May 28, 2013
The Inefficient Nightlife Of Bull Ants
Brett Smith for redOrbit.com - Your Universe Online
Nocturnal Australian bull ants have trouble commuting in the dark, making those that head out and return the most efficient workers, according to a new study that was recently published in the journal PLOS ONE.
“To cope with dim or dark environments, night-active ants have larger lenses and wider photoreceptors — to capture light — compared to their day-active relatives,” said co-author Ajay Narendra, at the Australian National University (ANU). “We found that even with this compensation, the nocturnal bull ants still take longer to get to their food sources or nests at night.”
“Our study shows that this is because these ants rely predominantly on surrounding landmarks to navigate, and that landmark information is less salient in the dark,” he added.
Narendra and his colleagues recorded the ants´ activity with model paints, infrared light sources and a video camera. They were also able to track the ants´ paths within a centimeter by using GPS markers that were placed every 2 inches behind a walking ant.
The team discovered that bull ant workers that leave the nest during early twilight to forage on a nearby Eucalyptus tree reach their destination faster than those that leave an hour after the sun goes down.
“The ℠late´ ants walked more slowly, stopped more frequently and paused for longer compared to the early foragers,” Narendra said. “They were also less able to travel in a straight path.”
While most of the ants returned to the nest at dawn, 10 percent of the workers began their journey home at midnight and took much longer to reach their nest, the researchers said in their report.
“The ants that return throughout the night are workers that have captured prey,” Narendra explained. “Once they´ve found a spider or a wasp, there´s no point in hanging around. The best thing to do is to store the prey in a safe place, and the safest place is home.”
To determine why the ants seem to have trouble in the dark, the researchers collected and displaced some of them outside of their typical foraging route in both light and dark conditions.
“If the displaced ants used a sky compass — the pattern of polarized skylight — for navigation, they would have travelled ignoring the displacement and headed to where the nest should be — even if it isn´t actually there,” Narendra said. “But once we released them, the ants that were let go in bright conditions went home directly and the ℠dark´ ants began searching for landmarks immediately.”
“This means that bull ants rely heavily on the surrounding landmarks and less on sky compass for navigating during both bright and dark conditions,” he concluded. “And in the dark conditions, landmarks are less salient — this is why they take much longer to complete their journey then.”
Narendra said the study demonstrated why most of the bull ant workers travel in the dim light conditions of twilight or dawn instead of complete darkness.
“So travelling in the dark comes at an enormous cost to these bull ants,” he said. “And given they are better navigators in bright light conditions, it raises the question of why they remain so stubbornly night-active throughout the year.”