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Most animals do not care for their young, and simply produce as many offspring as possible so that there is a higher chance at least some of them will survive. Mammals and birds are unique because rather than producing a ton of offspring, they invest time and energy into protecting and rearing their offspring so that each individual has a higher chance of survival. One of the ways they do this is by keeping their offspring inside their body for longer periods of time so they are able to be protected and grow more mature before being let into the world. This provides a unique survival advantage to mammals and has allowed them to become very successful.

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12y ago

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