For some mammals, large adult daughters, not sons, are the norm

Female elephant seals are not fragile creatures. They are undulating tubes of muscle and blubber that can weigh more than 1,000 pounds. Yet female elephant seals are definitely dwarfed by their male counterparts, who are typically at least three times heavier.

It’s an extreme case of what’s known as sexual size dimorphism, and fits within the long-standing narrative that male mammals are typically larger than female ones.

a new analysis of a diverse range of more than 400 mammal species paints a more complex picture. In 45 percent of mammal species, males outweigh females, scientists found. But in almost as many cases – 39 percent – ​​both sexes have the same mass. And in 16 percent of species, females are the heavier sex.

“What we found is that there is no standard,” said Kaia Tombak, a postdoctoral researcher at Purdue University and author of the new study, published Tuesday in Nature Communications.

The conventional wisdom about male size may stem in part from biases in the scientific literature, which has focused disproportionately on mammal species where larger males are common, said Dr. Tombak.

It is not the first study to do this challenge the size narrative, which dates back at least to Charles Darwin in the 19th century. But it highlights the need for more research into the varied mating systems, reproductive strategies and evolutionary forces that shape mammals, scientists said.

“There is a disconnect between what the data actually shows and the assumptions that many people, including many evolutionary biologists, have,” says Catherine Sheard, an evolutionary biologist at the University of Aberdeen in Scotland. involved in the research.

There is a well-established theoretical basis for the idea that male mammals grow larger than their female counterparts. The idea is that because female mammals invest so much in their offspring, they are the choosier sex. As a result, males may have to compete with each other for mates; if these competitions involve physical conflict, it could fuel selection for bigger and stronger men.

In elephant seals, for example, males compete against each other in brutal physical fights to gain access to large harems of females. Only a small percentage of the males – the largest and strongest on the beach – get the chance to mate.

That competitive dynamic between males and females is “quite common” in mammals, said Dr. Tombak, who conducted the research as a postdoctoral researcher at Hunter College. “But it is not the only force, and not necessarily even the strongest force, acting on relative body size.”

Dr. Tombak and her two co-authors, both based at Princeton University, drew on a variety of previous studies to collect and analyze detailed body mass data from 429 mammal species. (Body mass is not the only measure of an animal’s size, but it is a commonly used and readily available measure.) The final selection of species was intended to capture the richness and diversity of mammalian life.

“They opted for really high quality data, for data breadth,” said Dr. Sheard.

Larger males were most common among carnivores, primates and cloven-hoofed animals, three categories of mammals that are often the focus of research on size differences based on sex, the researchers noted. As for carnivores, 87 percent of the species included in the analysis had larger males than females. The same was true for 79 percent of hoofed animals and 62 percent of primates.

But rodents and bats are responsible more than half of the mammal species on this planet. And in 48 percent of rodent species in the study, males and females were the same size. (In 44 percent of rodent species, males were larger.) In 46 percent of bat species, females were the larger sex; Male bats were larger than females in only 18 percent of species.

“The diversity that bats and rodents represent is underappreciated and understudied,” V. Louise Roth, an evolutionary biologist at Duke University who was not involved in the study, said in an email. That could explain “why the idea that males are generally larger in mammals is so persistent,” she added.

There are several reasons why women can benefit from being tall, said Dr. Tombak. What sets bats apart from other mammals is that they fly, which takes an enormous amount of energy. Large females may be better equipped to carry their developing fetuses and young offspring through the air.

More generally, the “big mother” hypothesis, outlined by biologist Katherine Ralls in the 1970s, suggests that large females may have larger young, which are more likely to survive and provide better nutrition and care. If that favors selection for larger females, and sexual competition favors selection for larger males, the result could be that both sexes are about the same size, said Dr. Tombak. “You have two forces that increase body size on both sides,” she said.

The findings are based on just 5 percent of all mammal species and are far from the last word on the subject, the researchers said.

But the paper is a useful demonstration of how much variation exists among animals, says Robert Cox, an evolutionary biologist at the University of Virginia who was not involved in the study. “It’s nice to be reminded that some of the things we say as generalizations stop working when you look at the specific details,” he said.