We keep a lot of different mammals as pets, and each of them have their own life cycles. Often smaller animals have shorter lives, although that's not always the case. However, you can draw a pretty direct correlation between gestation period and life spans. Mammals with shorter gestation periods don't live as long as those who spend more time pregnant before giving birth.
A Dog's Life
In contrast to rodents and other types of mammals, smaller breeds of dog tend to live longer than larger breeds -- and just as the sizes of dogs vary wildly, so do their life spans. For example, a Great Dane lives 7 to 10 years while a Chihuahua lives 14 to 18 years. But the vast majority of dogs live somewhere between 10 to 15 years on average.
Smaller dogs generally breed earlier than larger dogs -- the little ones can reach sexual maturity at 6 months old while their bigger cousins sometimes don't start mating until 18 to 24 months old. Across the breeds, a dog's gestation period is 58 to 65 days with the average being 63 days. The puppies are born blind and deaf and open their eyes at 2 weeks old.
A Cat's Life
A cat's life span also depends on her breed, with ranges between 8 and older than 20 and an average of 12 years. Of course, a healthy cat who is fed a proper diet and kept at optimum weight likely will outlast her obese or unhealthy counterparts.
A cat can get pregnant at 4 months old, and will go into heat every two to three weeks. Her gestation period is similar to a dog's at 63 to 65 days, and she gives birth to two to five kittens who have their eyes closed for the first week of life.
A Rabbit's Life
Rabbits used to be considered rodents, but they're actually lagomorphs, an order that includes hares and pikas. They make good pets due to their quiet, affectionate nature and ability to be litter trained. Pet rabbits live an average of 5 to 10 years, with a potential to last 15 years.
Females can start breeding at 5 to 9 months old, and males at 6 to 10 months old. Her gestation period is 29 to 35 days before she gives birth to an average of 4 to 10 blind, hairless pink bunnies. Their eyes open at 10 days old and are weaned at 8 weeks old.
A Guinea Pig's Life
Guinea pigs, or cavies, are relatively large pet rodents that grow to be 8 to 10 inches long and weigh in at 1 1/2 to 2 pounds. Her average life span is 5 to 8 years. As a guinea pig ages, her toes start to roll and twist and she may develop tumors, but otherwise it's difficult to tell how old an adult guinea pig is.
Her gestation period is the longest of any rodent at 63 to 68 days, and the litters average two to three piglets, although up to six is not unheard of. While this may seem exceptionally long, it's because the youngsters develop more before making their debut -- babies are born fully furred with their eyes open and are completely weaned by 3 weeks old. They start to breed soon afterward, with females able to mate at only 5 to 6 weeks and males at 8 weeks.
A Hamster's Life
Hamsters are medium-sized rodents that kids often keep as their first pets. The common Syrian or golden hamster measures 4 to 7 inches when full grown. Hamsters have a relatively short life span of 1 1/2 to 3 years on average.
Hamsters start breeding at 6 to 8 weeks old, and her gestation period is 15 to 18 days. The average litter size is 5 to 10 pups, who are born blind and hairless but grow quickly. Their eyes open at 15 days. They are fully weaned by 3 weeks old, and within a few weeks the cycle begins all over again.