Brothers and sisters can be best friends or serious rivals, often both in a single day. Families do their best to make their children get along, because they'll be related for life. It doesn't work that way in all families, and can be quite opposite in some non-human families.
Living species have varying reproductive strategies. Plants and insects produce as many offspring as possible because most of them will not survive. Mammals produce fewer offspring, but devote lots of time and energy to keeping them alive. Then there are some species, like certain fishes and birds, that nurture their young while not getting too attached to them. They depend on sibling rivalry to cull the herd, so to speak, and keep only the fittest as their surviving progeny. MinuteEarth introduces us to some species that actually encourage offspring to kill each other. It seems a cruel strategy, but nature is only interested in what works. The video is only 2:25; the rest is an ad.
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