Activity patterns and thermic tolerance of two sympatric Acromyrmex species in lower delta of Parana river, Argentina

JIMENEZ, N; SCHILMAN, P.E.; CALCATERRA, L

Curitiba

Simposio; XXIII Simpósio de Mirmecologia; 2017

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 leaf cutter ants (Acromyrmex lundii and A. ambiguus). These 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 aim 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 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 3minutes, 1 meter from nest main entrance. Also, we collected the leave fragments carried by ants to estimate cut biomass, as dry weight. Maximum and minimum critical temperatures (CTmax and CTmin) were measured in 1 colony of each species in 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 allday, (6.6 grams/day), whereas A. ambiguus foraged only from 10:30am to 10:30pm, with a half of worker flowing and cuttingbiomass 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, CTmax and CTmin was mass ant dependent and inboth cases shown significant differences for each species. The thermotolerance range CTmax-CTmin 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.