Does it pay to be a lover or a fighter? The answer can depend upon the situation, of course, but some animals have evolved a greater tendency to war or woo, suggests a study in the latest issue of Nature Communications.
Senior author John Fitzpatrick, a lecturer on animal evolution at The University of Manchester, and his colleagues studied over 300 species and found that elephant seals are definitely more fight-prone.
"In groups like pinnipeds, in some species, like elephant seals, males are able use their investment in weapons -- in this case extremely large body size -- to keep rival males away from females and there is a trade off between investment in testes and weapons," he explained. "Essentially, the largest pinniped species have the smallest testes, i.e. they are fighters, rather than lovers."
The walrus is another pinniped that is more brawn and blubber than love machine. Both males and females use up a lot of energy sustaining their big bodies and tusks, which they aren't afraid to use in battles.
Acanthocephala is a group of parasitic worms, according to Fitzpatrick. As if that is not unattractive enough, these worms spend most of their energy in maintaining their body spines and fighting.
This is arguably the most un-romantic group of organisms in the animal kingdom.
Male pheasants "invest in both fighting and sex," according to Fitzpatrick.
"It is quite costly to invest in everything. You don't get something for nothing in evolution," he said.
Nevertheless, there are some animals, like pheasants, which seem to have evolved an even investment in the anatomy and behaviors required to fight and mate. Male pheasants that invest in being large fighters, for example, also develop large testes.
These tiny fish are scrappy, but sexy sometimes too. As for pheasants, male minnows are hard-wired to invest in fighting, often with their strong fins that push waves of water against rivals, but also in procreating. Male minnows often turn bright red or orange to attract females, and then go to a lot of trouble to find the perfect spawning site.
Male bush crickets have the largest testes compared to body mass of any known species on the planet. Males fight a lot too, though. In China, "cricket fighting" is viewed as a spectator sport on par with bullfighting and cockfighting. Cricket fights are even arranged according to weight class, with human viewers betting on which cricket will win the match.
Humans and other primates wind up in the category with crickets and minnows. Our canine teeth and other features can be used in fights, but significant bodily energy often goes into wooing too.
As Fitzpatrick said, "Compared with our primate relatives, human males don't exhibit particularly large investment in either weapons (which we measured as the difference between male and female body sizes) or testes size."
Albatross couples romantically dance and spend their entire lives together, devoted to a single, adoring mate. Males will wait for days, sometimes weeks, for their partners to arrive at mating sites.
Some snails are hermaphrodites, having both male and female sexual organs, but most still need a partner in order to reproduce. Ronald Chase, a professor of biology at McGill University, found that garden snails court anywhere from 15 minutes to six hours. It's a sexy tease that involves a lot of lip and genital nibbling. Like a natural cupid, one snail then builds up hydraulic pressure that causes the release of a sperm-containing "dart."
The result is that these creatures spend a lot of time procreating, as most gardeners have probably figured out.
Lovebirds (birds in the genus Agapornis) aren't always loving around strange birds and humans, but they are completely devoted to their mates. They may spend their days preening, dancing and cuddling. Mating can occur, on and off, for several days at a time during the breeding season.
Some lovebirds are endangered in the wild. Black-cheeked lovebirds, for example, are among the most endangered of all African parrot species.
"Understanding the ways animals reproduce is important as it helps us understand how species evolve and can prove important for conservation," said Fitzpatrick.