Thursday 5 June 2003

DRAGONFLIES PROVE CLEVER PREDATORS

Researchers at the ANU have uncovered a unique predatory ploy in the animal kingdom observing dragonflies, which can make themselves appear stationary by following a prescribed pattern of motion.

In the animal kingdom, the usual way a predator approaches potential prey (or a romantic male approaches an elusive female) is by stealth – moving very slowly, but moving nonetheless.

The ANU research has found that a predator can move rapidly towards its prey while appearing stationary, and therefore remain undetected. The ‘shadower’ animal moves in a way that produces the same image motion as a still object on the retina of its ‘shadowee’, in a tactic the researchers have described as ‘active motion camouflage’.

Dr Akiko Mizutani and Professor Mandyam Srinivasan, from the Visual Sciences Group in the Research School of Biological Sciences at the ANU, and Dr Javaan Chahl, a scientist with the Defence Science and Technology Organisation, recorded 15 encounters between two dragonflies on video camera and found six displayed clear examples of active motion camouflage.

Dragonflies were chosen for the study because of their propensity for aerial combat, their strongly expressed visual system and high manoeuvrability. But according to the researchers, this deft trick may not be confined only to the insects.

“Our findings raise a number of questions about the exquisite flight control and position sensibilities that are required to produce active motion camouflage,” their paper says.

“It is possible that the stratagem of active motion camouflage is unique to anisopterans [a suborder of dragonfly]. It is more likely, however, that motion camouflage will be found in other species such as predatory fish, birds and other insects, such as wasps, that operate in three-dimensional environments.”

The dragonflies studied in the paper were filmed on the shores of rivers and ponds around Canberra on mid-summer afternoons using a stereo camera system. The scenes were downloaded onto a computer and separated into individual fields. Three-dimensional trajectories were then reconstructed by digitising the head positions of both dragonflies in each field using specialised software.

As part of their study, the researchers calculated the apparent motion produced by the shadowing dragonfly on the retina of the shadowee and compared it with that produced by a stationary object.

“The results reveal that the two angular velocity profiles are very similar, indicating that the strategy is likely to be very effective in concealing the shadowers’ motion.”

In the dragonfly world, motion camouflage is most likely used by a male to ‘shadow’ a prospective mate, or to pursue, in stealth, a food item such as a fly.

Co-sponsored by the US Office of Naval Research, and Australia’s own Defence Science and Technology Organisation, the research has potential application in autonomous aircraft technology.

Dr Mizutani and her colleagues discovered it was likely the shadowing dragonfly uses all three types of motion camouflage – appearing to be a static object that is either in front of, or behind the shadower, or an infinite distance away from the shadowee.

“A shadower can also combine two types of motion camouflage in a single episode. [In one observation] during the first ten frames, the shadower imitates a stationary object close to the shadowee.

“Subsequently, the shadower imitates an object at infinity, by holding its position constant on the retina of the shadowee.”

In their paper, the researchers say the exact way the dragonflies achieve this camouflage effect remains to be discovered.

However, it is certain that the flight patterns are motivated by motion camouflage “and are not an artefact or by-product of another behaviour”.

Further Information

Tim Winkler
Media Liaison, Marketing and Communications
Tel: 02 6125 5001, Fax: 02 6125 8255
Email: tim.winkler@anu.edu.au

Amanda Morgan
Media Liaison, Marketing and Communications
Tel: 02 6125 5575, Fax: 02 6125 8255
Email: amanda.morgan@anu.edu.au