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With their mammaries, female mammals have infant nursing built in. Only where a male can contribute substantially – in protection and/or paternal care – does monogamy appear in mammals.
House Mice
House mice employ a complex olfaction-based communication network. Even without direct encounter, a mouse knows the sex and social status of another from the scent of its urine.
A pregnant female will spontaneously abort a litter she is carrying if a strange male mouse is in the house. However maladaptive this may seem, it is instead making the most of unfortunate choice.
Male mice tend to kill newly born mice that they did not father. Rather than risk carrying a litter to term that will be killed, a female reabsorbs her embryos; starting anew with a new male; thus raising the odds of offspring surviving.
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Owing to chronically limited food supplies, jackals are territorial. They are also monogamous, as defending territory is much easier by working together. Jackal males contribute parentally by feeding pups through regurgitation.
Prairie voles too are territorial and monogamous. The bond is lasting. Couples pair off and live together throughout their adult lives. The huddle and groom each other. They share parental care. While pair-bonded females are typically aggressive toward unfamiliar males, some will have a fling if the opportunity arises. Other vole species vary in their mating systems: some tending toward monogamy, others not.
Monogamous monkeys include the titi, night monkey, and the gibbon. The tiny tarsier is the standout monogamous primate.
Tamarins are squirrel-sized New World monkeys, arboreal inhabitants of tropical rainforests. They live in groups of up to 40, comprising 1 or more families, though small groups of 3 to 9 are more frequent. Each family group centers on a single dominant breeding female. Young females reaching sexual maturity either leave the group or remain and don’t breed. Multiple males mate with the dominant female. Twins are typical with tamarins. A few days after birth, the babies are carried by the males in the group and only handed back to mom for nursing.
Marmosets, closely related to tamarins, are similar in mating and parenting practices. Marmosets live in extended family groups of 3 to 15, including unrelated companions. Their mating system is variable, altering between monogamy, polygyny, and occasionally polyandry.
Marmosets practice alloparenting. Females other than the mother and males carry young through months of dependency. Marmosets scent-mark home ranges and defend them, but are not entirely territorial, as home ranges often overlap between groups.
In mammals, female gestation and lactation means that a male may have little to contribute in improving the chance of his offspring surviving. His reproductive interest may be in seeking more mates, not in parental care. Hence, promiscuity and polygyny are the most common mating system in mammals.
Polygyny predominates in primates. Males defend a small harem of mates whose long gestation and well-spaced receptiveness render them easily controlled.
A dominant male’s harem may be chipped away by promiscuity. Macaque females copulate on the sly with lower-ranking males when an opportunity arises.
Other than protection, primate mothers rear offspring unassisted. This typifies lemurs, capuchins, howler and leaf monkeys, macaques, and gorillas.
Several primates besides humans are quite promiscuous, including patas monkeys, baboons, and chimpanzees. A receptive female Barbary ape may be mated by as many as 10 males.
