Here's why:
* Evolutionary Advantage of Single Births: Primates, especially those with larger body sizes and longer gestation periods, tend to have single births. This is because:
* High energy demands: Raising a single offspring is already energy-intensive. Multiple offspring would put significant strain on the mother's resources.
* Increased risk of complications: Multiple pregnancies increase the risk of complications for both the mother and offspring, such as premature birth, low birth weight, or even death.
* Limited resources: Primates generally invest a lot of time and energy in raising their young. Multiple offspring would require more resources, potentially leading to competition and survival disadvantages for all involved.
* Species Variations: While twin births are generally uncommon, there are exceptions. Some primate species, such as marmosets and tamarins, often give birth to twins as a result of adaptations for their specific ecological niches.
In summary: While twin births are possible in primates, they are generally uncommon due to evolutionary pressures favoring single births.