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How Ants Use Vision When Homing Backward

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1 How Ants Use Vision When Homing Backward on Fri Jan 20, 2017 11:50 am


How Ants Use Vision When Homing Backward 1

Ants Have a Mighty Sense of Direction

When ants walk backwards, the Sun's position plays an essential role in helping them keep the right direction. 2 A new study sheds light on how some ant species ingeniously combine vision and memory to find their way back from foraging expeditions—whether walking forward, backward or sideways.

When your size comes down to millimeters and your eyes practically graze the ground, how do you know where to go? Scientists have long been intrigued by the way ants were able to stray dozens of meters from their nests—a long way from home for the tiny insects—yet still manage to find their way back. According to a new study by insect neuroscientists, computer scientists and ethologists based in Canada, Australia, the UK and France, the integrated use of vision and memory responsible for the navigational prowess of certain ant species proves far more sophisticated than previously thought.1 The researchers further show, by observing Spanish desert ants walking in sundry positions—forward, backward or sideways—that their bearings remain unperturbed by the direction that they face, pointing to the unsuspected complexity of ant brains and nervous systems.

Spread and conquer

If ants come in over 12,000 known species today, it is largely thanks to the diversity of their survival tactics, fostering colonies on every continent bar Antarctica. Among the strategies that vary between species are those adopted for returning to the nest after foraging trips. While some ants lay down pheromone scent trails, others, like the Spanish desert ant (Cataglypis velox), rely on vision offered by the multitude of interconnected lenses that make up their compound eyes, associated with an astounding ability to commit to memory scenery observed along a route. Scientists have previously hypothesized that ants navigate by matching the terrestrial cues in front of them to those in their memory bank, memorized from an egocentric perspective, in other words, when facing the path head-on. At the same time, such ants also show an ability to find their way home backward. So how do these two notions fit together?

Researchers decided to find out by using field experiments carried out in Seville (Spain) with the local desert ants. “These ants forage in summer when ground temperatures can reach 70°C,” explains Antoine Wystrach, an insect neuroscientist at the CRCA.2 “In this heat, the dead grilled arthropods they feed on lie everywhere, so ants disperse to forage alone rather than send out groups to the one patch.” This tactic accounts for the species’ solo navigation skills, making it an apt subject for demonstrating how individual ants make it home.

Reverse, full speed ahead

In a first experiment, the researchers placed barriers around an active nest to direct foragers, goaded by pieces of cookies, along a specific route. Once the ants were familiar with the itinerary, the scientists released them on a homebound leg that involved making a 90° turn. Insects with cookie pieces light enough to lift in their jaws while walking forward dashed home, turning as required. Ants with more unwieldy pieces, however, forced to tow them while walking backward, missed the turn and continued in a straight line—a clear indication that the route’s rear-facing view rang no bells. “This result supports the idea that an ant’s memories of the visual scenes are based on an egocentric perspective,” says the CRCA researcher. “But the question still remained: how then do ants manage to navigate correctly when walking backward?”

It was the same experiment that answered the question when some backward-moving ants made an unexpected show of resourcefulness: they dropped their loads, turned around to take a peek from a front-facing position—evidently calling on their egocentric memory of the correct route—before turning again to retrieve their booty and continue backward in a rectified direction. From this interplay of three types of memory—visual memory of the right track, memory of the cookie’s existence, and memory of where to head back—the ants demonstrated an “impressive ability to coordinate different pieces of information,” which also made their navigational behavior “surprisingly versatile.”

Also of note, the ants’ backward movements traced strikingly straight lines—quite an achievement, especially when dragging a load. “The steps of backward-walking ants are completely chaotic,” Wystrach points out, “so they had to use an external cue to line themselves up.” To test whether celestial cues had any bearing on this feat, the researchers conducted a second experiment in which they presented backward-moving ants with the Sun’s reflection in a mirror. The illusion of the Sun shifting to the opposite half of the sky had a visible impact. “The backward-walking ants changed direction,” recalls the researcher, “thus confirming Sun position as a major factor in how they keep a straight course when no frontward view is available.”

Indeed, no matter what walking position (even sideways), the desert ants stayed on a straight-line course. “Existing literature has argued that ant travel direction was coupled with body orientation,” details Wystrach. “But our results show otherwise: ants can face one direction but move in another. This decoupling implies that their bearings aren’t centered on their own bodies but on the outside world—a system that is particularly flexible as ants can then take account of all types of directional information, including information from egocentric visual memories, regardless of their body orientation.”

This study highlights the degree of sophistication of insect nervous systems. “Imagine the complexity of the motor control involved when the ant brain tells the legs to walk sideways in a straight line!” enthuses Wystrach. Furthermore, the ants’ behavior attests to “an unsuspected synergy between different insect brain regions.” For example, when an ant peeks frontward to check its position before continuing backward, this action requires “a transfer of information” between the ‘mushroom bodies’ (the part of the insect brain which stores memories on the visual field) and the ‘central complex’ (the one that picks up on the Sun’s position). Wystrach reveals that the team’s long-term goal is to “determine the extent to which these two brain regions work together” to orchestrate how ants find their way.

But in the shorter term, their research will be a little bit more light-hearted. “When a backward-moving ant drops its cookie to turn around and check its position, it remembers where to pick the cookie up again. But how much does it actually remember about the cookie? How will it react when we replace its large piece with a small one?” No mere prank, the test will help scientists detect the degree of detail of ant memory.

Ants can navigate over long distances between their nest and food sites using visual cues. Recent studies show that this capacity is undiminished when walking backward while dragging a heavy food item. This challenges the idea that ants use egocentric visual memories of the scene for guidance . Can ants use their visual memories of the terrestrial cues when going backward? Our results suggest that ants do not adjust their direction of travel based on the perceived scene while going backward. Instead, they maintain a straight direction using their celestial compass. This direction can be dictated by their path integrator  but can also be set using terrestrial visual cues after a forward peek. If the food item is too heavy to enable body rotations, ants moving backward drop their food on occasion, rotate and walk a few steps forward, return to the food, and drag it backward in a now-corrected direction defined by terrestrial cues. Furthermore, we show that ants can maintain their direction of travel independently of their body orientation. It thus appears that egocentric retinal alignment is required for visual scene recognition, but ants can translate this acquired directional information into a holonomic frame of reference, which enables them to decouple their travel direction from their body orientation and hence navigate backward. This reveals substantial flexibility and communication between different types of navigational information: from terrestrial to celestial cues and from egocentric to holonomic directional memories.

Ant navigation is often described as a tool kit of distinct behavioral strategies, in which the use of celestial and terrestrial cues (apart, perhaps, from wind are processed by independent modules weighted by simple rules and gated by simple motivational control . The current results depict a different story: ants walking backward must assess their accumulating uncertainty and eventually drop their cookie to peek forward for the time necessary to recover a direction; and this direction, obtained by egocentric, rotationally dependent processes based on memories of terrestrial visual cues, can be integrated (together with other directional information such as the PI vector) in a holonomic frame of reference and followed independently of the body orientation using the celestial compass. Whether these two processes (i.e., peeking forward to gather information using memories of the visual scene or moving along the computed direction using the celestial compass) are always achieved sequentially, or can be achieved simultaneously and continuously, remains to be seen. In any case, strategies of different kinds mingle, and navigational behavior appears to be a product of remarkably flexible control.


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