The gestation period is the time taken by a fetus to develop and grow. It starts with fertilization and ends with the birth of the fetus. The duration of the period though varies markedly in different species of animals. It also changes with seasonal influences, body size/mass, brain size and many biological and natural parameters.

Why is the length of gestation period different?

The two most important factors influencing the gestation periods are:

1. The size/mass of the animal

 Larger mammals generally have longer gestation periods because they tend to produce larger offspring. Elephants have a gestation period of 18-22 months whereas for Virginia opossum (the only marsupial found in the United States) the gestation period is only 12 days. Even within primates, the gestation period varies with size. The gestation period of humans is 266 days, 8 days short of nine months which is almost similar to Chimpanzees (8 months), gorillas (8-9 months) and orangutans (8-9 months). For Rhesus monkeys, it is 3-4 months and baboons 6 months.

2. The level of development of the newborn fetus

Depending on the level of development of infants at birth, mammals can be categorized as altricial or precocial. Altricial mammals like, marsupials or rodents, give birth to completely helpless offspring which require extensive parental care, like brooding or food provisioning. On the other hand, offsprings of precocial mammals (like sheep, horses and cows) are relatively mature, mobile and require limited parental care. Hence, altricial mammals generally have longer gestation periods than precocial mammals. However, recent studies have shown that most mammals lie within the altricial-precocial spectrum. So, the more developed the fetus is at birth, the longer becomes the gestation period. Longer gestation periods have also been associated with brain development and more intelligent animals.

Gestation period and its variations

The Gestation Period

Small mammals usually have short gestation periods, like, Stripe-faced dunnart (11 days), Virginia Opossum (12 days), Domestic white Mouse (19 days), Hamster (20 days), Meadow Mouse (21 days), Rat (22 days), Gerbil (24 days), Wombat (27 days), Muskrat (29 days), and Rabbits (33 days). The porcupines carry their offspring for almost 4 months, which is the longest gestation period among rodents.

For farm animals, like sheep and goats, the gestation period is about 5 months. For pigs it is almost 4 months, for cows, it is 9 months, for Buffaloes 10-11 months, and for Horses and Donkeys, it is 11-12 months. Dogs and other canines have a gestation period of about 2-2.5 months, which is similar to cats. Wild carnivores, like Leopards, Panthers, Foxes, Wolves and Jaguars carry their young ones for about 3 months. For Lions, Tigers and Hyenas, the gestation period is about 3 and half months. The gestation period of Bears is different for different species and also suffers seasonal influence. The Giant Panda carry their young for about 3-5 months whereas, for Polar bears, Black bears and Grizzly bears, it can be 6-9 months.

The gestation period for wild mammals like Sloths, Armadillos, and Anteaters are 5-8 months, 2-4 months, and 4-6 months, respectively. odd-toed ungulates like, Rhinoceroses and Tapirs, the gestation period is about 15-18 months and 13 months, respectively. For even-toed ungulates like Peccaries, it is about 5 months. Hippopotamus has a gestation period of 8 months, whereas, does carry their offspring for almost 7 months. Giraffes have gestation periods from 13-15 months. The giraffe mother gives birth standing up and so, the newborn needs to be strong enough to survive the long fall. Camels carry their young ones for about 15 months, whereas Alpaca carries them for 11-12 months.

Aquatic mammals like Dolphins carry their offspring for 13 months, Manatees for 13 months, and Whales for about 15-18 months. Among pinniped mammals (fin-footed, semi-aquatic marine mammals), Walruses have the longest gestation period of 15 – 16 months. For other pinnipeds like Seal and sea lion mothers, it is 11-12 months.

Last, but not least, Elephants have the longest gestation period of all mammals, 18 to 22 months before giving birth. They are not only the largest, but also have the biggest brain among all the land animals in the world, so, there’s a lot of developmental work to do in the womb. As discussed earlier, long developmental periods are common in more intelligent animals.

Did You Know?

1. In the case of Marsupials/pouched mammals, like Kangaroos and Koalas, the gestation period is almost a month for both these species, irrespective of their size difference. Their offspring (known as a joey) when born are very tiny and they stay in the pouch after birth to continue their development for another six (for Koalas) to nine (for Kangaroos) months.

2. Bats are the only flying mammals and they have a gestation period of 1.5 to 6 months.

3. More than 130 species of mammals can postpone the development of the fetus and wait until they have better conditions to deliver and rear their offspring. The suspended stage of pregnancy, also known as embryonic diapause is an enigmatic biological strategy to protect the newborns from unfavourable conditions, like, food shortage, lack of maternal fat stores, or unweaned older siblings. Seasonal embryonic diapause is common among Bears, armadillos, seals, otters and badgers.

References –

1. Scheiber, I.B.R., Weiß, B.M., Kingma, S.A. et al. The importance of the altricial – precocial spectrum for social complexity in mammals and birds – a review. Front Zool 14, 3 (2017). https://doi.org/10.1186/s12983-016-0185-6.

2. Hussein A.M., Wang, Y., Mathieu, J., Margaretha, L. et al.  Metabolic Control over mTOR-Dependent Diapause-like State. Developmental Cell, 52, 2 : 236 (2020). DOI: 10.1016/j.devcel.2019.12.018

3. https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/114-sexual-reproduction/gestation-periods.html

4. https://www.sjsu.edu/faculty/watkins/gestation.htm

5. https://www.worldatlas.com/articles/mammals-with-the-shortest-gestation-periods.html

6. https://www.msdvetmanual.com/multimedia/table/v4663439