Your spouse may baffle you at times, but does he latch on to your rear as a miniscule parasite 500,000 times smaller than you?
That’s what a male seadevil does. Is your honey 50 times your size and liable to eat you after a snuggle? Let’s hope not, else she’d be a garden spider.
The animal kingdom is full of amatory pairs whose extreme physical differences would give a matchmaker pause. But many of these dimorphic differences make good evolutionary sense, Daphne J. Fairbairn explains in her book Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom.
National Geographic Senior Writer Rachel Hartigan Shea spoke with Fairbairn, a biologist at the University of California, Riverside, about why in nature, love isn’t always one size fits all.
How did you become interested in sexual differences in animals?
For my PhD dissertation, I was going out day after day capturing mice in Vancouver, Canada. I discovered that males and females died at the same rates in the spring but for completely different reasons. The young male mice were getting into fights, but what was killing the females was getting pregnant. It struck me that no one cares what sex a mouse is, but for the mice it matters enormously to how they experience their lives. I never forgot that lesson.
Why are the differences between the sexes in some animals so extreme?
If you are coming into the world as a male, the way you get your genes into the next generation is getting your sperm to meet up with the eggs of females. So whatever it takes to do that is how the males are going to turn out. (Related Q&A: “Unlikely Animal Friendships.”)
So what does it take for male elephant seals, who are seven to eight times larger than females?
The way a male elephant seal passes on his genes is by having physical contests with other male elephant seals to keep them away from females so he can mate with them.A male southern elephant seal dwarfs a female in South Georgia Island. Photograph by Yva Momatiuk and John Eastcott, Minden Pictures/Corbis
His whole life history is structured so that he will be big, aggressive, and politically savvy on the breeding grounds. That means that he’s born larger [than females], he takes more milk from his mother than he would as a female, and he grows at a faster rate.
In order to sustain that growth rate, he ends up having to go foraging for a different kind of food that is found in a different part of the ocean [from where the females are foraging]. His whole life is different.
Male orb spiders, on the other hand, are much smaller than females. How does that affect their lives?
If you are a male orb spider, your biggest threat is that you won’t ever find a female to mate with. You have to leave your web and search for the trail of a female orb spider. A tiny body and long legs makes it more efficient to move through tangled vegetation. (See “Male Spiders Give ‘Back Rubs’ to Seduce Their Mates.”)
Across the animal kingdom, who’s bigger?
Females. If you think about a spider or a worm or an insect, they make a whole bunch of eggs, their bodies swell up with eggs, and they extrude the eggs in a big batch. It helps to be big.
What’s the strangest animal you studied for your book?
Bone worms. When scientists found these bone-eating worms [on whale carcasses at the bottom of the ocean], they couldn’t find the males until they looked inside the females. The males look like they’re single cells, just yolk and sperm. (See bone-worm pictures.)
Has your research given you a new perspective on the differences between men and women?
On the one hand, males and females are different from the get-go in their physiology, their behavior, their morphology. So to one branch of radical feminists I’d say, stop trying to say there’s no difference between men and women. On the other hand, compared with most animals, human males and females are hardly different at all.