Fernando A. Milesi

Polyandry in two South American harvester ants

Abstract.Although monandry (single mating) is the ancestral state in social hymenopteran insects, effective mating frequencies greater than 2 have been confirmed for a fair amount of ant species: Cataglyphis cursor, the leaf-cutters of the genera Atta and Acromyrmex, army ants of the genera Eciton, Dorylus, Aenictus and Neivamyrmex, and some North American seed harvester species of the genus Pogonomyrmex. This last genus spreads throughout open arid habitats from Patagonia to southwestern Canada. Whereas some North American Pogonomyrmex species are thoroughly studied, we know much less about these ants in South America. The objective of this study was to estimate the effective mating frequency of Pogonomyrmex inermis and P. pronotalis, two Pogonomyrmex sensu stricto species from the central Monte desert of Argentina. A total of 477 P. pronotalis workers from 24 colonies and 402 P. inermis workers from 20 colonies were analyzed using six and four highly polymorphic microsatellites, respectively. The multilocus analysis revealed that all colonies were monogynous and all queens multiply-mated. The effective mating frequency was 8.75 and 6.52 for queens of P. pronotalis and P. inermis, respectively; those values increased up to 15.66 and 9.78, respectively, when corrected for sampling errors. This is the first demonstration that queens in at least some members of the South American Pogonomyrmex sensu stricto are strictly polyandrous, with mating numbers per queen at least as high as those previously found for North American species. We suggest that multiple mating probably arose early in the evolution of the genus Pogonomyrmex and may be the basis of its ecological success and wide distribution.

Selection of Foraging Sites by Desert Granivorous Birds: Vegetation Structure, Seed Availability, Species-Specific Foraging Tactics, and Spatial Scale

Abstract Seed availability in the algarrobal of the Monte desert, Argentina, as in other deserts, is highly heterogeneous at small scales and is associated with vegetation structure. Granivores are expected to use space selectively (within the capacities of their foraging techniques), resulting in a heterogeneous effect on the seed bank. First, we describe the foraging repertoire of granivorous birds in the algarrobal to develop predictions about their expected use of space. Although the granivory guild as a whole tracked the temporal availability of seeds, species within the guild differed in foraging behavior and seasonal changes. We assessed selection of space by foraging birds through a two-scale bird-centered analysis, comparing the distributions of used and available sites. Granivorous birds, considered as a guild, used the whole range of available microsites, though as a result of an aggregation of contrasting partial patterns. Microsites with more cover of shrubs, grasses, and litter were preferred for predispersal consumption, which is consistent with the frequent technique of attacking grasses from low woody perches. By contrast, use of microsites for postdispersal consumption did not differ from random, which suggests that there are no safe microsites for seeds in this habitat. At a larger scale not particularly related to heterogeneity in food availability, a selective pattern was clearer: birds avoided mesosites with low shrub and litter cover, far from trees. In conclusion, patterns are not straightforward and depend on considerations of spatiotemporal scale and species-specific characteristics. Even when predictions seemed clear-cut and were tested with proper scale and target-group, granivorous birds did not select space according to environmental clues of food abundance.

Which food patches are worth exploring? Foraging desert birds do not follow environmental indicators of seed abundance at small scales: a field experiment

Consumers should show strong spatial preferences when foraging in environments where food availability is highly heterogeneous and predictable from its correlation with informative environmental features. This is the case for postdispersal granivores in most arid areas, where soil seed bank abundance and composition associates persistently with vegetation structure at small scales (e.g., decimeters to meters). We analysed seasonal single-seed removal by granivorous birds from 300 experimental devices in the algarrobal of the central Monte desert. Spatial selectivity was analysed by comparing the structural characteristics of used vs. available microhabitats and evaluated against bottom-up and top-down hypotheses based on our previous knowledge on local seed bank abundance, composition and dynamics. Seed removal, which showed its expected seasonal variability, was also explored for spatial autocorrelation and environmental dependencies at bigger scales. Postdispersal granivorous birds were less selective in their use of foraging space than expected if patch appearance were providing them useful information to guide their search for profitable foraging patches. No kind of microhabitat, as defined by their vegetation and soil structure, was safe from bird exploration. The only consistent selective pattern at this scale was closer to a top-down spatial effect by birds, i.e., a cause (and not a consequence) of the seed bank dynamics. Bigger spatial scales proved more relevant to describe heterogeneity in the use of foraging patches in this habitat. Closeness to tall trees, probably related to bird territoriality and reproduction or to their perception of predation risk, seems to determine a first level of selection that defines explorable space, and then microhabitat openness exerts an influence on which patches are effectively exploited (or more frequently explored) among those accessible.