When it comes to gestation, every mammal has its own time frame for how long it takes to develop their young between conception and birth. There are some natural tendencies, however, that will dictate an approximate length of time an animal will gestate. For the most part, the average length of time in the womb for most mammals is directly influenced by the size of the animal and its average lifespan.
This category consists of rodents and other small mammals. These mammals are small in stature, have a rapid heart rate and generally have a lifespan of just a few years. Opossums have the shortest gestation, coming in at 12 days, but also have the shortest lifespan of just under one year. Mice and rats have very similar gestational periods, though much longer than the opposum, coming in at around 21 days with an average lifespan of about five years.
Medium Sized Mammals
This category includes larger mammals such as predatory cats, common household pets, and larger rodents such as squirrels. Squirrels head up this list with the shortest gestation period, taking about 44 days to develop their young with an average lifespan of about 14 years. Bobcats, domestic dogs and cats tend to follow along the same gestational path coming in at 60, 61, and 63 days respectively and have a wide range of lifespans depending on the species and other environmental factors. The average lifespan for a bobcat is approximately 34 years while domestic dogs and cats average 16 years.
This category includes larger mammals such as humans, larger farm animals and some aquatic mammals. Grizzly bears start off this group with an average gestation period of 220 days and an average lifespan of 42 years. Humans are next with an average gestation period of 274 days, though we are among the longest lived mammals coming in at around 90 years for an average lifespan and are an exception to this rule. Horses are close behind with an average period of 284 days and an average lifespan of 50 years. Whales gestate an average 360 days before they will deliver their young. Their average lifespan ranges between 40-over 100 years depending on the species, which affords them plenty of time to produce many offspring.
Largest Land Mammals
This group of mammals take the longest time to deliver their young after conception. They also tend to have the longest lifespans. This group includes some of the largest mammals on earth such as rhinos and elephants. The first animal in this group are camels, who gestate for approximately 400 days before giving birth and live for approximately 50 years. Grey rhinos and black rhinos have different gestational periods with the greys averaging 480 days and the blacks coming in at 540 days, however they average about the same lifespan of approximately 47 years. The Indian elephant tops the list of the longest gestational period, carrying her baby for 624 days before giving birth to one live calf. The average Indian elephant can expect to live approximately 78 years.