Reproductive cycles

[Aoi Magi]

So why is gestation 9 months?

I'll try to explain this in simple terms, so it might not be scientifically accurate.

Let's say all buildings are made with similar units called bricks, arranged differently and in different quantities. So a house with just three rooms would take less time than an entire mansion. Now if the building process is automated, the building times will be quite similar for similar houses or mansions.

In animals, the concept is similar. So depending on the size and complexity of the animals, the time of the gestation varies. For example, Most apes with similar sizes and complexities have similar gestation periods, like chimpanzees with 237 days, gorillas with 257, orangutans with 260 days,and humans with 266 days. Where as dogs and cats have similar periods of around 60 days.

Some species, especially amphibians do show a gestation periods which seem to be partially determined by seasons, but the basic duration is still mostly based on their size and complexity.

[Anomalocaris]

So why is gestation 9 months?

I'll try to explain this in simple terms, so it might not be scientifically accurate.

Let's say all buildings are made with similar units called bricks, arranged differently and in different quantities. So a house with just three rooms would take less time than an entire mansion. Now if the building process is automated, the building times will be quite similar for similar houses or mansions.

In animals, the concept is similar. So depending on the size and complexity of the animals, the time of the gestation varies. For example, Most apes with similar sizes and complexities have similar gestation periods, like chimpanzees with 237 days, gorillas with 257, orangutans with 260 days,and humans with 266 days. Where as dogs and cats have similar periods of around 60 days.

Some species, especially amphibians do show a gestation periods which seem to be partially determined by seasons, but the basic duration is still mostly based on their size and complexity.

While there are undoubtedly rough relationships between body size and gestation period, the close similarity between the gestation period of the great apes is probably more the result of close evolutionary relationship.

[Aoi Magi]

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I'll try to explain this in simple terms, so it might not be scientifically accurate.

Let's say all buildings are made with similar units called bricks, arranged differently and in different quantities. So a house with just three rooms would take less time than an entire mansion. Now if the building process is automated, the building times will be quite similar for similar houses or mansions.

In animals, the concept is similar. So depending on the size and complexity of the animals, the time of the gestation varies. For example, Most apes with similar sizes and complexities have similar gestation periods, like chimpanzees with 237 days, gorillas with 257, orangutans with 260 days,and humans with 266 days. Where as dogs and cats have similar periods of around 60 days.

Some species, especially amphibians do show a gestation periods which seem to be partially determined by seasons, but the basic duration is still mostly based on their size and complexity.

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While there are undoubtedly rough relationships between body size and gestation period, the close similarity between the gestation period of the great apes is probably more the result of close evolutionary relationship.

Evolutionary cousins do show similar gestation periods mainly because they are generally similar in size and complexity. But evolutionary cousins with wide size gap do reflect the same in their gestation period difference, for example a rhesus monkey is 164 and a baboon is 187 days.

The primary factors that influence gestation periods are
- Complexity
- Size
- Breeding season
- Babies or litter size

[Anomalocaris]

That only holds when you are comparing relatively closely related animals who also happen to be similar in size. When you compare more distantly related animals of similar size, say between a grizzly bear (~220 days) and a horse (~350 days), the similarity in gestation period disappears.

When you compare relatively closely related animals of substantially different sizes, you see similar gestation periods. Black bears (200 lbs, 220 days) and grizzly bear (500 lbs, 215 days), for example.


clearly the evolutionary lineage of an animal is a more important factor in determining length gestation period.

[Aoi Magi]

As I mentioned, size alone isn't the deciding factor, in case of the bear, the 'breeding season' plays a prominent role. Apart from that there's the size of the baby to consider too, in case of a bear, the cub is lot smaller than it's mother while in case of the horse, the difference is comparatively smaller. You can see this same thing when comparing whales with elephants.

[Anomalocaris]

Yes, but the implication you overlooked is gestation period in families of animals reflects a separate evolutionary optimization Of traits required for adaptation to the family's life style than adult female size.

Human gestation period is 9 and half months not because the average human female is 100 lbs give or take, it is because humans are apes and has had a ape like lifestyle and metabolism for so long we are still constrained by many of adaptation to it. If we are the same size as we are now but had been descendants of predatory cats our gestation period would probably be three months.

Cat and dog gestation period is not 60 days purely because they are smaller then we. Lions and tigers bigger than we have gestation periods of only 100 days.

[Aoi Magi]

That is covered under "complexity". Yes you can break it down that way, and seriously there are also other factors which should be considered, like the environmental conditions which effects gestation period based on availability of food and such, but as I originally mentioned, I am trying to explain this in very simple terms so that the op can understand, and explaining this way means I won't be scientifically accurate.