Grasshoppers update escape response 'real time'

Two grasshopper species have been examined to see if they modify their fleeing strategies when repeatedly approached by a predator.

The study looked at Schistocerca alutacea, a medium to large grasshopper species (30-54mm body length) found in open woods and pastures on woodland edges, and Psinidia fenestralis, a smaller species (20-33 mm body length) found in sandy areas.

Curtin University's Dr Bill Bateman and Murdoch University's A/Professor Trish Fleming say the intention was to challenge the traditional belief that organisms converge on an 'optimum escape strategy' and test the theory that they can modify strategies in real time though risk/cost analyses.

"We found that in response to persistent pursuit by an assumed predator, two grasshopper species altered their behaviour from their initial tactics, switching to 'Plan B', which involved potentially riskier behaviours," A/Prof Fleming says.

"For S. alutacea, a strong flier, this involved abandoning the strategy of perpendicular movement and flying further and further away from predators in a direct line.

"P. fenestralis, on the other hand, began to initiate their escapes earlier, at longer distances from the predator, but their flight distances actually decreased."

The researchers suggest this could be due to decreased energy reserves.

P. fenestralis also modified its choice of landing areas.

"Of first escape attempts, 88 per cent were to bare sand. As the field was divided almost evenly between sand and grassed areas, this suggests that P. fenestralis were not landing randomly, but were initially selecting the sand," A/Prof Fleming says.

"P. fenestralis can seem to vanish immediately upon landing on sand due to their colouring, which can be enhanced by disorientation of the predator by the flashing and then disappearing of the coloured underwings upon landing, particularly when it occurs laterally to the predator's path and is only seen by peripheral vision.

"However, over successive escape attempts, when this camouflage was no longer a safe strategy, due to still being pursued, an increasing number of animals moved towards grassy refuges where they would move into the grass clump and become impossible to locate or flush."

The researchers found that grass is likely a least-preferred option in general for P. fenestralis due to ground predators and reduced camouflage to birds, but the best option in certain situations.

"The study suggests that even in 'simple' organisms like grasshoppers, escape behaviour is dynamic and variable and individuals can update their escape response in the face of changes in a predator's behaviour or persistence," they say.