Diminutive Dads of the Animal Kingdom; Fathers in many species play a minor role—and they don’t always survive the act of mating

June 14, 2013, 7:24 p.m. ET

Diminutive Dads of the Animal Kingdom

Fathers in many species play a minor role—and they don’t always survive the act of mating

By DAPHNE FAIRBAIRN

This weekend, children across the country will honor their dads with the usual assortment of cards, tools and ties, thus paying tribute to the influential role that fathers play in our lives. But human dads are unusual in their devotion to family, especially when compared with the rest of the animal kingdom. Though most bird fathers help care for their offspring, absentee dads are the rule in 90% of mammal species. Fatherly care is even less common in other animal groups. Also atypical is our expectation that the dad is the larger and stronger parent. Males are the larger sex in most birds and mammals, but among the vast majority of other animal groups, females are usually larger—often very much larger. Female deep-sea anglerfish, for example, outweigh males by a multiple of as much as 500,000. This makes sense when you think about the biology. Most female animals produce tens to hundreds of thousands of eggs at one time. Their bodies have to be large enough to accommodate these eggs, which can comprise as much as 25% to 75% of the mother’s weight. By contrast, reproductive tissues generally make up less than 1% of a male’s body weight.

While females are busy making eggs (or live young), males typically spend their time trying to secure mating opportunities. If this means competing with other males through flamboyant displays or physical combat, being larger is generally better. However, if the females are widely dispersed and difficult to locate, males need to focus on simply finding a mate. In these cases, their signature traits are small size, early sexual maturity and specialization for travel. Once they find a female, they often give her their all, sacrificing their small chance of future mating opportunities for maximal reproductive effort.

This is what happens with the yellow garden spider, one of the most common backyard spiders in the U.S. Adult females are up to an inch long and more than 50 times heavier than their diminutive mates. They produce batches of hundreds of eggs, and to fuel this tremendous reproductive effort, they spend almost all of their time trying to capture prey in their large orb webs. The tiny males have a very different life story. They mature weeks before the females and abandon their webs to search for a mate. Without a web, they cannot eat, and most starve or get eaten as they scramble through the vegetation. If they do succeed in finding a female and mating with her, they invariably die in the process. Their copulatory organs are single-use devices and almost always break off in the female’s reproductive tract, where their lifeless bodies are left to hang.

Blanket octopuses provide an even more extreme example. The females in this unusual group reach lengths of 6½ feet and are among the largest of all octopuses. Males, however, are among the smallest, topping out at only 1½ inches. The male’s copulatory organ, a modified arm called the hectocotylus, is longer than his entire body, including his other arms. This impressive organ breaks off during mating and is left within the female’s mantle, a separation that is fatal for the male. The giant females accumulate hectocotyli from several of these minuscule mates before spawning clutches of 100,000 to 300,000 eggs.

Not all males faced with finding rare and elusive females exhibit this suicidal mating tactic. Some, like Osedax worms, choose to live on or in their mates instead. These worms form dense colonies on the bones of decaying whale carcasses on the ocean floor. Females are several inches long, with feathery red arms and a thin, white body encased in a transparent tube. Males, by contrast, measure less than four one-hundredths of an inch long and live most of their lives in the females’ tubes; each female hosts a harem of tens to hundreds of them. The males don’t feed; they make sperm by using the nutrients in the yolk they carry with them from the egg. Once their yolk is exhausted, they die.

Fortunately, human dads aren’t required to sacrifice body parts, freedom or life itself to become fathers. Their extraordinary efforts come in the form of devotion to family. This makes them exceptional in the animal kingdom—and it is surely one of the keys to our unique success as a species.

—Dr. Fairbairn is professor of biology at the University of California, Riverside, and author of “Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom,” published by Princeton University Press.