Use of lures
There is a correlation between ‘feeding’ and using food as lure. Feeding in the early establishment phase will influence behaviour later-on. Animals are conditioned to finding food in their release cage or a feeding station and will come back if in need of supplementation or even just out of habit.
This learned behaviour highly improves the probability of detecting our target species. Reliably attracting them into the view of a camera is necessary to make cameras an effective monitoring tool.
Particularly when release numbers are still low, lures may be necessary to increase the propensity of animals to visit a camera site or the rate of sightings might be too small to be meaningful.
Whether the use of lures causes undue bias depends on the monitoring objectives. We specifically want to monitor our releases, their persistence, breeding activity and if possible their behaviour. Insights into interaction with brushtail possums and the effect on the latter when increasing/introducing a ringtail population and when providing some potential food source for them too, is an important side aspect. Attracting other species (e.g. phascogale) is not relevant for this investigation unless the species have implications for ringtails (e.g. predators).
Most scientific studies would require a randomised study design with unbaited camera traps, however those studies have very different objectives to our ‘citizen research’.
The biases caused by using lures might be no more significant than that caused by other factors that influence the detection of individuals within a species, e.g. camera placement. The placement of the cameras at non-random focal points such as at the release spot might already increase the chance of detecting some individuals over others. (Claridge et al, 2004) Ringtails are creatures of habit and individuals react differently to situations.
The familiar release cage itself could for some animals already function as a lure.
Lures also increase the time animals spend within the detection zone of the camera. A higher number of images occasionally even helps identify an individual.
Dominant females are likely to claim the supply and at times aggressively protect it, while at other times additional animals (male, previous offspring) are allowed to participate. The likelihood of frequent sightings of the same animal checking availability also increases.
Overall detection probability also increases through the active manipulation of the behaviour of the target animals. (Meek et al, 2012) Animals learn that there is some palatable, frequently ‘regrowing’ peppermint supply which might also attract animals from outer areas to check and that way helps attracting more animals which might otherwise never be detected by a camera.
Our cameras are left continuously in the same spot to examine the ongoing use of the site – with and without lure - and the interrelationship between the frequency of sightings and the availability of food in the various seasons.
The view that we are continuously ‘feeding’ by supplying a quantity of leaves once a week is weak criticism as what we provide would only sustain one animal for one night. If the animals cannot sustain themselves, this level of supplementation would not increase their chance of survival.
There seem to be 2 types of lures, those that can then be consumed by the target species and those that cannot.
Research was conducted with a variety of attractants, some as elaborate as a piece of cotton soaked in honey, raw linseed oil, vanilla essence, truffle oil and peanut butter but held in an enclosed container with holes so that the substance could be smelled but not consumed. (De Bondi et al, 2010)
Even when ringtail possums were the target species lures such as dough with rose essence and peanut butter and oat-based ‘universal bait’ were offered. Eucalyptus oil baits seem to be the closest to their natural food, however the type of bait (all not to be consumed) did not significantly affect capture rates of western ringtail possums. (Wayne et al, 2005)
As bait/lure in this research was intended to physically catch the animals and not just catch them on a photo, the trap setting might have deterred the animals though.
Lures/bait like the above would also have a higher risk of attracting predators than an offering of foliage.
An interesting experiment using either truffle oil or buried truffles for mammals that feed on them showed that buried truffles reduced the ‘by-catch’ of other non-truffles eating mammals and it was called ‘unbiased’ as no artificial attractant was used. (Vernes et al, 2014) In that sense our peppermint leaves should also produce unbiased results.
Rats seem common visitors to all types of bait/lure stations and they are also attracted to our cages and feeding/water stations. The particular attractant is unknown but it might just be the water that is usually available in close proximity to the bunch of peppermint.
Surprisingly it was worth noting in research into attractants that ringtail possums (and other species) were not deterred if rats visited the bait in the same 24-hours. (Paull et al, 2011) Ringtails and rats seem to use the same stations at the same time without issues.
