Murray Logan and Gordon D. Sanson of Melbourne's Monash University have published a report on their unique way of predicting the reproductive life of koalas in the latest issue of the Australian Journal of Zoology.
"It is very hard to measure actual reproductive activity of individual koalas since they rarely mate," Logan told ABC Science Online. "[And] it's hard to determine if the mating act led to successful reproduction."
Biologists have long used the degree of 'tooth wear' in koalas to assess how long they have left to live, but its relationship to koala sex habits had never been noted before. Koala teeth wear down as a result of their constant grinding of eucalyptus leaves, their primary source of food.
The blunter their teeth, the less food they can digest. Consequently, most koalas don't actually die of disease or old age - they die of starvation.
Logan wondered if tooth wear might also be a good indicator of a male koala's reproductive life.
He selected six wild koalas, all with the same weight and body condition, from a 25 hectare study area of coastal manna gum trees on Raymond Island, off the coast of the southern state of Victoria.
The only apparent difference between the animals was their degree of tooth wear. Two koalas had a low degree, two had medium degree, and two had a high degree.
He fitted radio collars with microphones to each koala, enabling him to follow each animal as it moved from tree to tree, observing it from a distance and recording how much time it spent bellowing - one of the ways male koalas assert their social dominance. The more bellowing a male does, the more dominant he is, and the more likely he is to mate, Logan said.
He studied the behaviour of each koala over two days and nights. This involved a month of field work, during which time Logan constantly wore the headphones, even while sleeping, in the pop-top caravan attached to his car.
Every time the koalas bellowed, Logan would hear them, and every time they moved, he would drive after them to keep the radio signal clear. "I didn't sleep for a month," he said.
Logan found that koalas with low and high tooth wear only spent 2 to 3 minutes in total every 24 hours bellowing, whereas those in their prime - with medium tooth wear - spent 25 minutes.
He backed up his findings by studying another measure of social dominance, scent-marking. This is where koalas rub against a tree as they climb, marking it with scent from a gland on their sternum. Logan compared the number of trees each koala climbed without eating leaves, deducing that these were trees used for scent-marking alone.
The results followed the same pattern as the bellowing observations. Those with low and high tooth wear climbed no extra trees, whereas those with medium tooth wear climbed 5-7 extra trees per 24 hour period.