April 1, 2010
In Panama, at the Smithsonian Tropical Research Institute’s new neurobiology laboratory, researchers are studying how the brain of the tropical sweat bee Megalopta genalis relates to the behavior of the species’ social queens and solitary queens. The study is helping scientists make large strides in understanding the insects’ social behavior.
After observing the bees during daily activities (gathering food and laying eggs), researchers found an interesting pattern in the brain region that is responsible for learning and memory. In social bee queens, who are responsible for coordinating a social network of bee workers, a larger portion of their brain is dedicated to learning and memory than in solitary queens, who have to do much of the work themselves.
We spoke with Adam Smith, a post-doctoral fellow on the study, to learn more about the species and what makes them tick.
There have been other studies that have looked at brain size among social and non-social animals. Why did you decide to focus on bees, instead of another social species?
Of the four major groups of social insects—termites, bees, wasps, and ants—bees are the only ones with species that can switch between being social and solitary. All ants and all termites are social. There are both social and solitary wasps, but, at least of the species investigated to date, no single wasp species can reproduce solitarily and socially, as the Megalopta genalis bees can.
Also, the neurobiology and development of the honeybee brain is very well studied, and a few other species have been studied to a much lesser extent. Together, these studies suggest that environmental influences, even on adult insects, influence brain development. This led us to suspect that the social environment of the Megalopta genalis might also influence brain development.
What is the difference between social bee queens and solitary queens?
The most important similarity between the two is that they both reproduce—that is, lay eggs. The major difference is that the social queens rarely leave their nest, and rarely forage for pollen and nectar. They only lay eggs. Solitary queens, on the other hand, have to do all the duties of reproduction. They must gather the food (pollen and nectar) for their offspring, as well as develop eggs in their ovaries and lay them in individual nest cells with the provisioned food. Social bee queens leave many of those duties to their workers. The other major difference between the social and solitary queens is that social queens must establish and maintain social dominance over their daughters, who stay in the nest as workers.
From the perspective of brain development, it is important to note that even social nests began as solitary nests: a female builds a nest and lays eggs, then the first generation of daughters either leave the nest to go initiate their own nests, or they stay in their natal nests as subordinate workers. Thus, while social queens rarely forage, they had to, at one point, in order to establish their nest. The dominance relationship associated with social nests, on the other hand, is unique to social queens.
Could you explain the social brain hypothesis, which you explored in this study?
The social brain hypothesis proposes that the complexities of social life—keeping track of dominance hierarchies, family relationships, individual identity—are so cognitively demanding that they require increased intelligence above and beyond what animals would otherwise need for the rest of their lives.
The basic prediction of the social brain hypothesis is that, all other things being equal, social species will be more intelligent than solitary ones. However, there are a few practical problems with this. One is that “intelligence” is not a specific trait that can be measured, so brain size, or the size of specific regions of the brain (such as the cortex in mammals) are usually measured instead. Another problem is that “all other things” are rarely equal between species. Even closely related species differ in a host of other traits. Lastly, it is difficult to quantify “sociality.” For instance, some species may live in large groups, but with little complex interaction between individuals. Other species may live in small groups, but with long-lasting, subtle relationships between individuals. Which of these would be more cognitively demanding? The difficulties inherent in between-species comparisons are what motivated us to use the Megalopta genalis, because the individuals within the species are very similar.
You found that the brain region responsible for learning and memory is bigger in social bee queens. Does that mean the brain itself is bigger, or that it works differently?
The brain region was not larger in absolute terms, nor were the brains themselves larger. What was larger was the ratio of one part of this brain region (the mushroom body neuropil) to another (the Kenyon cell bodies). In previous studies of bee brain development, higher values of this ratio result from increased cognitive challenges, such as learning new landmark locations around the nest. Thus, our data suggest that, as predicted by the social brain hypothesis, establishing and maintaining dominance over a social subordinate is more cognitively demanding than solitary life.
The last part of your question really hits at the heart of the matter: We don’t know what these differences mean in terms of how the brain works—either for the previous studies, which focused on more traditional learning challenges or our own, which focused on social differences. Future studies looking at the nature of the neural connections, rather than just the differences in brain development, are needed to figure out how the developmental differences lead to functional differences.
How is this information useful? How can it further future bee research?
In terms of future bee research, I hope it motivates more comparative studies. For instance, many bees in the same family as Megalopta are communal, meaning that they live together, but do not have dominance hierarchies. Do they show similar patterns of brain development? And even among the purely solitary species of bees, there are those who forage on just one type of flower, and others who gather a wide variety of pollen. Do the latter show more flexible patterns of brain development, while the former are more “hard wired” to forage?
This study should be useful for researchers interested in brain evolution because it shows that you don’t need to just use primates, with all the logistical, ethical, and scientific difficulties they entail, to study the evolution of social intelligence. Social insects as a group permit a wider range of comparisons than do vertebrates.
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