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.
Endodontic regeneration: hard shell, soft core
A loss of organs or the destruction of tissue leaves wounds to which organisms and living things react differently. Their response depends on the extent of damage, the functional impairment and the biological potential of the organism. Some can completely regenerate lost body parts or tissues, whereas others react by forming scars in the sense of a tissue repair. Overall, the regenerative capacities of the human body are limited and only a few tissues are fully restored when injured. Dental tissues may suffer severe damage due to various influences such as caries or trauma; however, dental care aims at preserving unharmed structures and, thus, the functionality of the teeth. The dentin–pulp complex, a vital compound tissue that is enclosed by enamel, holds many important functions and is particularly worth protecting. It reacts physiologically to deleterious impacts with an interplay of regenerative and reparative processes to ensure its functionality and facilitate healing. While there were initially no biological treatment options available for the irreversible destruction of dentin or pulp, many promising approaches for endodontic regeneration based on the principles of tissue engineering have been developed in recent years. This review describes the regenerative and reparative processes of the dentin–pulp complex as well as the morphological criteria of possible healing results. Furthermore, it summarizes the current knowledge on tissue engineering of dentin and pulp, and potential future developments in this thriving field.