Professor Russell Baudinette, head of the department of zoology at the University of Adelaide, believes he has found the answer to why it is possible for marsupials to carry up to 25 per cent of their normal body weight without showing any apparent signs of fatigue.
"What we have shown in previous work is that when wallabies hop they can increase speed without increasing their use of oxygen or energy. This is because of the spring-like function in their legs, primarily in the tendons," Professor Baudinette said.
"In this study we extended the work to see if this ability held up when animals carried loads. This is important as marsupials carry around 25 per cent of their body weight when the joey is in the latter stages of pouch life."
The scientists describe an experiment in the journal Nature in which four wallabies that had been trained to hop on a treadmill were given a load to carry in their pouches.
"Because of the risk of losing young animals, the load consisted of lead shot placed in a deformable latex glove carried in the pouch," the scientists say.
They assessed the metabolic rates of the animals when they hopped at different speeds both without loads and with the loads. When wallabies were carrying loads they found they used their elastic muscle tendons more by placing their hindfeet on the treadmill for longer to build up a greater stored force.
For loads up to 15 per cent of the animals' body weight, the wallabies did not use any extra metabolic energy. "In other words, wallabies are 'freeloaders'. This is unlike the situation with other animals, in which a 15 per cent load would result in a 15 per cent increase in energy cost," Professor Baudinette said.Reuse content