Activity patterns and thermal tolerance of two sympatric Acromyrmex species in lower delta of Paraná River, Argentina

JIMÉNEZ, NADIA; SCHILMAN, P.; CALCATERRA, L. A.

Curitiba, Paraná, Brasil

Simposio; XXIII Simpósio de Mirmecología, An International Ant Meeting; 2017

Universidad Federal de Paraná

The Lower Delta of Paraná River is one of the largest wetlands of Argentina. With its original coverage replaced by willows and poplars plantations, this region is the habitat of two sympatric species of leafcutter ants (Acromyrmex lundii and A. ambiguus). These ant species cut plant leaves to grown up a symbiotic fungus that results the main food source for their larvae. Their local coexistence could be related with their activity patterns, so the main objective of this work is to study (1) their daily and seasonal foraging activity patterns (and cut biomass) in the field and (2) their thermal tolerance limits in laboratory. Foraging activity was measured in each season, in 3 nests of each ant species during 48 hours. Every 3 hours, we recorded temperature and relative humidity and we counted number of laden ants going to the nest for 3 minutes, 1 meter from nest main entrance. Also, we collected the leave fragments carried by ants to estimate the cut biomass, as dry weight. Maximum and minimum critical temperatures (CTmax and CTmin) were measured in one colony of each species in the laboratory using an increase-decrease 0.25 °C/minutes ramp. Both species had similar foraging pattern in winter (7.5 grams/day) and summer (27 grams/day). In autumn, A. lundii foraged during all day, (6.6 grams/day), whereas A. ambiguus foraged only from 10:30am to 10:30pm, with a half of worker flowing and cutting biomass per hour. In spring, A. lundii and A. ambiguus showed a peak of activity at different hours and temperatures. In this season, A. lundii cut more than two times the biomass of A. ambiguus (30 and 13 grams/day, respectively). For both species, ant weight was related with TCmin and TCmax and in both cases shown significant differences . The thermotolerance range TCmax-TCmin of A. lundii (46.0-3.2=42.8ºC) contains the one of A. ambiguus (45.3-4.5=40.8ºC). The lower CTmin and larger thermotolerance range of A. lundii compared to A. ambiguus is not reflected on a wider temporal pattern of foraging activity. Our results show that both species did not differ temporarily in their forage activity due to their critical temperatures. However, they have different types and sites of nesting: A. lundii performs subterranean nests, frequently at the base of trees, in closer habitats, whereas A. ambiguus builds mound shaped nests in opener areas. These differences may help to explain the coexistence of these two species in the Lower Delta of Paraná River. (CONICET, PICT-FONCyT, PIA-BID)