Hugo Torres-Contreras

FOOD SELECTION IN AN HERBIVOROUS RODENT: BALANCING NUTRITION WITH THERMOREGULATION

The degu, Octodon degus (Rodentia, Octodontidae) is a diurnal herbivore inhabiting the semiarid and mediterranean environments of northern and central Chile. In the field, degus are constrained to specific foraging areas, mainly by their limited thermal tolerance and by environmental food quality. Consequently, we hypothesized that degus must balance their diet selection by maximizing nutrients/digestible energy intake, in the face of their time and digestive constraints and seasonal/spatial changes in food quality; and by minimizing thermoregulatory risk, in the face of their low evaporative water loss and seasonal/spatial changes in environmental temperatures among foraging areas. This hypothesis was tested in a series of diet selection experiments conducted in an experimental arena, as well as in nutritional trials, involving synthetic diets with different fiber content and thermal patches. As predicted, results of the arena food selection experiments, as well as the nutritional trials, demonstrated that O. degus prefers items of low-fiber composition. However, food selection is compromised by seasonal and spatial changes in food quality and environmental temperatures, coupled with feeding time and digestive and thermoregulatory constraints.

Participation of book lungs in evaporative water loss in Paraphysa parvula, a migalomorph spider from Chilean Andes

Small animals need efficient water conservation mechanisms for survival and reproduction, which is relevant for the spiders that have large book lungs with large respiratory surface. If lung evaporation is relevant to limit water loss, adjustments of the spiracle opening to metabolic demands should be expected. In this study, we measured the metabolic rate and total evaporative water loss mediated by the opening of the spiracles in the migalomorph spider Paraphysa parvula, a resident of fluctuating Mediterranean environments of the mountains of central Chile. We found that the metabolism of P. parvula was similar to other Theraphosidae and low compared to other arthropods. Carbon dioxide production and evaporative water loss increased with temperature, particularly at 40 degrees C. The total evaporative water loss at 40 degrees C increased dramatically to about 10 times that found with the lower temperatures. Thus, 40 degrees C will be the limit temperature for this species after which evaporative water loss starts to become damaging, so it has to avoid it. The exposition to hypercapnic environments had as a consequence an increase in evaporative water loss and the involvement of the book lungs in this loss was about 60%. The possibility of losing water could condition this species to seek temperate and oxygenated shelters under rocks.

Solitary Foraging in the Ancestral South American Ant, Pogonomyrmex vermiculatus. Is it Due to Constraints in the Production or Perception of Trail Pheromones?

Several North American species of Pogonomyrmex harvester ants exhibit group foraging, whereas South American species are exclusively solitary foragers. The composition of the secretions of the poison and Dufour glands in the South American species, Pogonomyrmex vermiculatus, were analyzed, and the secretions and their components were tested as trail pheromones in laboratory bioassays. The major compounds in the poison gland were the alkylpyrazines, 2,5-dimethylpyrazine, 2,3,5-trimethylpyrazine, and 3-ethyl-2,5-dimethylpyrazine. The Dufour gland contained five alkanes, from tridecane to heptadecane, with pentadecane being most abundant. In behavioral bioassays, poison gland extracts and the mixture of pyrazines produced a trail pheromone effect, whereas the Dufour gland extracts and the alkanes had no effect on ant locomotion. We conclude that group foraging in P. vermiculatus does not arise from the inability to produce or detect possible pheromones, but rather, from physiological and/or ecological factors.

Effects of diet and water supply on energy intake and water loss in a mygalomorph spider in a fluctuating environment of the central Andes

The metabolic and water evaporation strategies in spiders may be part of a set of physiological adaptations to tolerate low or unpredictable food availability, buffering spiders against environmental fluctuations such as those of the high mountains of the central Andes. The aim of this study is to analyze experimentally the variations in metabolic rate and the rate of evaporative water with food and/or water restriction in a high mountain mygalomorph spider population (Paraphysa sp.). We found that the low metabolism of this spider was not affected by water restriction, but its metabolism was depressed after 3 weeks of food deprivation. The spider did not show seasonal metabolic changes but it presented seasonal changes in the rate of evaporative water loss at high temperatures. Females with egg sacs reduced their metabolic rate and evaporative water at high temperatures. These findings constitute a set of possible adaptations to a highly fluctuating Mediterranean environment, which is completely covered with snow for many months and then progresses rapidly to a very dry climate with high temperatures.

Characterization of the thermal micro-environment of Paraphysa parvula Pocock 1903 (Araneae: Theraphosidae), a spider from the Chilean Andes

Abstract We characterize, in both the laboratory and the field, the preferential thermal microenvironments of Paraphysa parvula (Pocock 1903) (Araneae: Theraphosidae), a mygalomorph spider that successfully inhabits the high elevation environments of the Chilean Andes. We studied 116 spiders. Their average body temperature in the field was 31.02 ± 2.74° C, similar to the laboratory preferred temperature of 31.7 ± 2.31° C, and higher than the ideal temperature of reproductive females, 29.34 ± 2.81° C. In non-reproductive spiders, we found significant associations between body temperature and the temperatures of the air, substrate and rocks; however, the strongest association was between body and rock temperatures. Similar results were obtained in reproductive females, but there the best predictor of the body temperature was air temperature in the shelter. In both cases, the air temperature remained below body temperature and well below the temperature of the rocks and stones. Both situations show the importance of behavioral thermoregulation and the mechanisms of heat transfer into the microenvironment in the body temperature regulation of spiders. Conduction from the environment, heat transfer by small convection currents, and radiation from the hot stones constitute small environmental cues that allow these spiders to maintain an optimal temperature. The selection of shelters meeting specific temperature regimes appears to be a key condition for the optimization of female reproductive success and survival of females and juveniles in a high elevation environment.

Effect of Colony, Patch Distance, And Trajectory Sense on Movement Complexity in Foraging Ants

Animals display foraging trajectories when exploiting food patches. These displacements have been studied, particularly in organisms that forage from a central place. The complexity of a movement path may be analyzed by means of the fractal dimension, an index that estimates the tortuosity of a trajectory. In field experiments we studied the effect of trajectory sense, distance to the resource patch and colony on the movement complexity in a common ant of central Chile. We found that these factors and their interactions significantly affected the complexity of movement paths. We discuss whether mortality risk determines more linear trajectories when the resource patch is distant from the nest, and whether the information acquired from the environment by workers determines less complex return trips.

Fasting and chemical signals affect recruitment and foraging efficiency in the harvester ant, Pogonomyrmex vermiculatus

Summary Ants are central-place foragers, their foraging strategies varying from solitary to group foraging. Although the ancestral South American species, Pogonomyrmex vermiculatus, produces alkylpyrazines which attract foragers, this species exhibits solitary foraging behaviour in the field. This restriction was explored by evaluating the effects of fasting and presence of artificially applied trail pheromones on recruitment and foraging of P. vermiculatus. A circular arena connected to the nest at its center was divided into 12 equal sectors with a screen near the peripheral end of each sector. Prey was offered behind the screen in one of the sectors to ants exposed to different periods of fasting. A pyrazines trail was applied or not to the sector with prey. In general, the number of active foragers in the experimental arena depended on experimentation time. Preferential and time-dependent recruitment was observed in the sector with pyrazines. The colonies that used the pyrazines trail discovered the food patch faster and removed prey at a higher rate than colonies not exposed to pyrazines. Collectively, the results show that in Pogonomyrmex species, the ancestral character ‘solitary foraging’ shows behavioural plasticity when ants are confronted with trail pheromones stimuli and are under fasting stress.