Illuminating Behaviour

This blog post was written by postgraduate student Davena Watkin as part of the course, Research Methods in Ecology (Ecol608). Davena revisits a Lincoln University research area that looks at a study of how possums respond to moonlight in 2011.

The moon has a face like the clock in the hall;
She shines on thieves on the garden wall,
On streets and fields and harbour quays,
And birdies asleep in the forks of the trees.

The squalling cat and the squeaking mouse,
The howling dog by the door of the house,
The bat that lies in bed at noon,
All love to be out by the light of the moon.

– excerpt from The Moon by R. L. Stevenson

For our species, the moon is a theme of many literary and cultural ideas. And it's more than just waxing lyrical; we also have a long history of attributing human behaviour to moon phases. Calling someone a ‘lunatic’ is a reference to the historic belief that prolonged exposure to moonlight caused insanity and epilepsy. In folklore, a full moon in some instances could also mean turning into a hairy, rampaging beast; lycanthropy is popular in many modern fictions.

Morepork: a native nocturnal predator.
Photo by russellstreet

As it turns out, ecologists are similarly fascinated with the effects of lunar light on behaviour, though for more down-to-earth reasons. Light, including moonlight, is an important regulator of the circadian rhythms of animals. Nocturnal animals are known to alter their foraging behaviour under higher levels of illumination, and such studies have their own moon-related jargon. Animals that avoid moonlit nights are known as ‘lunar phobic’, while animals that are more active on moonlit nights are known as ‘lunar philic’. 

Typically, small terrestrial mammals that are preyed on by higher-order nocturnal predators exhibit lunar phobia, while the opposite is usually true of their predators. In fact, contrary to Robert Louis Stevenson’s poem, mice and some bats are both lunar phobics. Lunar phobia is thought to be a form of predator avoidance, as more light increases the ability of visual hunters to detect prey.

The Ecology Department at Lincoln University have their own literary works on such behaviour. A recent example is a study that was undertaken by University of Göttingen masters student, Jessica Parisi, in 2011. The research sheds new light on the activity levels of brushtailed possums (Trichosurus vulpecula) in New Zealand under full moon and new moon phases. 

Brushtailed possums are native to Australia, where they are preyed on by snakes, eagles, foxes and dingos. As such, Australian possums display anti-predator behaviour by avoiding well-lit open areas. It was expected that the possums in Jessica's study would therefore be more active during new moons. 

New Zealand possum nocturnal behaviour:
a dramatic re-creation of the study findings.
Picture from Jeff Carter (modified)

In fact, Jessica found the opposite: that possum activity was lessened during new moons in forested areas. Possum activity was also still high in open scrub-land even during full moons.

Possums are perfectly capable of clambering around tree tops in the dark, so possum foraging strategies must have changed since their introduction into New Zealand. Jessica's results suggest that New Zealand possums don't need to be lunar phobic to avoid predators. Possums don't have many predators in New Zealand, but young possums are sometimes preyed on by feral cats. The feral cat is an ambush predator. It is likely that a preference for high visibility prevents a possum from being attacked out of the black by feline foe.

Caught on camera trap: possums 

interacting with wax tags, which

are used to monitor possum density.

Photo from Parisi, 2011

Brushtailed possums make regular appearances on this blog for one big reason: they are a notorious pest in New Zealand. As opportunistic omnivores, they cause significant tree defoliation and pose a threat to large endemic invertebrates and native chicks. Their widespread distribution also facilitates the spread of bovine tuberculosis. Hence, a great deal of research goes into finding the most effective ways to control possums in New Zealand. Pest management policy mandates that possum density is kept below a certain threshold and, as such, continual monitoring of possum numbers is required to show how effective ongoing control methods are and when further control needs to be undertaken. 

This research has important implications for the timing of possum control. Poisoning and trapping during times of greatest activity will conceivably increase possum encounter rates with control measures. Control operations may therefore achieve threshold values faster if populations are targeted during peak lunar phases. Similarly, monitoring using indirect measures should achieve greater precision if standardised to particular times of the month. Focusing control on open areas may also be particularly beneficial regardless of moon phase. 

More than anything, the study highlights the importance of behavioural studies for applied ecology. "Unless we fully understand pest animal behaviour, how can we effectively manage them?" asks James Ross, the supervisor of the study. Spoiler alert: we can’t. Behavioural ecology is essential for informing conservation and pest management, and helps us sort fact from fiction. As such, it is a valuable scientific pursuit. To believe otherwise is lunacy.

"AWOOOooo!" 
Picture by DCW

For more works from Lincoln University on the effect of the moon on behaviour, including the one discussed here, see:

Lennon, J. S. (1998). The Effect of Moonlight Intensity and Moon Phase on Feeding Patterns of Common Brushtail Possums. (Master's thesis, Lincoln University, 1998.) Retrieved from http://hdl.handle.net/10182/3062
 
Parisi, J. D. (2011). The Influence of Lunar Phase on Indirect Indices of Activity for the Common Brushtailed Possum (Trichosurus vulpecula) on Banks Peninsula, New Zealand. (Unpublished master's thesis). Lincoln University, Lincoln, New Zealand.