Infertility of Norns, Ettins, or Grendels can be caused by many factors. Poor health or old age may prevent pregnancy from occurring. If the breeding limit has been reached, the game enforces infertility, even though it continues to allow kiss-pops.
In Creatures 2, Ettins and Grendels were infertile by default and required a COB and genome update to make them breedable. In addition, some Creatures 2 genomes had a bug whereby a gene intended to cause heart disease when a norn's consumption of fat was too high was accidentally placed in the uterus, often causing that organ to die off before the norn came of age.
Radiation and heavy metals can cause infertility of the Norn, but they are also slowly destroying its' immune system.
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