In a new paper published Thursday in the journal Science, researchers suggest that the effects of climate change are causing some bumble bees’ tongues — yes, their tongues — to shrink. In fact, they found that tongues on two alpine species of bumble bees in the Rocky Mountains have shrunk by nearly 25 percent in approximately 40 years. And it might sound bizarre, but this tongue-shrinking could actually have big implications for both the bees and the flowers they pollinate.
Tongue size is important in bees because it controls which flowers they can visit for nectar. Bees with longer tongues are able to collect nectar from flowers with longer corollas (that’s the tube shape a flower’s petals form, protecting the tasty nectar inside).
Bees with more medium-length tongues tend to pollinate many different species of flowers. But bees with long tongues are often considered specialists, meaning they only pollinate flowers with deep corolla tubes — and this can be a beneficial arrangement for both the bee and the plant. The bee gets to collect nectar from flowers that insects with shorter tongues can’t access, meaning it has less competition for food. And the flower is pollinated by bees that are only visiting other flowers of the same type, meaning there’s a better chance its pollen is getting transferred to the correct species.
This tactic works best when food is abundant. But the researchers on this study found that rising temperatures are causing flowers (of all sizes) to decline in the mountains, putting more stress on the bees when it comes to finding food.
With fewer flowers to choose from overall, it makes less sense to be so specialized, says Nicole Miller-Struttmann, lead author of the new study and an assistant professor of biological sciences at SUNY College at Old Westbury. She and her colleagues believe the bees’ tongues are shrinking to allow them to be more generalized when it comes to the flowers they visit, giving them a wider range of food sources to choose from.
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.
The researchers conducted their study by measuring bumble bee tongues from two different time periods: bee specimens collected between 1966 and 1980, taken from museum collections, and bees collected between 2012 and 2014. They focused on two different bumble bee species, Bombus balteatus and Bombus sylvicola, which are among the most common species at high elevations in the Rocky Mountains. They found that tongue length, between both species, has declined by 24.4 percent in all, or about 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.”
The researchers had several ideas about what might be causing the tongues to shorten — the effects of climate change on floral density was one of their theories, but not the only one. They also investigated whether a reduction in the bees’ overall body size could have caused the tongues to shorten, as well as whether an increase in short-tubed flowers or an increase in food competition from other organisms might have caused the bees to evolve.
But the strongest evidence pointed to 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.
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.”
In this way, the tongue evolution could be a boon for bees, helping them adapt to their changing landscape. But the tongue-shrinking could have negative effects for the long-tubed flowers these bees 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.
And Richardson added that similar studies should be conducted in other locations to see if the trend holds up. While the species sampled in this study, which live at high elevations and may be fairly isolated, showed a strong reaction to environmental pressure, “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.”
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