Soil temperature, digging behaviour, and the adaptive value of nest depth in South American species of Acromyrmex leaf-cutting ants

Abstract

In leaf-cutting ants, workers are expected to excavate the nest at a soil depth that provides suitable temperatures, since the symbiotic fungus cultivated inside nest chambers is highly dependent on temperature for proper growth. We hypothesize that the different nesting habits observed in Acromyrmex leaf-cutting ants in the South American continent, i.e. superficial and subterranean nests, depend on the occurrence, across the soil profile, of the temperature range preferred by workers for digging. To test this hypothesis, we first explored whether the nesting habits in the genus Acromyrmex are correlated with the prevailing soil temperature regimes at the reported nest locations. Second, we experimentally investigated whether Acromyrmex workers engaged in digging use soil temperature as a cue to decide where to excavate the nest. A bibliographic survey of nesting habits of 21 South American Acromyrmex species indicated that nesting habits are correlated with the soil temperature regimes: the warmer the soil at the nesting site, the higher the number of species inhabiting subterranean nests, as compared to superficial nests. For those species showing nesting plasticity, subterranean nests occurred in hot soils, and superficial nests in cold ones. Experimental results indicated that Acromyrmex lundi workers use soil temperature as an orientation cue to decide where to start digging, and respond to rising and falling soil temperatures by moving to alternative digging places, or by stopping digging, respectively. The soil temperature range preferred for digging, between 20 degrees C and maximally 30.6 degrees C, matched the range at which colony growth would be maximized. It is suggested that temperature-sensitive digging guides digging workers towards their preferred range of soil temperature. Workers' thermopreferences lead to a concentration of digging activity at the soil layers where the preferred range occurs, and therefore, to the construction of superficial nests in cold soils, and subterranean ones in hot soils. The adaptive value of the temperature-related nesting habits, and the temperature-sensitive digging, is further discussed.

Leaf-Cutter Ant Parasitoids: Current Knowledge

“…For Acromyrmex, climatic conditions can explain aspects of the mentioned differences regarding the presence/absence of a mound [18] and dump location either interspecifically (Farji Brener, pers. com.)…”

Evolution of cold-tolerant fungal symbionts permits winter fungiculture by leafcutter ants at the northern frontier of a tropical ant–fungus symbiosis

“…Growth rates of tropical Attamyces fungi are optimized around 25°C; growth stagnates below 20°C, and prolonged exposure to temperatures below 10°C can be lethal to tropical Attamyces (12)(13)(14). Tropical leafcutter ants therefore construct fungal chambers that maintain gardens within a temperature window of 20 to 30°C, typical temperatures for tropical rainforest soils (15)(16)(17)(18)(19). Moreover, leafcutter ants possess an antennal sensitivity to temperature gradients that is far more acute than in any other insect (20), and this sensitivity enables leafcutter workers to rapidly assess temperature gradients.…”

“…Leafcutter ants can protect their gardens against some environmental fluctuations (18,19), but the northernmost A. texana populations experience harsh winters in which soils can freeze for prolonged times to significant depths (Table S1). A. texana ants can prevent garden desiccation by foraging for groundwater in their nests' deep tunnel systems [reaching as deep as 32 m (27)] and by supplying gardens with moisture during fungiculture [e.g., manuring of gardens with droplets of liquid feces (11,28,29)].…”

First-year nest growth in the leaf-cutting ants Atta bisphaerica and Atta sexdens rubropilosa

“…In this perspective, we can hypothesize that nest exposed in grassland should have deeper fungus chamber than nest under shade of trees or inside the woods, given that soil temperature is negatively correlated with soil depth (Rosenberg et al, 1983). For leaf cutting ants, is indicated that soil moisture and temperature acts together: (i) Bollazzi et al (2008) verified that workers' thermopreferences lead to the construction of superficial nests in cold soils, and subterranean ones in hot soils; and (ii) Pielström & Roces (2014) verified that soil moisture also varies according to soil depth, and demonstrably affects the digging behavior of leaf cutting ants.…”