Bufonid toads of the genus Melanophryniscus represent one of several lineages of anurans with the ability to sequester alkaloids from dietary arthropods for chemical defense. The alkaloid profile for Melanophryniscus stelzneri from a location in the province of Córdoba, Argentina, changed significantly over a 10-year period, probably indicating changes in availability of alkaloid-containing arthropods. A total of 29 alkaloids were identified in two collections of this population. Eight alkaloids were identified in M. stelzneri from another location in the province of Córdoba. The alkaloid profiles of Melanophryniscus rubriventris collected from four locations in the provinces of Salta and Jujuy, Argentina, contained 44 compounds and differed considerably between locations. Furthermore, alkaloid profiles of M. stelzneri and M. rubriventris strongly differed, probably reflecting differences in the ecosystem and hence in availability of alkaloid-containing arthropods.
Oviposition-site selection plays an important role in the reproductive success of amphibians. In unpredictable environments where resources vary within a season, amphibians should select oviposition sites using parameters that can be easily evaluated, or spawn in several ponds to increase offspring survival. Melanophryniscus rubriventris (Vellard, 1947) uses shallow ephemeral ponds in an unpredictable montane-forest environment. During 40 consecutive days, we surveyed potential spawning sites and measured several biotic and abiotic factors to determine if any of these factors influenced breeding-site selection. We also described the spawning behavior of this species. Water temperature and pond level (flooded or not) were significant predictors of whether a pond was used or not. Warmer ponds would permit accelerated development of larvae in habitats where ponds are ephemeral and their presence unpredictable. Because of the short and unpredictable hydroperiods, it will be better to select a pond full of water instead of evaluating other pond characteristics that can be very variable. Mating pairs spread several egg masses to different sites but in the same pond. This behavior is likely a consequence of pairs avoiding interactions with intruding males and not as a strategy to enhance offspring survival.
Climate may play a direct role in determining patterns of reproductive activity in anurans. Investigating the responses of males and females attending a breeding site to environmental variation are of particular interest regardless the possible effect of climate change on the decline of populations. I studied annual variation over two years in the breeding population size and breeding phenology of the diurnal toad, Melanophryniscus rubriventris, in a seasonal environment characterized by temporal and spatial uncertainty due to climatic conditions. Breeding activity of this toad was opportunistic throughout a prolonged spring-summer breeding season. Direct influence of rainfall on the onset and extent of breeding activity was detected in both years. Spawning activity showed a clear pattern of two three-day peaks mostly related to heavy rainfall events. Males always significantly outnumbered females at breeding sites. The disparity of rainy season extent between the two breeding seasons studied resulted in different annual patterns of arrival and length of residence of adults. Males and females remained several days longer during the second year than during the first year. Because adults showed highly variable annual patterns of breeding activity studies involving only a few years may yield misleading results concerning the nature and extent of the decline of the species or populations. Future studies on the ecology and conservation of the species should include long-term monitoring.
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