eli5 what determines if an animal will have 1 offspring or an entire litter?
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Evolution. The number of offspring is the number with the best success rate, success being measured by how many of those offspring make it to reproduce themselves. If a mother has 10 babies, and she can’t feed them, protect them, whatever, and only 1 survives, she passes her “have 10 babies” gene on to 1 offspring. If another mom of the same species has 3 kids and is able to raise 2, she passed her “smaller litter” gene on to twice as many babies. Over many generations the number will trend to the ideal for that environment.
Clarify. Are you asking about a specific animal, or are you asking how you can tell how many offspring various species usually have?
As others have indicated, there are multiple answers to this question.
– What determines if one pregnancy will result in one or multiple offspring? That can depend on how many eggs are in the uterus at the time of conception, or sometimes one egg can split into two+ embryos early on in pregnancy. In humans, “two eggs in the uterus, fertilized by two sperms” produces fraternal twins, and “one embryo, fertilized by one sperm, splits into two” produces identical twins.
– What determines the average litter size of an animal?
Well, if you’re asking “Why do dogs have 4 babies at once, but humans usually have one?” then the answer (as stated elsewhere) is evolution.
However, if you’re asking “How can I know how many offspring a mammal will have?” then I have a neat trick for you: the half nipple rule! This rule applies to most mammals and states that a mammal will have an average litter size amounting to half of its nipples, with a typical maximum litter size of the full number of nipples. For example, humans have two nipples, which means that (on average) they will bear one child at a time, with a (typical) maximum of two. Dogs have 8-10 nipples, which means an average litter size of 4-5 and a typical max of 8-10.
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