In nature, the single-parent family is an important rule — and that parent is almost always female. From building a nest and gathering food to giving birth and protecting their young, females do it all. Male involvement rarely extends beyond the moment of conception.

　　There are, however, a few notable exceptions to this rule. In 90% of bird species, the male provides some form of parental care. In fishes, males occasionally take on a custodial(看管的) role at the nest site. And for animals living in harsh environments, the efforts of two parents are often required for the young to survive.

　　So this Father’s Day, remember the millions of dads hard at work in the animal kingdom!