Digging effort in leaf-cutting ant queens (Atta sexdens rubropilosa) and its effects on survival and colony growth during the claustral phase

Abstract

Nest foundation in the leaf-cutting ant Atta sexdens is claustral, and the single queen completely relies on its body reserves throughout, approximately, 9 weeks until the first workers emerge and initiate foraging. Nest digging is much time-and energy-consuming, and it is an open question how queens decide on the length of the tunnel they dig and therefore the depth of the initial chamber. Shallow founding nests may be energetically cheaper to dig, but queens may be more exposed to changing environmental variables. Deeper nests, on the other hand, may be climatically more stable and suitable, but more expensive to dig. We hypothesized that the maximal nest depth excavated by Atta founding queens may represent the outcome of an evolutionary trade-off between maximizing nest depth and minimizing energy expenditure during digging, so as to save energy for the long claustral phase. We tested this hypothesis by comparing the fitness consequences of increased digging effort in queens that were experimentally stimulated to excavate a complete founding nest either once, twice or three times consecutively compared to control queens that did not dig. Fitness was quantified as mortality rates, rates of egg-laying and offspring production, and size of the fungus garden until the emergence of the first workers. Results showed that, in contrast with the initial expectations, fungus growth, egg-laying rates and offspring production were not affected by the increased digging effort in the experimentally induced successive excavations. However, a significant higher mortality was observed in queens with increased digging effort, i.e., those that dug two or three nests consecutively. It is argued that in queens a behavioral mechanism for the control of nest depth has evolutionary been selected for as a trade-off between maximizing nest depth, to favor protection of the queen against unsuitable environmental variables, and minimizing energy expenditure during digging, which significantly affects survival.

The determination of nest depth in founding queens of leaf-cutting ants (Atta vollenweideri): idiothetic and temporal control

“…Queens precluded for several days to initiate digging showed a slower digging rate, probably as a result of partial exhaustion or desiccation, and excavated tunnels of reduced length, yet they invested a similar time, roughly 20h, until switching to chamber digging. Analogous results were recently obtained in A. sexdens in another context: queens that successively excavated two or three founding nests dug significantly shorter tunnels than those of their first nest, yet they spent a similar time digging (Camargo et al, 2011).…”

“…A hypothetical control system that would trigger the excavation of a chamber only after a preset tunnel length is reached, i.e. lacking temporal control, would have the disadvantage that in harder soils, longer digging times and higher digging efforts need to be spent: queens would be exposed to predators for longer, and the increasing digging efforts may compromise their survival (Camargo et al, 2011). A control system using both idiothetic and temporal information, as demonstrated in the present study, allows a flexible reaction to different soil conditions so as to preclude excessive time and energy investments.…”

“…Deeper chambers may counteract these effects and offer a more stable microclimate, but they require more time and energy for their construction, with the concomitant extended exposure of the queens to predators while digging (Autuori, 1950). In addition, although it does not compromise the rate of offspring production, a larger digging effort by queens significantly decreases the probability of their survival after nest founding (Camargo et al, 2011).…”

Energy substrate used by workers of leaf-cutting ants during nest excavation

“…Furthermore, Camargo et al (2011) evaluated the excavation effort by the queens of A. sexdens, reporting an impact on the queens' survival when they are experimentally subjected to a greater excavation effort. On the other hand, a single worker of A. sexdens, in a group of 10 workers isolated for 48 hours, carries approximately 0.56 g of soil in 33 trips .…”

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

“…In Atta, the work of nest foundation is performed by a single recently fecundated queen that constructs the first chamber (Autuori, 1942;Ribeiro, 1995;Camargo et al, 2011;Fröhle & Roces, 2012). After the first worker ants initiate foraging, they assume activities within and outside of the nest, including the excavation of new tunnels and chambers (Autuori, 1942;Amante, 1972); so, the nest enlargement is thus the responsibility of adult offspring.…”