Combining direct methods (PIT tags and radio-telemetry) with an indirect method (mtDNA) to measure movement and dispersal at different scales in North American tarantulas (Aphonopelma spp.).
Hamilton, Donna E.
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Despite their conspicuousness and importance as predators in arid and semi-arid ecosystems, almost nothing is known about the behavioral ecology and genetic structure of populations of North American tarantulas (Araneae, Theraphosidae). Additionally, two members of this genus are threatened and listed on CITES Appendix II. We used three methods of measuring movement and dispersal in these animals –the direct methods of radio-telemetry and passive integrated transponders and the indirect method of mitochondrial DNA analysis. We radio-telemetered 18 adult female tarantulas, Aphonopelma hollyi, in order to gather information regarding their time budgets, the effect of environmental factors on behavior, interactions between colonial females, and individuality in behavior between tarantulas. Date and cloud cover were the only environmental factors measured that affected activity levels in the tarantulas. Equilibrium behavior frequencies indicate time budgets that indicate the spiders spend more than 96% of their time inside the burrow, even in their peak activity season. We introduce a novel method for measuring the individuality of behavior between animals by comparing individual equilibrium behavioral frequencies with the frequencies estimated for the entire study group. We found no indications of individuality in behavior in the tarantulas monitored. We used Passive Integrated Transponders (PIT tags) as permanent, internal markers in order to monitor burrow fidelity and movement within a colony of the tarantula Aphonopelma hollyi over the span of 14 months. We used two data sets of weather variables in the analyses; one collected at the study site and the other collected 9.75 km from the study site. All the variables were statistically the same between the data sets except for days of measurable precipitation per month. These data were significantly different with t = 3.318; df = 13 and p = 0.0028. To determine how typical the period of the study was in relation to the previous ten years, we performed an ANOVA on data that included the past 10 years and there was no significantly different year found in the eleven years tested (p = 0.1543; df = 49). Over the span of the fourteen months of the study, all sixteen tagged females abandoned their burrows. Using multiple regression we determined the model including maximum air temperature, cumulative monthly precipitation and days of precipitation per month accounted for 48.82% of the variance explaining the percentage of PIT tagged females remaining in their burrow at each monthly time interval. The p value was 0.096 and so not quite significant for the model as a whole. Days of measurable precipitation were the only variable that was found to make a significant contribution (p = 0.018) to the model. Because North American tarantulas are relatively unique in their natural history when compared to other spiders, even member of the same family, diversity studies previously published are not generally applicable to members of the genus Aphonoplema. Here we present the first data (16S) that can be used to evaluate specimens within the genus Aphonopelma for hierarchical structure in genetic diversity in order understand the long-term patterns of movement and dispersal in these animals. We were able to isolate contiguous 16S sequences from 27 animals that were 430 base pairs in length. We were able identify a total of 9 haplotyes present in the 27 animals analyzed and there were 28 polymorphic nucleotides found in the 27 16S sequences analyzed (6.4%). The only significant FST values were found at the generic level of analysis (FST = 0.546). There was no apparent relationship between geographic and genetic distance within the genus; although there are three clear divisions apparent in the neighbor-joining network, which we believe to represent natural species divisions. Our analyses show a pattern of divergence that is consistent with the presence of three species: A. hentzi, A. hollyi and an unidentified species A. sp. And there was an average of 4.42% difference between species. We found no evidence of regionally selective pressures on the 16S gene.