Application:
• The average 38 week pregnancy in humans can be positioned on a graph showing the correlation between
animal size and the development of the young at birth for other mammals
For mammals, the gestation period is the time taken for a foetus to develop – beginning with fertilization and ending with birth
- The duration of the gestation period will differ markedly between different species of animal
Generally, there are two main factors that contribute to the length of the gestation period:
- Animal size / mass – larger animals tend to have longer gestation periods (as they tend to produce larger offspring)
- The level of development at birth – more developed infants will typically require a longer gestation period
The level of development at birth for mammalian infants can be described as either atricial or precocial
- Altricial mammals give birth to relatively helpless, undeveloped offspring that need extended rearing
- Precocial mammals give birth to more developed offspring that are mobile and independent and require minimal rearing
- Generally, altricial mammals (e.g. marsupials and rodents) require shorter gestation periods than precocial mammals (e.g. ungulates such as cows, pigs and rhinoceroses)
While the length of a gestation period does appear to positively correlate with size and development, other factors also exist
- Some mammal species may have similar gestation periods despite having significantly different body masses
Length of Gestation versus Mammalian Mass
