How come animals can give birth to so many children at once e.g. 6 or more but this is so rare in humans?

Question: How come animals can give birth to so many children at once e.g. 6 or more but this is so rare in humans?
- Keywords:
  children,
  evolution,
  offspring,
  reproduction
Asked by marialovesmusic to Charlotte, Jo, Kevin, Louise, Valeria on 15 Jun 2012. This question was also asked by anniew.
- Kevin Mahon answered on 15 Jun 2012:
  
  This is a ‘mating strategy’ of which there are many, it’s a field of biology in of itself! In a nutshell, animals that give birth to a very high number of individuals tend to not look after them as much and the ones that die are seen as a trade off for not investing parental care. Some fish, like cod give birth to hundreds of offspring, the vast majority of which will die.
  
  Animals that give birth to a very few offspring (primates area good example) tend to offer a lot of parental care in order to ensure the one or two survive.
- Joanna Cruden answered on 15 Jun 2012:
  
  If you compare like for like, primates in general only have 1 -2 babies and we are primates, mammals that are lower down the food chain have more offspring (babies) because they have a lower survival rate, but in general mammals tend to have fewer offspring than animals like fish, insects and amphibians that tend to have hundreds of offspring/eggs with a very poor survival rate.

Question: How come animals can give birth to so many children at once e.g. 6 or more but this is so rare in humans?