“The California mouse led the list [percentage of full sibling pairs], with a 100% rate of full siblings, while humans had a 66% rate. Bottlenose dolphins, orcas, macaques and black bears took many of the bottom spots with 4% or lower rates. The sheep ranked lowest, with 0.6%.”

https://www.wsj.com/science/biology/monogamy-mammal-study-humans-f9d899f2