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
Underground nest building: the effect of CO2 on digging rates, soil transport and choice of a digging site in leaf-cutting ants
By reacting to local environmental stimuli, ant workers excavate a nest that offers suitable climatic conditions. Workers may face increasing CO2 levels while digging across the soil profile, and it is an open question whether these levels are used as cues during nest excavation. Here, we explored the influence of different underground CO2 concentrations on digging behavior in the leaf-cutting ant Acromyrmex lundii. We first quantified digging rates and transport of excavated material as a function of CO2 levels, ranging from atmospheric values to 11% CO2. The mass of both the excavated soil and the pellets transported out of the digging chamber were quantified. CO2 preferences for excavation were investigated in a second experiment, in which workers were presented with a choice of two digging sites that differed in their CO2 levels, ranging from atmospheric to 4% CO2, and the mass of excavated material was quantified. Digging rates were similar for all tested CO2 levels up to 7%, and only significantly lower for 11%. The transport of excavated soil increased with increasing CO2 levels up to 7%, and then decreased at 11%. Workers preferred digging at 1% CO2 and avoided levels of 4%. We suggest that the observed CO2 preferences are likely driven by the requirements of their symbiotic fungus, and could be one of the reasons why colonies of A. lundii excavate superficial nest chambers.
Beeinflussung des Gedächtnisses der Honigbiene durch Narkose, Kühlung und Streß
Summary1.It has been shown by olfactory as well as by visual training that RA (retrograde amnesia) occurs if, shortly after training, the bees are exposed to supercooling, CO2 narcosis, or to a stress situation caused by captivity.2.Loss of memory was revealed in a decrease in positive decisions for the signal the bees were trained for before the treatment. The loss amounts to about 50%.3.Memory was not recovered within 24 hours.4.If a bee is isolated, a recovery of positive decisions is maximal; this decreases as soon as a bee is kept together with more than two other bees.5.The extent of the RA after a 15-minute isolation is nearly the same as that after a 2-hour isolation.6.In the discussion, the reverberation hypothesis of Hebb (1949) (according to which an RA is caused by the temporary standstill of the brain waves after narcosis or supercooling) is questioned.
 The experiments led to the following hypothesis: The mechanism causing an RA is the same as the mechanism causing a retrograde inhibition: The retrograde inhibition is caused by the addition of new information; the RA is caused by the addition of “nonsense information”. It can be assumed that supercooling, CO2 narcosis, and stress produce discharges in the central nervous system which generate the same physiological processes as new information.Zusammenfassung1.Es konnte bei Duft- und optischen Dressuren gezeigt werden, daß eine retrograde Amnesie (RA) eintritt, wenn Bienen kurz nach der Dressur einer Unterkühlung, einer CO2-Narkose oder einem Streß durch Haft ausgesetzt sind.2.Der Gedächtnisverlust zeigt sich in einem Rückgang der positiven Entscheidungen für das vor dem Eingriff dressierte Merkzeichen und beträgt etwa 50%.3.Der Gedächtnisverlust bleibt 24 Stunden erhalten.4.Werden Bienen allein inhaftiert, dann ist der Rückgang der positiven Entscheidungen maximal, er wird geringer, wenn mehrere Stockgenossinnen zusammen eingesperrt werden.5.Das Ausmaß der RAist nach einer 15 min dauernden Haft etwa genau so groß wie nach zweistündiger Haft.6.Die Versuche führten zu folgender Hypothese: Der Mechanismus, der zur RA führt, gleicht dem einer retrograden Hemmung — verursacht durch zusätzliches Einschleusen von „Nonsense Information“.