AUSTRALIA: Mums kept on the hop

What defines devotion to the cause of research? Try this: two postgraduate students perched atop a three-metre tower outside a 22-hectare paddock, watch kangaroos through telescopes for several hours a day, seven days at a time.

And there's more: using digital voice recorders, the observers note at three-minute intervals what the females carrying joeys are doing, how many bites of grass they take and the number of steps they take, an indication of whether each kangaroo is after better-quality grass. The students compare the behaviour of mother kangaroos with that of another group of females who were 'on the pill' to prevent them becoming pregnant.

The study took nine months, with students Jemma Cripps and Michelle Wilson and occasionally their supervisors, spending a week at the site early in each joey's life, again at the mid-point of its time in its mother's pouch and towards the end before it left mum and was not allowed to hop back in.

The novel investigation into the feeding habits of kangaroos carrying their young was the first of its type and involved a population of kangaroos at the Serendip Sanctuary outside Melbourne.

It revealed surprising results: kangaroos with joeys in their pouch almost double their food intake and significantly reduce the time they spend resting to meet the nutritional needs of their rapidly growing babies. The bigger the joey became, and the more of the mother's milk it drank, the more time she had to spend foraging.

"But, by increasing their food intake, the mothers also increased the risk of gastro-intestinal parasite infection because it is harder to avoid faecal-contaminated pasture while consuming so much more forage," says Dr Graeme Coulson, who led the investigation with Professor Mark Elgar, both zoologists at the University of Melbourne.

"This elevated risk of parasite infection may be a very significant additional cost for reproducing females. We have been able to prove that reproducing females altered their behaviour in direct response to the energetic demands of reproduction."

Scientists have long assumed that reproducing female mammals change what they usually do to compensate for the increased feeding demands of their offspring. But, until the Melbourne researchers had completed the study, no one had been able to prove this.

"This is the first field experiment that has been able to comprehensively compare the behaviour of females with young and females without young at the same time and place," says Jemma Cripps.

"In our study, we manipulated reproduction by giving some females within the group birth control but leaving others to reproduce naturally. This allowed us to develop a better understanding of the energetic and health costs in populations of kangaroos, and other mammals more generally, and can therefore help us better manage species."

The first weekly observations began after the team had captured 10 female eastern grey kangaroos that roam wild within the sanctuary, sedated each one, implanted a birth control device and attached different coloured ear tags to each so it could be easily identified.

Another 10 females were also caught, sedated and a different set of coloured tags attached to their ears: they would soon become pregnant and within 30 days would give birth to a tiny joey.

The young kangaroo is born alive at a very immature stage when it is only about two centimetres long and weighs less than a gram. Immediately after birth, it crawls up the mother's body and enters the pouch where it attaches its mouth to one of four teats.

This then enlarges to hold the joey in place and, as it slowly grows, the little animal becomes more active and gradually spends more and more time outside the pouch, which it leaves completely at about nine months.

"The long-standing and much broader question we were addressing is the cost of reproduction," Elgar says. "In terms of understanding an animal's life history and strategies, it is important to demonstrate that reproduction has a range of costs and part of that is lactation: an animal that is providing nutrients to its offspring has to find more food or take that out of its own reserves.

"Documenting how those costs are manifested is problematic because if you just compare an animal lactating with one that isn't, there are other factors that may cause a difference over and above the one you're investigating. One animal might be healthier than another to begin with and the differences may reflect lactating or some other quality, such as age or body condition.

"Alternatively, if you observe lactating and non-lactating animals at different times of the year, the results might differ for a number of reasons. So an experiment is the way to go".

Coulson had been involved in a long-standing investigation into the impact of birth control mechanisms to regulate kangaroo populations. This led to the idea of an experiment in which a control group of kangaroos could be fitted with an implant to prevent pregnancy and its behaviour compared with a group of females with babies in their pouches: an experiment no one had done before.

"The study has much broader implications than just with kangaroos," Jemma Cripps says. "Kangaroos are a great model system to work on because we can see what stage of reproduction the female is engaged in. Unlike other mammals like cattle, the kangaroo's offspring is born very young and can be observed once it is in the pouch."

In a report on the study, published in the British Royal Society journal Biology Letters, the researchers say mother kangaroos during peak lactation spent 16% more time foraging and took 20% more bites than did the non-reproducing females. They ate almost 50% more during daylight hours and this disparity may have been even greater "if nocturnal activities were taken into account".

Surprisingly, the females with joeys did not lessen their vigilance for predatory foxes and dogs despite being more vulnerable and reducing the time spent resting. The researchers say the implications of this are unclear but the fact the mother kangaroos ate much more and were less selective about where they ate during peak lactation may magnify the risk of acquiring gastro-intestinal parasite larvae.

This was because "avoiding patches of faecal contamination while trying to consume more forage would be challenging. The effect of increased risk of parasitism in female kangaroos during lactation requires investigation and could highlight a further, largely neglected, cost of reproduction," they say.