A blog post highlighting the article written by D.C. Cardoso, M.P. Cristiano, C.B. da Costa-Milanez and  J. Heinze in Insectes Sociaux

Written by Aniek Ivens

Sometimes a chance encounter leads to a new scientific discovery. Let me tell you the story of four biologists in Brazil who were looking for fungus-growing ants and then discovered that these ants’ fungus gardens got stolen by other ants: thief ants. This discovery is more than just a fun fact; below you’ll find how it may contribute to a better understanding of the farming practices of ants, which are cases of mutualism and how these mutualisms persist despite the threat of parasites.

Our historical transition from a hunter-gatherer to a farming-based lifestyle contributed significantly to our success as a species.

We humans weren’t the only beneficiaries when we transitioned to farming. The crops and animals that we keep and farm also benefited from this interaction. For example, in some countries there are at the moment more pigs than humans. It is hard to imagine that such vast numbers of pigs could be maintained in the wild without human assistance⁴. These reciprocally beneficial cooperative relationships between the farmers and the farmed species are called ‘mutualisms.’

Given the mutual benefits to farmers and the species they farm, it is not surprising that we are not the only organisms that practice agriculture or husbandry. Nature provides many examples of such non-human farming: there are ants that farm aphids as we do cattle, damselfish that grow little gardens of algae in the sea, and even amoebae that farm bacteria. Perhaps the most frequently grown crop out there is fungus: we find ‘mushroom growers’ among termites, beetles, sloths, snails, and, of course, ants.

Figure 1: A worker of fungus-growing ant Trachymyrmex intermedius carries a leaf to the nest as substrate for its fungus. Photo: Alex Wild (www.alexanderwild.com)

Fungus-growing ants are sophisticated farmers. They build subterranean nests in which they grow gardens of fungi, for food. To grow the fungus they bring in substrate from outside the nest, often flowers or cut leaves (Fig. 1). They also maintain the garden by applying their excrement as manure and planting new tufts of fungus.

The ants and fungi together form thriving little communities from which they both profit. Unfortunately, their success also puts them at risk: any thriving mutualism will attract parasites that reap the benefits without paying the costs. Yet, many mutualisms persist and how they defend themselves against these parasites is a major question in biology. Studying the interactions between mutualists and their parasites can shed light on this question.

It is no surprise that the ants’ fungus-garden risks parasite invasion. We’ve long known that ants need to actively weed out and even apply pesticide to parasitic fungi, which try to profit from the ants’ care without providing food. In recent years, it has become clear that the fungus-gardens also risk ‘agro-predation’, in which other ants come in and steal the entire garden⁵. This is of course a major loss – imagine you have carefully planted a patch of strawberries and once they are ripe, somebody comes in and steals all of them!

In the study highlighted here, the biologists discovered by chance that this is exactly what happens to Mycetophylax (My) fungus-growing ants. The biologists set out to collect some colonies of My ants from sand dunes near Ilhéus, Brazil. However, in one case, they found that the fungus-garden was inhabited by a different ant, Megalomyrmex incisus (Me) (Fig. 2). No Mycetophylax ants were in sight. Knowing that other Me can be agro-predators⁵, or ‘thief ants’, they hypothesized that this nest indeed had been usurped by the Me ants and brought it to the lab to test this hypothesis.

Figure 2: Thief ant Megalomyrmex incises seen from the front (‘frontal view’, a) and its left side (‘lateral view’, b). Photo: Cardoso et al. 2016

The researchers first confirmed that the found Me ants were parasitic ants, by testing whether these ants were able to rear the fungus garden themselves. As it turns out, the Me ants could not. Although they ate the fungus, they did not provide the fungus with substrate and did not weed it. As a result, the fungus died within three weeks.

Next, the scientists gave the thief Me ants the opportunity to steal a fungus, by providing the colony with a piece of fungus garden including about 20 workers of its original farmers, the My ants. Turns out that raiding a fungus garden is indeed what the Me ants are very good at: within an hour, they had taken possession of the garden and expelled all My ants by employing an arsenal of aggressive weaponry. The thieves bite, sting and pull the My ants (see video below). Presumably in response to the venom the Me ants produce⁶, the My ants mostly play dead – and the thief ants just carry them off their garden.

Why didn’t the My ants, the mutualists, evolve to protect their garden better? The reason is probably the same as the reason why nobody observed this case of agro-predation before: the chance of these Me ants encountering a My colony is just extremely low. This is because Me ants are rare and the habitats of these two different types of ants hardly overlap. Evolution is only able to shape better defenses when an attack happens often enough.

Even though it only happens rarely, this case of agro-predation by Me ants can still be very valuable for science. Combined with other known cases of garden-stealing by Me ants^5,7,8, it will allow us to study the strategies of parasites – and their victims’ defenses against them – in more detail. Ultimately this will contribute to a better understanding of the fragile balance between mutualists and parasites and how best to protect mutualist crops (and maybe even our own) from being stolen by other species.

