HA! I think most women would agree that at least 50% of men have a hard time being monogamous… Though later in life, many settle down in solid, long-term relationships.
Males in these species have higher metabolic rates and shorter life spans than females, and they basically put zero effort into caring for kids. In other words, all a female gets from a mate are his genes, and females select for males with good genes. This has led to the evolution of conspicuous, costly displays in males that advertise good genes. These features—flamboyant facial coloration, big capes of hair, silver backs—are the primate equivalents of the peacockery of peacocks (a classic polygamous species).
Because fertile females will mate with multiple males, male-male competition extends to sperm competition. By primate standards, polygamous male primates have large testes (as a percentage of body weight) and high rates of sperm production. And they happily mate with anyone in the county who is ovulating.
Things are quite different among monogamous “pair bonding” primates. Critically, males do much of the infant care. Thus, you don’t see a male indiscriminately mating left and right (or fighting for the chance to do so), since he’ll be doing a lot of work if there’s a child. In these primates there isn’t a high degree of “sexual dimorphism”—sex differences in body size, musculature, metabolism and life span—and males don’t have those garish secondary sexual characteristics of males of polygamous species. Testes are small, sperm count low, mating infrequent.
These profiles are consistent. If 10 seconds into watching a newly discovered type of primate you see that males are twice the size of females and have flashing neon noses, the issue is settled: it’s a polygamous species. If you spend forever trying to tell the sexes apart, they’re monogamous.
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