A report by Discovery News with a rather straightforward title:
I just hope they can find a little love during the upcoming Valantine’s Day.
Ants may be tiny, but they still have brains. In fact, a recent study by Amador-Vargas et al. has shown that studying these tiny brains can yield interesting discoveries about colony processes and, potentially, the evolution of division of labor in insect societies.
The researchers set out to test two hypotheses, the “task specialization hypothesis” (TSH) and the “social brain hypothesis” (SBH). In the context of ants, the TSH predicts that growth in colony size will favor unequal growth in brain size, with ants in different roles exhibiting growth in different regions of the brain (corresponding to their different tasks). In other words, large colony sizes promote task specialization, which in turn drives cognitive differentiation between individuals. Alternatively, the SBH predicts that due to an overall increased requirement for cognitive function (e.g. nest mate recognition) in larger social environments, total brain volume should increase throughout the colony, regardless of task, as colony size increases. Here, Amador-Vargas et al. particularly focus on mushroom bodies (MB), the “integration centres of the brain”, in the acacia ant Pseudomyrmex spinicola.
The study produced several interesting findings, but the key discovery is that, at least in these ants, an increase in colony size does seem to drive some cognitive differentiation (in accordance with the TSH). That is, larger colonies tended to include ants that more consistently stuck to one task, and also exhibited increased differences in brain sizes between ants dedicated to defense and those tasked with foraging for food.
The exciting thing about this finding is that it may be one step towards understanding how a division of labor evolves in insect societies. Potentially, a growth in average colony size for a species may increase the likelihood that it will evolve castes – that is, workers that are morphologically and (often) functionally distinct. The species studied here contains workers that are “monomorphic” (i.e. without physically distinct castes), as opposed to “polymorphic” (i.e. with physically distinct castes). However, future comparisons of the brains in different species spanning a full range of polymorphism would help to elucidate whether or not the trends observed in this study represent one case of a general phenomenon in which an increase in colony size drives not just behavioral but also cognitive differentiation between individuals. Such cognitive differentiation may provide a link between monomorphism and polymorphism in ant societies.
When I tell friends and acquaintances that I studied the impact of bison on ants, a common first expectation is that the bison are stomping on or eating the tiny gals. This is not really the case (bison actually increase plant diversity, which is expected to indirectly increase ant diversity), but I’ve always thought the image of bison eating ants a rather amusing one:
Grinath came upon another predator-plant connection while studying the partnership between ants and treehoppers on a common plant called rabbitbrush. These tiny sap-sucking insects secrete a sugary liquid the ants eat in return for taking care of the treehoppers. One summer, a bear moved into Grinath’s study site and started digging up the underground ant nests, eating both larvae and adults. So he decided to see what effect the bears had on his study subjects. …
The ants aren’t directly harming the plants, he and colleagues concluded after a series of field experiments. Instead, the presence of the ants scares off predatory insects, in turn enabling treehoppers and other plant-munching insects to thrive and take a serious toll on plant growth. “The ants are providing an enemy-free space for all these herbivores,” Grinath says. Where bears have eaten the ants, predators return and help protect the plants, he and his colleagues reported online ahead of print in Ecology Letters.
Such indirect interaction cascades are part of what makes ecology so interesting, but also rather daunting. Grinath was lucky to have been in the right place at the right time to observe the bears eating ants (the more times I get to write that phrase, the better). Otherwise, who would have proactively investigated the impacts of bears eating ants? Bears eating ants is not something one expects. In fact, even this case of bears eating ants involves a relatively simple system with a limited scope – who knows the full breadth of impacts caused by bears eating ants? Plus, it took four years for the researchers to discover what they did about bears eating ants. Still, it is exciting to know that bears eat ants, and that this consumption is an important factor in the reproductive success of local plants.
Two days ago, a paper was published in Ecosphere that describes a perhaps unexpected relationship between arboreal (tree-dwelling) ants and small detritivorous creatures underfoot. The authors, Clay et al., focus on the Neotropical ant species Azteca trigona. According to the authors, this species “rains refuse out of its hanging nest onto the leaf litter below”. Essentially, A. trigona just throws its poop and trash onto the ground below its nesting tree.
While it may be considered impolite for humans to do such things, the ants’ refuse is enriched with various nutrients (Phosphorous, Potassium, and Nitrogen) that limit decomposition. That is, the detritivores in the soil and leaf litter are limited by a limitation of these nutrients, so that an increase in the abundance of the nutrients would increase the rate of decomposition, and therefore the rate of nutrient cycling, in the ecosystem. Consistent with this fact, Clay et al. found that in the areas below the trees that housed A. trigona nests, the rate of decomposition was increased 1.2-fold (i.e. 20%). So, by throwing to the ground all that junk inside their trunk, ants can actually provide an important ecosystem service.
