Science can be creative and elegant.
To illustrate this fact, I want to bring to your attention a groundbreaking review paper that was recently published in Myrmecological News, written by Michele Lanan of the University of Arizona.
Usually the terms “groundbreaking” and “review paper” aren’t paired with one another. Review papers usually codify existing ideas, propose some new ones that may fall flat. And, if you chat with an editor, you’ll learn that good reviews really improve a journal’s impact factor.
Then there’s this amazing review I loved so much I had to write this post about it. Even if you don’t know a thing about ants, I’m betting you’ll love how the paper draws a clear and simple explanation from complex interacting phenomena.
Ant people are asked about foraging behavior quite often. How and why do ants make trails? Why do some species make trails and others don’t? Until now, our answers were vaguely correct but relied heavily on generalizations. Now, after Michele Lanan scoured pretty much every paper that’s ever collected data on foraging behavior and ecology, we have a quantitative and robust explanation that is powerfully simple and elegant.
We’ve known that foraging behaviors are structured by that ways in which food is available. Among all ants, there’s a huge variety of foraging patterns. Some are opportunistic hunter-gatherers, others are nomadic raiders, and some use trunk trails, as in the figure below. These patterns reflect differences in food availability.
Figure from M. Lanan 2014, Myrm News 20:50-73.
How, exactly, is it that the properties of food availability can predict how ants forage? In an analytically robust and predictable manner, that works for all ants throughout the phylogeny? It doesn’t require an n-dimensional hyperspace to understand foraging patterns of ants. It only needs a 4-dimensional space.
Figure from M. Lanan 2014, Myrm News 20:50-73.
Lanan took into account four properties of food items: size, spatial distribution, frequency of occurrence, and depletability. She arranged these variables along four axes (as on the right), and showed how this this 4-dimenstional space foraging patterns in the figure above.
How do these foraging patterns distribute across the major ant subfamilies? Are some lineages more variable than others, and what might account for these differences? What other beautiful figures and photographs are in the review that illustrate the relationship between spatiovariability of food and foraging biology? As they say on Reading Rainbow, you’ll have to read the review to find out!
As a disclaimer, I should mention that the author of this paper is a collaborator and friend of mine. And she is leading The Ants of the Southwest short course this summer which I’m also teaching — and spaces are still available!
But that’s not why I’m featuring this paper. I am enthusiastic about this paper because it so obviously resulted from a labor of love for the ants, and is a culmination of years of reflection. This is just a downright gorgeous piece of science, and the more people that see it — and the more recognition that the author gets — the better.
I’m a bit surprised she left out the detailed work of Tschinkel (Tschinkel, W. R. 2010. The organization of foraging in the fire ant, Solenopsis invicta. J. Insect Sci. 10:26) and in doing so also left out a strategy: those species that maintain ‘absolute territories’ as in the manner of fire ants. There are other species that likely do this and their foraging strategies would not seem to fit neatly into any of the recruitment or foraging strategy categories she describes. Although there is certainly some overlap with a number of the ‘modular units’ of foraging in Fig. 1. Otherwise, awesome review, though!
I saw in the supplementary data that she listed S. invicta as having both long-term subterranean trails and short-term trails. This fits my observations, which are not nearly as comprehensive as yours.
I was thinking of ‘foraging’ as ‘spatiotemporal mode of food collection’ as something distinct from ‘territoriality’ as ‘exclusion of competitors from space near the nest.’ Clearly, territoriality is more likely to be paired with some foraging strategies more than others. However, the phenomenon of using behavioral dominance to exclude competitors from a certain space is distinct from the spatial and communication networks used to acquire food across space.
I disagree in the case of highly territorial species. Foraging and territoriality in species like the fire ant are intertwined and a product of worker behaviors. I suggest a read of Tschinkel would make that clearer, especially about the role of ‘scouts’ as the ‘fingertips’ (my word) of the colony. It also suggests that ‘territoriality’ as we think about it in ants, is probably very closely aligned to ‘foraging strategy.’ Anyhoo, just some thoughts on the subject.
That’s an excellent paper that should have made it into the final version of my review, and in fact appeared in a previous draft of the supplementary info. If you are curious about the reason (my excuse, if you will), it’s this: I have a much, much larger and more detailed data set with notes from 1250 papers that needed to be greatly shortened to fit in the published supplementary info. For ant species that had multiple references reporting diet or foraging type, I generally went with the earlier reference for the shortened table and removed extra later references. In this case Tschinkel (2010) should have been kept because it contains so much more detail, and I just didn’t catch the mistake in the second-to-last draft. My first response to your comment was thinking “I love that paper! how did it get cut out?” Then I had to go and check to make sure it wasn’t there.
Because I wanted to keep the number of total strategies low, I would classify this example as a combination of units. No organizational scheme will perfectly fit the variation we see in nature, but it’s a good starting point. The behavior Tschinkel (2010) describes seems somewhat similar to that of Pachycondyla sennaarensis, described by Dejean and Lachaud (1994), which also use underground tunnels to deliver foragers throughout the foraging territory (discussed in the section on trunk trails and columns). Tschinkel (2010) could have been nicely discussed there as well.
Territory is also an important part of foraging strategy, which I chose not to spend much time on in this review for the sake of brevity. I refer to Hölldobler and Lumsden (1980), who very nicely divided territorial strategies among “absolute” territories such as those Tschinkel describes in S. invicta and “spatiotemporal” territories such as those of Myrmecocystus. There are extra levels of complexity in thinking about territorial strategies because ants also vary in whether they only defend against conspecifics or against other species. Many authors have also used the term “territory” to refer to the colony foraging area, without knowing whether the ants defend it at all. To adequately treat territorial behavior I would also want to consider things like worker spatial fidelity (or lack thereof), habitat complexity (is absolute territoriality more likely to occur in complex 3d habitats such as forest canopies or simple habitats like barren ground?), whether extremely large polydomous colonies or supercolonies can be considered to have territories in the same way, the role of structures such as outstations and tunnels in territory defense, etc. I ultimately decided that there was so much to think about with territorial behavior that it would be too much to cram into one review, so I just focused on the foraging behavior within the territories. The use of territories would be a prime topic for a future review, which is an idea I’ve been batting around.
Michele, I hope you didn’t find my thoughts to be critical – indeed, I think your review is brilliant! But I was “thinking out loud” about the absolute territoriality subject and, specifically, the example of the fire ant because I’m not sure that it is possible to delineate “foraging” from “territory patrolling,” which in of itself opens a pandora’s box of interesting thoughts about how the behavior of individuals outside the nest are stereotypical under a number of situations. In any case, I found your review to be very interesting. Your writing is excellent. And I completely understand the need to constrain the topic of the review. Keep up the great work!
Don’t worry, I thought it was a good point and thus worth a good response! There are clearly many unanswered questions regarding territoriality and foraging behavior, and plenty of food for thought.