As ants roam around on a plant, they can help their leafy companion by killing any herbivores they find. Ants often do just that, because many ants need meat in their diets.
Some species of ants are more aggressive than others, and many plants don't have any choice about which species visit.
Researchers report for the first time that when plants supply ants with nectar, it boosts the ants' desire for meat, potentially making them better bodyguards for the plant.
"If you have enough birthday cake or soda pop, you're eventually going to want something of substance," said team leader Joshua A. Ness, an ecologist at the University of Arizona in Tucson.
The same is true of ants, he said. And the most convenient protein packets for ants on plants are often insects that are there to munch on the plant.
Nectar isn't just found in flowers. Many plants exude nectar from little pores on the plant, called extrafloral nectaries, that attract ants.
"The plant wants ants to be protein eaters while they are on the plant," said team member Judith L. Bronstein, a professor in UA's department of ecology and evolutionary biology. "The plant gets them really jacked up on carbs so they're desperate for protein."
Ness's presentation on the team's findings, "Contrasting the diet and aggressiveness of the ant bodyguards tending an extrafloral nectary-bearing plant," will be given on Wednesday, Aug. 4 in Portland, Ore., at the annual meeting of the Ecological Society of America. Ecologist William F. Morris of Duke University in Durham, N.C., is also a team member. The research was funded by the National Science Foundation and the National Institutes of Health.
Ness and his colleagues study mutualisms, beneficial partnerships between species. In this case, the team studied the behavior of four species of ants that visit extrafloral nectaries on a common Sonoran desert plant known as barrel cactus, or Ferocactus wislizeni.
The carbohydrates and water supplied by the cacti are not readily available to ants elsewhere, Bronstein said.
In one experiment, the researchers sealed up the nectaries with rubber cement to see if the ants would abandon a nectary-free cactus.
"The ants were very put out and pulled off the rubber cement to get at the nectar," Bronstein said. "By hook or by crook, they will be going up there to get that nectar."
The ants clearly treasure the nectar available from the cacti. The researchers wanted to know what kind of value the ants gave the plant in return.
To see whether ants ate more meat when exposed to sugar, the researchers put baits of sugar and of meat at the base of barrel cactus plants and at the base of plants without extrafloral nectaries. For all four species, ants foraging by the cacti had a stronger preference for meat than did the same species of ants when foraging near other plants.
However, the ant species differed in the strength of their preference for meat.
To see how that might translate into plant protection, the researchers put caterpillars on cactus plants and measured how quickly each ant species recruited its fellows to the fresh meat and how many ants it took to vanquish five caterpillars in 30 minutes.
Within five to 15 minutes, ant numbers rapidly increased, and ants were dismembering caterpillars. But although all four ant species were equally good at recruiting their kindred to attack caterpillars, the species differed in their effectiveness at killing. Ant species that had shown greater preferences for meat in the previous experiment were also better protectors.
Even so, all four species did kill some caterpillars, so hosting even the wimpiest ant species was better for the plant than having no ants at all. The researchers also found that numbers matter: a lot of the wimpy ants could eventually do as good a job of protection as a few of the more aggressive ants.
In addition, the results suggest that loading up a mild-mannered ant on nectar makes the ant better at plant protection by increasing its cravings for meat.
Ness said, "If you pay your bodyguard up front, he or she could abandon you later, in your time of need. Here the act of collecting the reward makes the performance of the service more likely. No one has shown that before with ant bodyguards."