By Marlene Cimons, National Science Foundation
Virtually all living things, from the smallest organisms to humans, are vulnerable to infectious diseases. Most humans know they have to stay away from other infected people if they don’t want to get sick themselves. But how does nature figure it out?
The social behavior of honeybees may provide some important clues.
“The bee society is a complex society, and they get a lot of diseases naturally,” says Dhruba Naug, associate professor of biology at Colorado State University. “Evolution and natural selection must have played a role in creating a social structure responsive to the threat of diseases. If you can understand the behavioral mechanisms that restrict the spread of disease, including how their social structure and contact network looks like, maybe you can gain an understanding of how such processes work in other complex societies, including humans.”
Honeybees, especially older ones, can discriminate well between different odors. Older bees tend to cluster with other bees that smell like them. This allows these older bees, who forage, increasing the likelihood they will bring back an infectious agent from the outside, to restrict disease among them.
But things change once they become sick.
“Infectious agents often drain the host of nutrients, and hosts need energy to fight these infections, so when a bee is sick, it becomes hungry,” Naug says. “Hunger alters their smell, just like we have keto smell [the bad breath caused by hunger or exercise] when we’re starving. And this makes sick and hungry bees drawn to other sick and hungry bees, while healthy and well-fed bees hang out with other healthy and well-fed bees. And, once again, this might restrict the spread of a disease.”
In other words, because hungry bees are sick bees, they tend to interact with other sick bees that smell like them, and avoid the healthy bees that don’t smell like them. “Maybe that’s nature’s way of preventing disease transmission,” Naug says.
Naug is studying the social structure of bees and its influence on disease transmission under a National Science Foundation Faculty Early Career Development (CAREER) award, which he received in 2009 as part of NSF’s American Recovery and Reinvestment Act. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding his work with about $650,000 over five years.
His findings could prove significant, since bees are critically important to the nation’s agriculture. Bee pollination represents $15 billion in added crop value, particularly in specialty crops, such as almonds and other nuts, berries, fruits and vegetables, according to the U.S. Department of Agriculture. Moreover, about “one mouthful in three” in the human diet directly or indirectly results of honeybee pollination, according to USDA.
“Understanding the influence of social structure on honeybee diseases makes this study timely to address the recent concern about the health of honeybees, a topic of national importance, but also important for extending our still incipient knowledge of biological networks in general,” Naug says.
The health of honeybees has been declining since the 1980s, as a result of new pathogens and pests, as well as well as to the mysterious phenomenon known as Colony Collapse Disorder, in which worker bees suddenly disappear from a beehive or colony, and never return.
“We use the bee colony as a model of complex social structure to see how a disease might be expected to spread in a social group,” Naug says. “I’m not suggesting that a bee society mirrors a human society--there are a lot of differences--but you can get an understanding of the basic processes involved in spreading a disease. Also, you cannot obviously examine these processes from an experimental standpoint in humans.”
Naug and his students infect their bees with Nosema, a fungus that afflicts the gastrointestinal system, causing diarrhea. By infecting bees with a model pathogen, they can observe how it changes the behavior of the bees as a result of the infection.
They have found that the infection creates an “energetic stress,” where “the bees are more hungry,” he says. “They try to find food, and beg more for food, making many more contacts with each other than they normally would do. We also find that once they get food, they do not seem to share it with others.”
To track the movement of food, the researchers set up a feeder with sugar syrup containing a marker, sometimes a colored particle or a radioactive tracer. “They eat the food and take it back to the colony, and start passing it to each other,” Naug says. “You can track and see how the food is moving through the colony. This is the way you can model the spread of the disease.”
They also tag the bees with numbers, and video them. “From that, we start to see who interacts with whom,” he says. “We try to build a contact structure inside the colony to see whether we can predict the transmission pattern from that contact structure. And based on our results from the odor experiments, we try to determine why A interacts with B and not with C. What are the underlying mechanisms by which this contract structure is built?”
In another series of experiments, the researchers also found that energetic stress affects cognitive ability and sensitivity to risk. One group of bees made to “learn” an association between two stimuli died four and one-half hours earlier than a control group, suggesting that “when you are energetically low, you do not learn as well, and you are also more risk-prone, trying to meet the energetic demand on you,” Naug says.
“Learning involves spending energy, and it appears that the energetic spending you do can kill you faster, especially when you don’t have a lot of energy to begin with due to an infection,” he adds.
While Naug does not study Colony Collapse Disorder directly, he wonders whether the behavior associated with energetic stress is related.
“Energetic stress could also affect navigational learning,” he says. “When you have a disease, you will be energetically stressed, and the impaired learning might make it more difficult to find your way back home.”