Sentence:  In Homo sapiens we find two types of polygamous <WSD>mateships</WSD>, polygyny and polyandry, and social structures based on these are ordinarily called “families.” Relatively rare in man in an institutionalized form, polyandrous <WSD>mateships</WSD> appear to be absent in infrahuman primates. 
Meaning of the word marked with <WSD> tags:  A relationship based on mating.