How deep to dig?
In an intriguing paper in this issue of Insectes Sociaux, Römer (2018) and her colleagues ask: How do leaf-cutting ants, when constructing their nest, respond to carbon dioxide levels? Leafcutters in the genus Acromyrmex farm fungi, so for these ants maintaining homeostasis involves regulating conditions for both the ants and their crop of fungi. CO2 is ubiquitous, benign to most animals at atmospheric concentrations, and, potentially lethal at higher concentrations. Photosynthetic organisms require CO2 as the basis for building sugars, and heterotrophs release CO2 into their surroundings as a metabolic product. Unventilated areas, such as nests of social insects, can accumulate levels of CO2 that may become lethal. In response to the buildup of metabolic by-products in nests—CO2, water, and heat—many social insects have elaborate architectural systems to enhance air circulation and regulate concentrations of these by-products as well as ensuring a supply of oxygen to support metabolism (Ocko et al. 2017). Social insect architecture, however, does not result from following a design like an architectural drawing. Instead, each worker makes a decision about its construction work based on information it gathers from its immediate surroundings. The resulting structure is the compounded result of thousands of minute responses. Differences in these simple response patterns among species result in dramatic interspecific variation in nest architectures. The evolution of nest architecture is a fascinating area of study in social insect biology and it is safe to say that we have only the beginning of an understanding of the processes and evolutionary tradeoffs that guide nest construction. CO2 is a particular problem for tunneling animals, and ants are capable of detecting CO2 levels in their environment using sensillae on their antennae. This ability, while not unique to ants among the insects, is well developed in ants, and is an unsurprising feature as they may encounter high CO2 in subterranean nests. Ants could respond to high CO2 levels either by avoiding nest construction where CO2 is elevated or by constructing ventilation turrets that facilitate air exchange. The question of response to CO2 concentrations in leafcutting ants is complicated by their need to ensure not only their own survival, but also the survival of their fungal garden. High levels of CO2 inhibit fungal growth, potentially feeding back to diminished colony productivity for the ants. CO2 levels in soils tend to be higher, by two or three orders of magnitude, than atmospheric concentrations, and increase with depth below the soil surface. It would be reasonable to hypothesize that ants would seek to construct nest chambers at depths that optimize CO2 concentrations. However, there may be significant trade-offs in terms of soil temperature and moisture that cause ants to choose a compromise nest chamber depth that balances CO2, temperature and moisture. Römer et al. (2018) test the excavation behavior of Acromyrmex lundii ants at ranges of atmospheric CO2 concentrations up to 11%. They used two experimental designs; one captured the dependence of digging rate and soil removal rate on CO2 concentration and the other assessed choices of digging areas based on CO2 concentration. This species differs from many other Attines in building its nest chambers close to the soil surface. Digging rate was not affected by CO2 concentration except for inhibition at the highest level, 11%, a level that would not be expected in soil layers where leafcutters excavate their nests. Soil removal, on the other hand, increased with rising CO2 levels up to 7%, and then showed an inhibitory decline at 11%. Digging choices suggested a preference for excavation in soils with CO2 concentrations that favor fungal growth. These results offer an explanation for this species’ nest chambers near the soil surface—that the ants select locations that offer conducive CO2 growth conditions for their brood and fungi. Other ant species place chambers much deeper in the soil, where they encounter higher CO2 levels, but more Insectes Sociaux
Carbon dioxide sensing in the social context: Leaf-cutting ants prefer elevated CO2 levels to tend their brood.
Social insects show temperature and humidity preferences inside their nests to successfully rear brood. In underground nests, ants also encounter rising CO2 concentrations with increasing depth. It is an open question whether they use CO2 as a cue to decide where to place and tend the brood. Leaf-cutting ants do show CO2 preferences for the culturing of their symbiotic fungus. We evaluated their CO2 choices for brood placement in laboratory experiments. Workers of Acromyrmex lundii in the process of relocating brood were offered a binary choice consisting of two interconnected chambers with different CO2 concentrations. Values ranged from atmospheric to high concentrations of 4% CO2. The CO2 preferences shown by workers for themselves and for brood placement were assessed by quantifying the number of workers and relocated brood in each chamber. Ants showed clear CO2 preferences for brood placement. They avoided atmospheric levels, 1% and 4% CO2, and showed a preference for levels of 3%. This is the first report of CO2 preferences for the maintenance of brood in social insects. The observed preferences for brood location were independent of the workers' own CO2 preferences, since they showed no clear-cut pattern. Workers' CO2 preferences for brood maintenance were slightly higher than those reported for fungus culturing, although brood is reared in the same chambers as the fungus in leaf-cutting ant nests. Workers' choices for brood placement in natural nests are likely the result of competing preferences for other environmental factors more crucial for brood survival, aside from those for CO2.
Analogy, Supposition, and Transcendentality in Narrative Argument
Rodden writes, “How do stories persuade us? How do they ‘move’—and move us? The short answer: by analogies.” Rodden’s claim is a natural first view, also held by others. This chapter considers the extent to which this view is true and helpful in understanding how fictional narratives, taken as wholes, may be argumentative, comparing it to the two principal (though not necessarily exclusive) alternatives that have been proposed: understanding fictional narratives as exhibiting the structure of suppositional argument, or the structure of a kind of transcendental argument. Three key aspects of understanding a fictional narrative as an argument from analogy are identified. First, the argument will be relativistic or depend in an essential way upon the circumstances or intentions of the auditor or author. Second, in view of the first aspect, the argument will be loose and subjective, and accordingly less likely to yield knowledge. Third, the argument will not exhibit a distinctive structure applicable only to fictional narratives. I find that the third, and sometimes the first and second, of these same three aspects apply to understanding fictional narratives as suppositional arguments. I present considerations that point to a way of establishing that some extended fictions exhibit the structure of a kind of transcendental argument that is neither relativistic nor subjective, is knowledge-generating, and is uniquely applicable to fictional narratives. This supports literary cognitivism—the thesis that “literary fiction can be a source of knowledge in a way that depends crucially on its being fictional.”