Ever since tamarins were first captured from the wild to serve as research models in laboratories, we have been curious about their use of odour for communication. These miniature monkeys are superlative olfactory communicators – covering every inch of their enclosures with secretions from scentglands, drawing out each application into an elaborate affair, and letting no scent go un-sniffed.
At the time, we could only guess at what they were saying to each other, but given the effort expended on marking, it must have been important. As we moved toward studying them in the wild however, it soon became evident that there was a more chilling consequence to their penchant for leaving scented trails across their sylvan homes.
They could be tracked.
In the Amazon rainforest, where their territories are large and it rains frequently and hard, wild tamarins spend long hours leaving complicated messages to each other. But they share these forests with numerous predators, including several small felines and a smorgasbord of snakes. Leaving a trail of evidence behind you wherever you roam might not be a survival-friendly move. Like Hansel and Gretel, these little primates walk a delicate line between marking their territories and giving away their nightly sleep trees, where they are most vulnerable.
As the sun sets each evening, they adopt a cloak-and-dagger sequence to rival Houdini. Without warning, they sprint through the forest in conspicuous silence, frequently doubling back on their own trail. Below them, observers such as me are running frazzled circles through the toughest spots in the jungle, only just keeping up. Despite watching them turn in every day for ten months straight, that particular moment at which they begin their night-time routine still takes me by surprise.
One minute they’re lounging about, chattering softly to each other …
…and then suddenly they are stealth ninjas…
When they finally quit running and near their final sleep tree for the night, they sometimes post a lookout. The lookout moves away from the spot, creating a distraction to anyone who might be watching, while others leap into a hole in the chosen tree. Then, with every deceptive ploy it can muster, the last tamarin sneaks into the sleep tree and disappears from sight.
They rarely, if ever, use the same sleep tree twice. Someone could be watching. But worse still, someone could be tracking their scent!
In Sydney Harbour National Park in Australia, a group of scientists set out to document how attractive scent really is to predators. They picked the local assemblage of insectivorous bats as the model for their study, since bat guano is noticeably awful smelling. They began the investigation during the bat breeding season, during which several hundred animals gather at roosts each night.
After capturing several bats to obtain guano from them, these scientists determined that a bat produces about a tenth of a gram or 0.1 g of faecal matter in a day. Armed with several hundred such samples, they then created 90 artificial roosts within the National Park and proceeded to set up an elegant experiment.
They divided these sites into three sections of 30 roosts each. In roosts from section 1, they placed 0.1 g of bat faeces to simulate a single bat. On the second night, they removed the faeces from half of these roosts, to simulate roost switching. Switching is a process by which a bat takes a possible precaution against predators and changes its sleep site.
For another 30 roosts, they repeated the experiment with 1 g of faeces, or the equivalent of 10 bats roosting in a location. Half of those roosts were “switched” as well, on the second night.
The third section of 30 roosts had no faecal matter in them, and served as controls for the experiment.
For 5 days, they recorded all movement at the entrance of every roost via motion-sensitive camera traps. These cameras are awesome, and are now one of the most popular ways to census for wildlife that is hard to find on foot. In this case, they were looking for bat predators.
The scientists thought it quite likely that the black rat (Rattus rattus) would make an appearance, for they are known to predate upon bats, and they were right. But they had sorely underestimated the others that tracked bat perfume.
The study reports that predators visited roosts with faeces in them significantly more often than they did the control roosts. It didn’t matter if the faeces were in there for one night, or several nights, nor did it matter how much of it they placed in a roost – the predators screened them more often than they did the control roosts. If it smelled like even a single bat could be in there, predators were tracking them.
Although black rats were captured on film scouting out roosts, the most common visitor was the vegetarian, leaf-eating, ringtail possum (Pseudocheirus peregrinus). Other nocturnal prowlers incuded several native birds, including the lovely pied currawong (Stepera graculina). Perhaps they are attracted to the roosts as possible sleep sites, or maybe its the insect-life that is feeding on guano that draws them near. It is possible that predation isn’t the primary goal of all who came to the roosts. One thing, however, is certain – bat guano is drawing them there.
In fact, putting just a tenth of a gram of bat faeces into a roost made predator visits shoot up by 30% after the first day. After day 2, this rise was 44% more visits.
But what about large gatherings of bats? Intriguingly, predators visited roosts with 1 g of guano less often than they did those with only a tenth the amount of faecal matter. This is not really intuitive at all – shouldn’t they be attracted to a larger gathering of prey? The only likely explanation is that larger groups of bats are harder to hunt, and a solitary predator cuts its losses and moves to a different roost. In short, for a bat, it pays to live in a group.
If you are a bat, or a nice-smelling monkey, here are a few things to keep in mind:
a. Never take your group for granted – being alone is a LOT harder than it looks.
b. If life lets you down and you’re alone anyway, refrain from having a bowel movement in bed OR scent-marking anything within 20 m of your sleep tree (just to be safe). Even a tiny accident will imply not just changing the sheets, but entire rooms for safety.
c. Since bats are literally everywhere, you had better hope that conservation efforts today include the preservation of habitats with MANY roosting spots. Besides being better for the bats, it’s also better for the predators – after all, they need the occasional bat to survive, and group-living bats are a whole different ball game.
d. Ring-tailed possums are allegedly leaf-eaters. Until, that is, they start showing an unhealthy interest in bat poop. My advice? Treat as Predator Until Otherwise Proven!
Threlfall C., Law B. & Banks P.B. (2013). Odour cues influence predation risk at artificial bat roosts in urban bushland, Biology Letters, 9 (3) 20121144-20121144. DOI: 10.1098/rsbl.2012.1144