When it comes to pregnancy lengths, animals have a wide range of gestation periods. For some animals, pregnancy lasts only a few weeks, while for others it can last over a year. So which animal has the shortest pregnancy of just 12 days? Let’s take a look at some quick answers to this question:
Quick Answers
The animal with the shortest known pregnancy length of just 12 days is the American opossum. Opossums are marsupials that give birth to highly underdeveloped young that complete their development attached to a nipple inside the mother’s pouch.
Other animals with very short pregnancy lengths include:
- Rabbits – 28-35 days
- Hamsters – 16 days
- Mice – 19-21 days
- Rats – 21-23 days
- Guinea pigs – 59-72 days
Most animals have longer pregnancies lasting several months. Elephants have the longest pregnancy term of all mammals at 22 months.
The American Opossum’s 12 Day Pregnancy
The Virginia opossum or North American opossum (Didelphis virginiana) is the only marsupial found north of Mexico. Like all marsupials, the American opossum gives birth to highly underdeveloped young after an extremely short gestation period.
For the American opossum, the pregnancy length is just 12-13 days. This is the shortest pregnancy term of any mammal.
The opossum embryos develop for just 2 weeks in the mother’s womb. They are born extremely small, weighing only 0.13 grams each – about the weight of a bee. The newborns are deaf, blind, hairless and lack any immune protection.
These tiny, underdeveloped jellybean-sized babies must then crawl unaided into the mother’s pouch where they each attach to a teat for the remainder of their development. They will spend the next 2-3 months attached to the teat, where they continue to grow and develop.
Once developed enough, the joey opossum will detach from the teat and ride on the mother’s back. At about 100 days old the joey becomes independent.
This strategy allows opossums to bear and nurse large numbers of young. It also enables their short pregnancy and rapid birthing of offspring. Up to 20-25 opossums may be born in a single litter.
Why Do Opossums Have Such Short Pregnancies?
Marsupials like opossums have evolved a reproductive strategy very different from placental mammals. Their pregnancies are extremely short, but development continues externally in the mother’s pouch.
Several evolutionary factors have led to opossums developing this strategy:
- They evolved from small mammals where short pregnancy was advantageous.
- Shorter gestation periods allow more litters each breeding season.
- Their young develop externally, requiring minimal time in the womb.
- Short pregnancies reduce risks to females and young.
- Development in the pouch optimizes care and resources.
For primitive, mouse-sized ancestors, a 12 day pregnancy was more beneficial. It enabled breeding multiple litters annually. This adaptation has continued even as opossums grew larger in size.
Short pregnancies also enable opossums to produce more litters each season. Female opossums can give birth to up to three litters per mating season. This high reproductive rate is facilitated by the minimized time embryos need to develop in the womb.
Additionally, since opossum joeys complete development externally, they require a minimal time in utero. The pregnancy only needs to progress until live birth is possible. Extended development happens outside on the teat, not internally.
The short pregnancy may also reduce risks to both the mother and embryos. Extended pregnancies pose higher risks of complications, stress, metabolic costs and mortality. By minimizing the time young spend in utero, marsupials reduce these risks.
Finally, development within the pouch provides maximum care and resources. Attached to the teat full time, joeys get constant nourishment. They also bond closely with their mother and receive protection in the pouch. This optimized external development likely played a key role in the evolution of short gestation periods.
How Do Short Pregnancies Work?
In order to bear live young after just 12 days of development, opossum embryos undergo specialized changes:
- Rapid cell division and growth
- Accelerated organ development
- Premature birth while organs continue to develop
- Development of adaptations to survive extreme prematurity
To achieve their fast growth, opossum embryos undergo cell divisions at a highly accelerated rate. They also employ special developmental adaptations to speed up organ system formation.
Opossums are able to give birth after just 12 days because their young are still highly premature. Internal organs and systems are minimally developed at birth. The newborns are still at an embryonic stage requiring extended development outside the womb.
Specific adaptations help the tiny, premature opossums survive. These include:
- Hardened mucus on skin for protection
- Hands, feet and mouth perfect for grasping teat
- Strong instinct to search for mother’s pouch unaided
Together, these specialized adaptations make the opossum’s extreme prematurity and short pregnancy possible.
Other Animals with Short Pregnancy Lengths
While opossums have the shortest known pregnancy at just 12 days, other animals also have quick gestation periods of less than a month. These include:
Rabbits
Pregnancy length: 28-35 days
Rabbits have a relatively short pregnancy period that lasts about one month. They are able to give birth to multiple litters per year due in part to their short gestation period.
Hamsters
Pregnancy length: 16 days
Hamsters have gestation periods lasting approximately 16 days. Like other rodents, their small size facilitates rapid reproduction with short pregnancies.
Mice
Pregnancy length: 19-21 days
Mice have pregnancy lengths ranging from 19-21 days. Their tiny size enables their embryos to develop quickly in the womb.
Rats
Pregnancy length: 21-23 days
Another small rodent, rats have gestation periods of 21-23 days on average. Their reproductive strategy depends on continually rapid breeding.
Guinea Pigs
Pregnancy length: 59-72 days
Guinea pigs have an average pregnancy term of 63 days. While longer than other rodents, their pregnancy is still short compared to larger mammals.
These small mammals demonstrate why a short pregnancy period can be advantageous. Their reproductive strategies rely on breeding multiple litters in quick succession.
How Do Opossum Pregnancies Compare to Humans?
Attribute | Opossum Pregnancy | Human Pregnancy |
---|---|---|
Length | 12 days | 280 days (40 weeks) |
Number of offspring | Up to 25 | Usually 1 |
Offspring maturity at birth | Extremely premature, embryo-like | Fully developed baby |
Offspring mobility at birth | Must crawl unaided into pouch | Helpless, unable to crawl |
Subsequent development | Attached to teat for 2-3 months | Develops externally for 18+ years |
When comparing opossum and human pregnancies, some huge differences quickly become apparent. While opossums have 12-day pregnancies and give birth to many naked, premature offspring, humans have 280-day single pregnancies and give birth to developed babies.
However, both species feature extensive development outside the womb. For opossums this happens in the pouch, while humans develop slowly over 18+ years.
Nonetheless, the opossum’s pregnancy shows how extremely short gestation periods can be. No other mammal has pregnancies and births quite like those of the American opossum.
Conclusion
The American opossum has the shortest known pregnancy of any mammal at just 12 days. Opossums are marsupials, giving birth to highly premature offspring that complete development while nursing in the mother’s pouch.
Several factors have led opossums to evolve this reproductive strategy. Their small ancestors benefitted from short gestation periods that enabled breeding multiple litters per season. The minimal time embryos spend in the womb reduces risks and metabolic costs to mothers. And development on the teat optimizes resources for vulnerable newborns.
Accelerated embryonic growth, extreme prematurity and specialized adaptations enable opossums to maintain pregnancy lengths of less than two weeks. While other small mammals have pregnancies lasting 3-4 weeks, no other animal can match the opossum’s 12-day gestation period.
This short pregnancy exemplifies the extreme adaptations mammals can evolve to facilitate reproduction and survival of their young. The opossum takes speedy reproduction to the limit with the shortest pregnancy term found in nature.