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In most living animals, egg cells are vastly larger than sperm cells. In humans, for example, a single egg is 10 million times the volume of a sperm cell.
In a new study, Northwestern University researchers found that competition and natural selection drove this curious size discrepancy.
Using mathematical modeling, the researchers considered a time very early in evolution when primordial species reproduced using external fertilization. In the model, bigger reproductive cells, or gametes, presented a competitive edge because they could hold more nutrients for a potential zygote. Smaller gametes, however, required fewer resources to make, which put less stress on the parent.