Monday, May 5, 2014

Safety in numbers: bees use social cues to avoid predatory threats

There is safety in numbers.  Anyone who has ever had to walk home solo at night knows how vulnerable we can be when alone.  Animals find safety in numbers too: animals often associate into herds to reduce their probability of being attacked and to more easily spot predators.  But even though we know that this happens, very little is known about the behavioural experiences that contribute to the aggregation of individuals in response to predation.

Pollinators, like bees, also have predators, and have also evolved ways to avoid predation.  Bees are threatened by ambush predators, like the crab spider (Family Thomisidae), which does not build webs, but wait on flowers to, you guessed it, ambush their bee prey.  Some species of crab spiders are able to change their coloring, to camouflage themselves on flowers (below, left).  Others sit on leaves, but look like inconspicuous bird droppings, tricking their prey into a false sense of security (below, right).  But most crab spider ambushes are unsuccessful, and so bees are able to learn from their own experiences to avoid risky-looking flowers.  But avoiding sketchy flowers also has the potential to reduce foraging efficiency, so why would this be an adaptive trait?

File:Crab Spider Thomisus Female 5741.jpg   File:Phyrnarachne sp.jpg        

Well, a new study examined this phenomenon.  Bees use social cues to communicate between individuals in a colony.  For example, honeybees use a “waggle dance” to share information about the direction and distance to areas with lots of pollen and nectar, water sources, or new housing locations.  As it turns out, bumblebees also use social cues, like the “lessons learned” of their bee friends, to identify those flowers that are safe and those that are not.

Erika Dawson and Lars Chittka set up three bumblebee colonies in an artificial meadow where pollen sources were held in false flowers whose colors could be interchanged from white to yellow.  Bees were encouraged to forage freely in order to familiarize themselves with the area, prior to being trained to recognize flower color with either a reward or a predation risk.  One half of the bees were exposed to yellow flowers as the “safe” flowers, while the other half had white flowers as their “safe” flowers, to control for color preference.  The “dangerous” flowers were equipped with foam-coated pincers that could rapidly close to trap, but not harm, bees that landed there.  No other cues were provided other than flower color. 

Following training, the authors exposed the bees to only one flower color.  What they found was that the bees were far more likely to choose a “dangerous” flower to gather pollen if that flower was also occupied by other bees.  Those exposed to their identified “safe” flowers were equally likely to choose a free or occupied flower.  The authors conclude that because bees ignore the “safety in numbers” rule in the absence of a threat, and ignore their own personal experiences in the presence of a threat, that they are actively deciding when to use social information to avoid predation.  

I find this study really interesting, because I love to think about the complex lives of other organisms.  The only thing that stands out here, is that the researchers report really small sample sizes (n = 14 for each of the three treatment groups in the “buddy system” part of the experiment).  Now, I’m not a behavioral ecologist, so I’m not sure if this actually is an appropriate sample size or not, but I would be wary of generalizing these results.  But when taken in the context of learning and social interaction among bees, this study definitely strengthens the body of evidence available to us on the secret lives of bees.  It also shows that bee behavior is not a hard-wired set of rules, but can be adapted based on social cues and habitat conditions.

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