ASU Research E-Magazine: King of the Hill

When the topic is successful mating strategy among tarantula hawk wasps in the Sonoran Desert, you can be sure that size does make a difference.

Finding romance often involves dealing with the competition. But what makes a superior suitor? Often, one or two traits can make the difference.

Picture this scene: Hoping to snare a gal, a male decides to survey the social situation. He mills around in search of a mate. Not far away, he notices a hub of activity. The females are heading right past him and straight to the action. He sees the big boys sitting comfortably, just waiting for the girls to come along. Might he find his own place among the social elite?

Our hero searches for a spot to fit in. But each time he finds a corner to call his own, one of the other guys sends him on his way. They are all a lot bigger. And they’re ready to throw their weight around.

Outsized, he gives up the competition. Defeated, he flies away.

Yes, flies away.

Except for the flying part, the actors in this scenario might be twenty-somethings at a trendy singles bar. Instead, this drama is enacted by wasps, tarantula hawk wasps to be exact.

John Alcock is a Regent’s Professor of biology at Arizona State University. For more than 20 years he has watched tarantula hawks play the mating game. Alcock’s research examines the social tradeoffs that drive their mating behavior.

Over the years, the ASU scientist has developed an ever-growing familiarity with generation after generation of tarantula hawk wasps. He studies the insects in the Usery Mountains, just an hour northeast of ASU’s main campus. Each spring, Alcock spends several days a week hiking the area’s scrubby peaks. The wasps stake out mating territories along palo verde-studded ridges.

Some aspects of tarantula hawk behavior are more similar to humans’ than you might expect.

“Insects are not automatons at all,” Alcock insists. “They’re highly flexible. And that’s reflected in the behavior of the wasps.” Being flexible is one of the best ways a lustful tarantula hawk can ensure himself success.

The Never-ending Search
A male tarantula hawk wasp’s most momentous social challenge is finding a mate. The easiest way to attract a mate is to get access to the places females visit. Getting access to these locations is a struggle for smaller males.

Like people, tarantula hawks favor hilltop real estate. When mating time arrives, female wasps fly uphill to the highest point. Males meet them at the top of the hill. The males keep lookout in the palo verde trees or creosote bushes growing on the peaks.

Alcock has found that peak top territories are highly contested. The outcome of competitions for these prized sites is determined mostly by size, which means that larger males almost always win. This is a serious problem for the smallest males. The size disadvantage that they face is staggering.

“Males vary in body weight by at least a factor of five,” Alcock explains. “There are little tiny guys, and there are some great big bruisers. Side by side, you wouldn’t think they were members of the same species.”

Alcock’s “big bruisers” check in at almost 2 inches long. Their miniature counterparts are runts less than half an inch long.

Small males pay a steep price for their inadequate stature. Smaller wasps get stuck with low-quality, low-altitude territories. Some little males have no territory at all. They spend mating season wandering from tree to tree.

Bottom-of-the-hill losers struggle for success. They often try to catch females racing toward the peak top territories.

Why the size difference between males? Peewee tarantula hawks can blame their mothers.

Being Little: Logic or Luck?
Tarantula hawk wasps earn their name by preying on tarantulas and other large spiders. The female wasp is armed with a long, powerful stinger. During a successful hunt, she will sting and paralyze a spider. With the spider unable to move and at her mercy, the wasp lays a single egg on the spider’s body. Then she drags the spider into its own burrow, or buries it in a hole. When the larva hatches a few days later, a massive meal is waiting. It slowly eats the spider alive.

Full-bellied larvae that feed on large spiders become large adults. Unlucky larvae hatch with only a snack-sized spider waiting for dinner. They eat every bite, but grow up to be little wasps.

Tarantula hawk moms aren’t intentionally cutting their smaller sons short. In fact, they should prefer to make the biggest babies they can. They have more to gain by producing large sons, says Alcock.

Bigger sons are more likely to find mates and produce their own offspring. The mother wasp’s grandchildren will carry some of her genes. And this is the most important measure of a wasp’s success in life—she who spreads her genes most shapes the evolution of her species in her own image. So why even bother making a small son?

A major obstacle stands between a female tarantula hawk and her gene-spreading success: Spiders are not easy to come by. Nabbing a big one depends on the luck of the draw. Sometimes the hunter has to settle for less than the biggest and best.

“If the choice is between exploiting a large spider or a small one, of course the wasp would go for the large one. But there may be no choice. My guess is that these wasps hunt long and hard to even find a spider of any size,” Alcock explains. “If she finds a small one, well, she lays an unfertilized egg on it and so produces a son. He’s going to be small, but he might get lucky.”

Little males can’t really fault their mothers for their size deficiencies. But they still must struggle as underdogs in the battle for love.

Conditional Love
A big, burly male tarantula hawk’s mating strategy is a no-brainer. He scouts out a prime palo verde tree at the top of a hill. He claims the tree as his territory and waits for the females to come to him.

Small males don’t have the brawn to compete for top territories. Alcock says that undersized males use different approaches. They don’t waste their efforts in a failing pursuit of the “big boy” method. Instead, they try tactics better suited to their abilities.

Alcock calls their adaptable behavior the product of a “conditional strategy.” He argues that evolution has favored males able to adjust their behavior to make the best of a bad situation.

“Males use a conditional strategy to make tactical decisions that will give them the best possible chance,” he says. “Smaller males tend to occupy the lower, less attractive sites.”

The downside of living down-slope is that females are in short supply. However, small males who stake out these modest properties don’t have to fear eviction by larger competitors.

A second group of smaller males are vagabonds. Rather than making any particular tree or shrub their home, they wander among other males’ territories. Alcock calls tarantula hawks using this approach “patrollers.”

“Either they never snag a territory or they refuse to accept the lower ranking ones and keep on the move. Some patrol a site repeatedly, attempting to intercept females on their way to the top,” Alcock says.

The males are opportunists. “Take the large males out of the territories and the small males are perfectly prepared to move into them,” he adds.

A particularly confident mid-size male tarantula hawk may even try to oust a bigger wasp from his roost. If he is in good condition, he may win the challenge.

Most of the time, however, the intruder is unsuccessful, Alcock explains.

“Typically, a resident male will challenge any intruder. They fly about the tree, the resident returns, and the other guy keeps going.”

Occasionally, the invading male is more persistent. These encounters can escalate into dramatic battles.

“The fights are spectacular,” Alcock says. “The wasps don’t grapple or wrestle. They engage in ritualized aerial displays that you must be see to believe.

“Perched high on a branch, the resident wasp meets the intruding male and they fly around the tree. Then they go zooming upwards. As they fly up, they spiral around each other. I’ve watched them go upwards out of sight of my binoculars. Most of the time they don’t go up that high, but 100 feet is common,” he explains.

“They go racing up until one of them starts back down again. They literally turn around and just dive back down to the tree. If the intruder comes in a second time, back they go. When they are serious about winning, these upward spiral flights can number in the hundreds and last up to an hour.”

Competing males don’t actually engage in physical combat. The real challenge of the spiral flights is paying the energy cost of an exhausting encounter. The winner is probably the male with the most endurance and best body condition.

Tarantula hawks assess whether they have the stamina to succeed in these bouts. Small males must choose their battles wisely. For them, challenging the king of the hill is not a wise proposition.

“The males essentially sort themselves out,” Alcock explains.

“The males of real competence head for the top sites. Males of lesser competence head for the low sites. There’s a bit of a tradeoff. If you go to the top sites, you’re going to be confronted with more intense competition.”—Danika Painter