A new paper published Thursday in Science looks at how climate change is (out of all things) making the tongue of some bumble bees shrink. Two species of alpine bumbles in the Rocky Mountains already show a decrease in tongue volume of nearly 25 percent in the last 40 years; and smaller tongues could spell big trouble for the flowers that rely on bumble bees for pollination.
Well you see, if you’re a bumble bee, size matters — more to the point, tongue size matters because it dictates which flowers the insects can harvest for nectar. This sweet liquid pools in the corolla, the long tube-y shape petals form at the base of the flower, and the taller the corolla, the longer the tongue needed to scoop out the tasty treat.
While bees with medium-length tongues tend to pollinate many different species of flowers, the ones boasting a longer tongue tend to feed off of only a few species, specializing in flowers with deep corolla tubes. This benefits the plants and the bees themselves, as the hungry insects get exclusive access to the nectar and the flower enjoys selective pollination — visiting only flowers with long corollas, the bees carry only a few types of pollen, increasing the chance that it will be transferred to the right species.
This tactic works really well when food is abundant, but the researchers on this study found that rising temperatures are causing flowers (of all sizes) to decline in mountainous regions, putting more stress on the bees when it comes to finding food.
When there are fewer flowers to choose from, specialization becomes a curse rather than a blessing, says Nicole Miller-Struttmann, lead author of the new study and an assistant professor of biological sciences at SUNY College at Old Westbury. They believe that shrinking tongues can be explained by an evolutionary attempt to make the species more generalized, giving them a wider range of food sources to visit.
“[The study is] a beautiful piece of work that shows the first incidence of climate affecting an important functional trait in the bees,” said Sydney Cameron, a professor of entomology at the University of Illinois at Urbana-Champaign, who was not involved with the study.
Shorter and more versatile
The study involved measuring Bombus balteatus and Bombus sylvicola (two species of bumble bees) tongues from two different spans of time: samples collected between 1966-1980 taken from a museum collection were compared to more recent ones, from 2012-2014. Balteatus and sylvicola were chosen as they are the most commonly encountered species at high elevations in the Rocky Mountains, and the results were quite shocking: tongue length of both species has declined by 24.4 percent overall, for an average of 0.61 percent each year.
Cameron believes it would be a good idea to conduct the same study again in five years, just to be sure that the tongue-shrinking is a long-term trend and not just “short-term cycling.” But if the trend holds true, it represents an instance of surprisingly rapid evolution in the bees.
“It’s a very short period of time to have seen such a strong shift,” said Leif Richardson, a postdoctoral research fellow at the University of Vermont’s Gund Institute for Ecological Economics, who was not involved with the study.
“It suggests that these bees may have an especially low effective population size and that they could have been through an evolutionary bottleneck, allowing very rapid change in these traits,” he added.
Climate change was not the first cause of this shortening the team thought of — they investigated whether a reduction in overall body size (which would mean the muscle shortened, but remained proportional with the bumble bee) occurred, if there was an increase in short-tubed flowers that would allow the species to harvest more food sporting shorter tongues, they even looked for an increase in food competition from other organisms might have caused the bees to evolve. Their theories didn’t pan out, and the team was puzzled.
Then they looked at the effects of climate change on floral resources in the area. According to the researchers, minimum summer temperatures in the mountains they sampled have increased by about 2 degrees Celsius since 1960. That means it’s become more common for temperatures to get warm enough to cause flowers — of all sizes — to decline. And, in fact, the researchers found that total food resources for bumble bees in the region have fallen by about 60 percent since the 1970s. It fit, and it explained the shortening — it was an evolutionary response, allowing the bees to make the most out of dwindling food supplies.
So the tongue-shrinking seems to be an adaptation that allows the bees to better cope with dwindling food supplies.
“When resources are low, it’s more advantageous to go to lots of different flowers because there’s more resources that way,” Miller-Struttmann said. “And it takes less energy to get to them because you don’t have to search them out as much.”
But versatility is the death of specialization
Great for the bees, but terrible news if you’re a flower — the shrinking could have catastrophic effects on the long-tubed flowers that the bumbles used to pollinate.
“It is possible that, at the same time, plant species that depend on these bees are receiving less effective pollination service,” Richardson said.
It’s possible that the plants could also adapt to the bees’ new behavior, perhaps by evolving shorter corollas, said Miller-Struttmann. But for now, it’s still unclear how the flowers will be affected.
“It will be really interesting to use some models to see how sensitive some of these species we see are to changes in bumble bee behavior,” she said.
Richardson said that similar studies should be conducted in other locations to see if the trend holds up. The areas they sampled are fairly isolated and thus show a strong reaction to environmental pressure, but the trend should be visible to one degree or another in other areas affected by climate change.
“Bumble bee species that live in lower mountainside habitats and have larger populations might be buffered from these very strong selective pressures,” he said.
So, while the researchers have uncovered an intriguing trend in one instance, work remains to be done.
“We documented something that has happened, but we’re not exactly sure what’s going to happen going forward,” Miller Struttmann said. “That’s true from both the plants’ and the bees’ perspective.”