References

1. Cardoso, D. C., Cristiano, M. P., Costa-Milanez, C. B. da & Heinze, J. Agro-predation by Megalomyrmex ants on Mycetophylax fungus-growing ants. Insectes Sociaux 63, 483–486 (2016).
2. Larsen, C. S. Biological changes in human populations with agriculture. Annu. Rev. Anthropol. 24, 185–213 (1995).
3. Diamond, J. Evolution, consequences and future of plant and animal domestication. Nature 418, 700–707 (2002).
4. Aanen, D. K. As you weed, so shall you reap: on the origin of algaculture in damselfish. BMC Biol. 8, 81 (2010).
5. Adams, R. M. M., Norden, B., Mueller, U. G. & Schultz, T. R. Agro-predation: usurpation of attine fungus gardens by Megalomyrmex ants. Naturwissenschaften 87, 549–554 (2000).
6. Adams, R. M. M., Jones, T. H., Longino, J. T., Weatherford, R. G. & Mueller, U. G. Alkaloid venom weaponry of three Megalomyrmex thief ants and the behavioral response of Cyphomyrmex costatus host ants. J. Chem. Ecol. 41, 373–385 (2015).
7. Adams, R. M. M. et al. Chemically armed mercenary ants protect fungus-farming societies. Proc. Natl. Acad. Sci. 110, 15752–15757 (2013).
8. Adams, R. M. M., Shah, K., Antonov, L. D. & Mueller, U. G. Fitness consequences of nest infiltration by the mutualist-exploiter Megalomyrmex adamsae. Ecol. Entomol. 37, 453–462 (2012).

A blog post highlighting the article written by J. C. Makinson, T. M. Schaerf, A. Rattanawannee, B. P. Oldroyd and M. Beekman in Insectes Sociaux

Written by Rachael Bonoan

Decision making is hard. Decision making in a group is even harder. The vultures from Disney’s The Jungle Book come to mind. What we gonna do? I don’t know, whatcha wanna do? And so it goes.

Honey bees are an example of a superorganism. Not only do they work together to run their large and complex societies, they also work together to decide on a new home.

When honey bees decide it’s getting too cozy in their hive, half of the bees will leave with the old queen and swarm to an intermediate location. The remaining bees will stay home with a newly raised queen.

Rachael Bonoan with a swarm outside a hive entrance. Photo: Salvatore Daddario

While the bees are clustered in their swarm, special members of the colony, aptly named scout bees, check out possible new homes in the area and report back to each other via dancing. In their dances, the scout bees encode the location and quality of each potential new home. Eventually, the scout bees decide on a new home and, after a consensus is reached, the swarm takes off. Before the swarm takes off, it is vital that all the bees agree on where they are going. In European honey bees, we know a lot about this process. Until recently however, we didn’t know how Asian honey bees (Apis dorsata) make this important decision.

Unlike European honey bees, Asian honey bees nest out in the open and their colony’s population size is not constrained by a nest cavity. As such, Asian honey bees tend to swarm to find a home with more food rather than to find a home with more room for all those bees.

Asian honey bees nesting on a tree branch. Photo: Wikimedia Commons

Asian honey bees are much quicker at making decisions about a new home than European honey bees (hours vs. days respectively). How do Asian honey bees make a group decision so quickly? Recently, James C. Makinson and colleagues asked the question, how does group decision-making in Asian honey bees differ from group decision-making in European honey bees?

Swarm board and video camera set up. Photo: Makinson et al. 2014.

To investigate this question, the research team first created Asian honey bee swarms which were released onto a swarm board. Equipped with a video camera, the researchers filmed the scout bees as they searched for new home sites and made their decision. The researchers measured dance and flight activity, and to get an idea of individual behavior, they labeled the scout bees with colored paint.

Like European honey bees, individual Asian honey bee scouts take flight in between dances, and before lift-off, dances converge in a similar direction. Also, in both species, the duration of a scout’s dance is directly related to the quality of the new home site.

Unlike European honey bees however, Asian honey bee scouts do not exhibit a phenomenon called dance decay when narrowing down their choice. In European honey bees, a scout visits a potential new home multiple times and each time, the duration of her dance shortens. Another scout follows the dancer’s directions to check out the site herself. This recruited scout will also visit the site multiple times; she too will shorten the duration of her dance with each visit. Since scouts do longer dances for more favorable homes from the start, scouts dancing for higher quality homes will continue dancing even after dances for lower quality homes have ceased. Eventually, dance decay results in only dances for the most favorable home site. This is when the bees take off.

Asian honey bees use a different means of coming to a consensus. Makinson and colleagues found that scouts dancing for a “non-chosen” location change their dance direction after observing the dance of a “chosen” location. Thus, Asian honey bee scouts switch their dances—or change their minds—without visiting the potential new home themselves. These “switchers” simply trust what the other scout bees are telling them. This is likely how Asian honey bees make their decision so much faster than European honey bees. It also suggests that checking out the site themselves isn’t as important to Asian honey bees as it is to European honey bees. Based on their nesting behavior, this makes sense. Since European honey bees nest in cavities, the bees check out the cavity to make sure it’s the right shape, size, height, etc. Since Asian honey bees nest in the open, they have less factors to debate about when making their decision.

It seems that Asian honey bees are efficient at group decision-making because they pay attention to only the pertinent information. They don’t let irrelevant factors (in their case, shape, size, height, etc. of the home site) get in the way. They stay focused on the specific task at hand: find a new home.

References

Makinson JC, Schaerf TM, Rattanawanne A, Oldroyd BP, Beekeman M. 2016. How does a swarm of the gian Asian honeybee Apis dorsata reach consensus? A study of the invidual behavior of scout bees. Insectes Sociaux 63: 395-406.

Makinson JC, Schaerf TM, Rattanawanne A, Oldroyd BP, Beekeman M. 2014. Consensus building in giant Asian honeybee, Apis dorsata, swarms on the move. Animal Behavior 93: 191-199.

Seeley TD, Visscher KP, Passino KM. 2006. Group decision making in honey bee swarms. American Scientist 94: 220-229.

About the author:
Rachael Bonoan is a PhD student at Tufts University in Medford, Massachusetts, U.S.A. You can tweet to her at @RachaelEBee or check out her website: www.rachaelebonoan.com where she writes her own blog.