Just watched the Nature episode “Born Wild: The First Days of Life”, and got a dangerously large number of ideas for an artificial life simulation.

Here are (some of) the forces that seem to be in play:

• Food and energy affect all aspects of birthing strategies. All these make it more likely at least some young will survive, but take energy in return:
• Producing more than one fetus.
• Long gestation periods.
• Making yolk inside eggs.
• Producing milk.
• If you're not top of the food chain, your babies need to be born ready to run. A longer gestational period is in order.
• A baby can be left to fend for itself, if it's born smart enough to find food. This means no milk production or babysitting, but probably also means longer gestation.
• Birds let the strongest infant feed first - it's most likely to survive. The others get the scraps and if they live too, great. Sometimes the strongest sibling kills the weaker ones. Sometimes the parents themselves do.
• If the father stays to help, it brings extra food energy into the equation - he feeds the kids or sometimes the mother. Sometimes this means bigger litters, or in the case of emperor penguins, it simply means survival.
• Sometimes the mother herself is the food energy - one species of spider willingly lets the babies eat her alive.
• For males, killing the cubs of your rivals is a good way to get their harems ready to mate with you. Of course, you have to prove your genetic superiority by fighting off the father first.
• Social animals help defend each others' young, IF they're from the same father. Children from other groups get attacked, though.