Mammalian Reproduction and Offspring Care

The Sexual Reproduction of Mammals, Monotremes and Marsupials

Oct 2, 2009

Mammals generally invest heavily in raising their offspring and this determines how they choose their mates and also how often they can breed.

All mammals reproduce sexually, which produces offspring that are unique because they have a mixture of genes from both parents. Offspring with strong genes are more likely to survive environmental extremes in weather, predation, disease and competition for resources.

Sexual reproduction does take time and effort, from finding a suitable mate to caring for young. Male mammals must compete for female attention. Because females want the fittest and strongest males, males will display their strength and fitness in ways such as fighting or elaborate ritualistic displays, in addition to physical characteristics, like having large antlers or a big mane.

Mammals generally invest heavily in raising their offspring and this determines how often they can breed. Many reptiles leave after laying eggs and never even meet their young compared with mammals that suckle their young and care for them for months or even years.

Mammals that live in complex societies such as humans or apes will produce one baby every few years. Mammals that live shorter lives, like mice, invest less time in raising offspring and are capable of breeding at an incredible rate.

Monotreme Reproduction

Monotremes such as the duck-billed platypus and the echidna lay young inside of eggs. The eggs are similar to birds or reptile eggs except that the monotreme eggs are much softer. The young hatch after a few days and continue to grow as the mother nurses them. Echidnas nurse their young in a pouch on the underside of their belly and the platypus nurse young in an underground nest. Monotremes don’t have teats and secrete milk onto their fur for the young to lap up.

Marsupial Reproduction

Marsupials give birth to live, tiny underdeveloped offspring that continue their development inside the mother’s pouch. Once outside the birth canal, the offspring make their way through the mother’s fur to find her nipples, which are often inside the pouch.

In red kangaroos after the first joey is born and crawls to the pouch, the mother can become pregnant again. Because there is a joey suckling at her already, the new embryo’s development is arrested in the uterus. This is called embryonic diapause and lasts around 235 days. When the joey leaves the pouch, development of the embryo continues and that joey is born around one month later.

Once the second joey is born the mother can become pregnant again and the new embryo’s development will become arrested until the second joey leaves the pouch. The mother can have three joeys at different stages of development to care for.

Placental Mammal Reproduction

Placental mammals (eutherians) have prolonged gestation in which the embryo remains in the uterus to develop fully, receiving nourishment from the mother via the placenta. Length of gestation varies depending on the mammal. Some gestation times of mammals include:

Generally, the larger the mammals, the longer the gestation period, however, there are exceptions to this rule. Baleen whales are the largest mammals and gestation time for them is 12 months, half that of the elephant and only a bit longer than human gestation of 9 months!

Generally, the larger the mammal, the lower the number of young in a litter. While small rodents produce several offspring in a litter every season, large mammals, like the elephant, may only have 4 calves over its entire life of around 50 years.

Hickman, Roberts, Larson & I’Anson, 2004, ‘Diversity of Animal Life’ in Integrated Principles of Zoology, 12th edition, ed. McGraw Hill, New York, chapter 28.

The copyright of the article Mammalian Reproduction and Offspring Care in Mammals is owned by Roberta Goli. Permission to republish Mammalian Reproduction and Offspring Care in print or online must be granted by the author in writing.