive success, with reproduction primarily limited to dominant individuals, while others adopt more egalitarian or opportunistic strategies.Stearns, Stephen C. (1992). The Evolution of Life Histories. Oxford University Press. pp. 130–180
In addition, reproductive strategies in mammals are often shaped by life-history trade-offs between producing many offspring with low individual investment and producing fewer offspring with higher parental investment. Species with high parental care typically exhibit lower reproductive rates but increased offspring survival, whereas species with minimal parental investment may reproduce more frequently with higher offspring mortality.
Reproductive timing strategies Fertilization occurs within hours of insemination in most mammalians. Reproductive timing in mammals varies widely and is influenced by body size, ecological conditions, and life-history strategies. The length of gestation generally increases with body size but there are exceptions to this general trend. Larger mammals such as primates tend to have longer developmental periods and extended parental care . External factors such as seasonality, latitude, and access to resources also play a role in reproductive timing. In many species, reproductive events are synchronized with favourable environmental conditions to maximize offspring survival. Four general reproductive timing strategies are recognized in mammals: delayed fertilization, delayed development, delayed implantation, and embryonic diapause. These strategies differ in when fertilization and development take place.
Delayed fertilization occurs when sperm are stored in the female reproductive tract and fertilization is postponed until a later time. . This strategy is well documented in temperate bat species, where mating occurs in late summer or autumn, but fertilization is postponed until after hibernation in the spring. Species such as the little brown bat (Myotis lucifugus) store sperm during hibernation, allowing fertilization to occur when environmental conditions improve. This delay enables births to coincide with periods of high food availability, such as peak insect abundance in early summer.
Another strategy is delayed development, where the embryo implants normally but develops at a reduced rate over an extended period. This has been observed in several bat species, including the Jamaican fruit bat (Artibeus jamaicensis), where reproduction is timed so that young are born when food resources, such as fruit or insects, are most abundant.
A related mechanism is delayed implantation, in which the embryo develops to the blastocyst stage but does not immediately attach to the uterus. Instead, it remains free within the reproductive tract until conditions are suitable for continued development. This phenomenon is well documented in species such as the American black bear (Ursus americanus), where implantation is postponed so that cubs are born during winter, when mothers are in dens and able to provide protection.
In embryonic diapause, development is temporarily paused, usually at an early stage. This phenomenon occurs in several mammalian lineages, including many marsupials and numerous carnivores, and allows birth to coincide with favourable environmental conditions, such as better access to food. In species such as the Tammar wallaby (Macropus eugenii), embryonic development is paused until the blastocyst stage. During this period, embryonic development is temporarily suspended until environmental conditions become favourable, after which development resumes.
Mating systems
Mammalian mating systems represent adaptive strategies reflects differences in reproductive investments under specific ecological conditions. They are broadly categorized based on the number of mates an individual has during a breeding season. Monogamy: One male and one female form a persistent pair bond, often cooperating to defend a territory and rear offspring. This system is relatively rare in mammals. It often evolves where male parental care is critical for offspring survive or when females are so widely dispersed that a male cannot effectively defend more than one. Social monogamy is a key prerequisite for the evolution of cooperative breeding, where non-breeding helpers assist in rearing young, as it ensures high genetic relatedness within the social group. Social monogamy is relatively rare in mammals, occurring in up to 5% of species. Prairie voles (Microtus ochrogaster) is one of the few mammalian species where stable pair bonds are formed, and both male and female care for the offspring. Polygyny: One male mates with multiple females. This is one of the most common mating systems in mammals, and occurs when males monopolize access to females, or maintain control of the resources that females depend on. In many species, reproductive success is highly skewed, with dominant males securing the majority of matings. Polygyny is common in mammals such as gorillas and elephant seals, where a small number of dominant males control access to groups of females, or harems. Polygyny is often associated with sexual size dimorphism, where males are visibly larger than females. Intense competition among dominant males for females favours the evolution of a larger body size, and other traits that increases the likelihood of mating success. Promiscuity: Both males and females mate with multiple partners without forming lasting pair bonds. This system is common in species where females live in large populations. It is often associated with high levels of sperm competition, leading to the evolution to relatively larger testes in males. In species with multi-male mating systems, sperm competition can strongly influence male reproductive strategies. Increased sperm competition has been linked to the evolution of larger testes size, and altered mating behaviour. Lekking: In a lek mating system, males gather in traditional display areas to perform courtship displays and compete for the attention of females. This rare system is seen in some ungulates, such as the fallow deer and Uganda kob. This system emerges from simple, local interactions between males, such as attraction to a traditional site, mid-range attraction to each other, and short-range repulsion for territory defenses.
Evolutionary and ecological drivers
Sexual selection plays a central role in shaping mammalian mating systems. Differences in male and female reproductive investment often lead to sex-specific competition and mate choice, driving the evolution of traits such as sexual dimorphism and courtship behaviour.
The type of mating system that evolves in a mammal species is primarily determined by the interaction of social distribution of females and the defensibility of those females or the resource they need. While ecology is a major driver, phylogenetic analyses show that shared evolutionary history also plays a significant role in constraining or facilitating transitions between different mating systems.
Male infanticide, in which incoming males kill dependent offspring that they did not sire, has also been proposed as an important selective pressure influencing mating systems. Some males would also engage in infanticide to bring back females into estrus, thereby increasing the chances of siring offspring. Evidence suggests that this behaviour is thought to have evolved in response to competitive pressures, particularly male-biased sex ratios, with traits like increased testes size emerging as a byproduct rather than a cause. Sexual selective pressures influence the structure of mating systems, often driving females to form stable associations with males to reduce the risk of infanticide, potentially promoting social monogamy. Some researchers have criticised that a focus on male-male competition may come at the cost of underrepresenting female choice and adaptive strategies in shaping mating systems.
Reproductive system In many social animals, reproduction is often restricted to a dominant breeding pair, while subordinate individuals may experience behavioural or physiological reproductive suppression. In socially structured mammals, reproductive suppression may be mediated by neuroendocrine mechanisms, including stress-induced inhibition of the hypothalamic–pituitary–gonadal axis.
Placental mammals
Male placentals