Here are the average lengths of pregnancy for some mammals, in alphabethical order:
Ok I realize that an interval isn't an average value, but I didn't come up these numbers myself; someone else did.
Mammal No. of days
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Camels 406
Cats 62
Cows 280
Chimpanzees 237
Dogs 62
Dolphins 276
Elephants (African) 640
Ferrets 42
Foxes 52
Giraffes 395-425
Goats 151
Guinea pigs 68
Hamsters 16
Hedgehogs 35-40
Horses 337
Humans 266
Hyenas 110
Kangaroos 40
Lions 108
Mice 21
Minks 50
Monkeys (rhesus) 164
Moose 235
Opossums 13
Orangutans 245-275
Pigs 113
Rabbits 32
Rats 21
Reindeers 215-245
Seals (northern fur) 350
Sheep 148
Skunks 62
Squirrels (gray) 44
Tigers 105-109
Whales (blue) 315
reference: ne.se, supermemo | 