Clay, N. A., J. Lucas, M. Kaspari, and A. D. Kay. 2013. Manna from heaven: Refuse from an arboreal ant links aboveground and belowground processes in a lowland tropical forest. Ecosphere 4(11):141. http://dx.doi.org/10.1890/ES13-00220.1
A recent paper by Sasaki and Pratt, published in Biology Letters last week, describes an elegant study that expands our understanding of ant decision-making. Their simple experimental design is shown in Figure 1 (taken from the paper):
The focus of their experiment was to determine if, when choosing which site to make a nest, ants shift the weights of attributes they consider as a result of previous experiences. Several past studies focusing on many different species have looked at the impact of weighting choices on decision-making, but this paper is, according to the authors, the first to document potential shifts in weighting multiple variables due to experiences. Specifically, Sasaki and Pratt looked at preferences for entrance size and light availability, because the ant species of interest, Temnothorax rugatulus, is known to prefer nests that have smaller entrance size and lower light availability.
What the researchers found was that when the ants were first exposed to only a standard (ideal) nest and another nest that varied in only one attribute (light or entrance size), the ants later exhibited a significant increase in preference for nest sites that were more desirable in that one attribute, but less desirable for the other. For example, the ants that were first made to choose between a standard nest and a nest with higher light availability consistently chose the standard, more ideal nest. Then, when presented with a choice between a nest that had a small entrance, but more light, and one that had less light, but a larger entrance, the ants chose the nest with more optimal light conditions at the expense of less ideal entrance size conditions. Revealingly, the ants that were first made to choose between a standard nest and one with a larger entrance size later chose the nest with the more optimal size conditions. Therefore, potentially due to the perception of rareness of one nest trait, the ants increased the weight of that trait when seeking out a new nest in the future. The results of this tight experiment seem to me to provide compelling evidence that ants somehow collectively use a decision-making process that incorporates past information to shift the weights of preference in later choices.
Perhaps the most interesting bit about the study is that the ants’ style of decision-making has also been documented in humans. As the authors note, past research has shown the influence of scarcity in social psychology, where people will more strongly seek out something that they perceive to be rare. Although the underlying mechanism used to incorporate scarcity into decisions may be different between ants and humans, it is interesting to consider that our mental processes may mirror those of the small creatures underfoot.
Sasaki, T. and S.C. Pratt. 2013. Ants learn to rely on more informative attributes during decision-making. Biology Letters 9: 20130667.
I am pleased to announce that the first fruits of the collection trip to Xishuangbanna has emerged. Happy Birthday to Bannapone scrobiceps, a new species in the very rare genus Bannapone! Previously, this genus was known from only one dealate (not winged) queen, and our discovery of not only two individuals of the worker caste but of a whole new species is exciting indeed!
Figures 2 and 3 from the manuscript, respectively:
Citation: Guénard, B., B. Blanchard, C. Liu, D.R. Yang, and E. Economo. 2013. Rediscovery of the rare ant genus Bannapone (Hymenoptera: Formicidae: Amblyoponinae) and description of the worker caste. Zootaxa 3734 (3):371-379. Access here (behind subscription barrier).
The long, long, long list of useful ant applications to human life may soon grow even longer. A recent study reported in Scientific American reveals that a certain species of ant, Camponotus sanctus, may be the secret to hibernation at room temperature. The greater mouse tailed bat (Rhinopoma microphyllum) shifts to almost exclusively eating the fatty queens of this ant species when they emerge for the nuptial flight, the mating event between males and queens. This targeted consumption by the bat precedes a five-month hibernation in caves at room temperature, during which they neither eat nor drink. The timing indicates that something about the fat in these ants may enable this relatively novel behavior.
The article ends on this note:
We used to think about hibernation as something related to freezing temperatures, but mouse tailed bats definitely change this perception: hibernation is possible at room temperature and it is probably also related to diet composition. Induction of hibernation in humans is still impossible and would be very important for long journeys into space and to “freeze” people suffering from still incurable diseases. The ability of the mouse tailed bat to hibernate at room temperature makes it a great model organism to understand hibernation and perhaps one day apply it to humans.
In other words, the likely key to this hibernation secret – ants – may make time travel and deep space flight possible. If this comes to fruition, we all know where the first destination would be: The Ant Nebula.