From Creatures Wikia
In Creatures 2, Ettins and Grendels were infertile by default and required a COB and genome update to make them breedable. In Creatures 3 they were fertile, but the only way to get a female Grendel or male Ettin was to splice existing members of the species.
Radiation and heavy metals can cause infertility of the Norn, but they are also slowly destroying it's immune system.
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