“Many animal species are more monogamous (or less polygynous) than humans. Quite a few likely qualify as “truly monogamous.” The greater the degree of polygyny, the greater the sexual dimorphism in physiology and behavior. Sexual dimorphism is a function of mating competition. The greater the level of polygyny, the greater the skew in male reproductive success, and the more intense male mating competition will be. The exaggerated size, elaborate ornamentation, and armaments seen in the males of highly dimorphic species are great investments of energy and other somatic resources that facilitate success in male mating competition. Because each individual has limited energy and resources, there is a trade-off between investment in mating competition and investment in paternal care of offspring and reproductive partners. Polygynous species tend to have less, if any, paternal investment.So, the more monogamous a species is, the more similar you would expect females and males to be in physiology and behavior. One of my favorite examples is Emperor penguins, in which females need to return to the sea to feed for two months after laying a large energetically expensive egg. The males care for the egg over the harsh winter with no food, huddling together for warmth and losing considerable weight. Both sexes have an enormous physiological investment for offspring, and are very similar in appearance (they have low sexual dimorphism). You probably cannot tell which are females and which are males, and in fact the penguins themselves cannot identify individuals by appearance. The have a unique little dance that they use to identify themselves and recognize their partners. Of course Emperor penguins are serially monogamous, they are faithful to their mate through the course of the year, but usually have a different mate in the next breeding season.”

More about Kruger talks about where else monogamy happens in the animal kingdom. >