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Over 150 years ago, C. Bergmann observed a within-species pattern of increasing body size as elevation and latitude increased. Bergmann’s data came from numerous endothermic taxa, but the pattern has since been documented among numerous poikilotherms and invertebrates. The ferocious waterbug Abedus herberti Hidalgo (Hemiptera: Belostomatidae) lives in isolated populations distributed across gradients of latitude and altitude, and as such is an excellent model organism to examine the application of Bergmann’s Rule to a large aquatic insect.
This study documents the variance in adult length and width of fifteen disparate populations of Abedus herberti in southeastern Arizona, USA and northern Sonora, Mexico. It also examines the life-history traits of two cadres of populations: those living in mountain runoff creeks, the typical habitat for this species; and those living in spring-fed desert ponds and marshes, called ciénagas.
During 2004-2006 my colleagues and I collected and measured 611 individual adult A. herberti from fifteen populations in the Madrean Sky Islands. I took live insects from six populations back to Oregon and raised their offspring in a common-garden experiment, to measure length, width, and time between molts. Temperature probes were placed in the pools from which these six populations were taken, to provide a record of annual thermal fluctuation.
I used these data to examine several hypotheses concerning body size and growth in insects:
Are there significant size differences between mountain-stream bugs and those living in ciénagas?
Do these insects obey Bergmann’s Rule, i.e. does body size increase as local mean temperature decreases, as predicted by altitude and latitude?
Does a larger final body size correlate with a longer development time?
Are the size differences between bugs from different habitat types resulting from phenotypic plasticity or are the traits heritable?
How does Dyar’s Rule apply to this organism?
Does body size provide any evidence of isolation or speciation within these populations?
I also used these data to create a rearing and care program for Abedus herberti that will serve future researchers working with this organism. This program, as well as an investigation of the latter two hypotheses, is relegated to the appendix of this work.
I found the mean sizes of individuals within Abedus herberti populations to be consistent with Bergmann’s Rule. Adult length and width increased with the altitude of the population. Even omitting the ciénaga populations from the analysis produced significant results. As the populations were distributed over a minor gradient in latitude, I tested for significant correlation but found only a weak relationship between latitude and length, and no significant relationship between altitude and width.
The thermal regimes between mountain and ciénaga habitats differed in the following ways: higher annual mean temperature and greater thermal stability in ciénagas, lower annual mean temperature and greater fluctuations in mountain streams. These differences may contribute to selective pressures that result in size disparities between conspecifics living in the two habitats.
The mean adult length and width varied significantly between mountain-type populations and ciénaga-type populations. These differences were perpetuated by offspring raised under identical conditions, suggesting the traits in question are inherited, rather than resulting from phenotypic plasticity.
Larger-sized nymphs took significantly longer to develop than smaller nymphs. Intermolt time between hatching and first molt, and between first and second molt, was longer for the mountain-type populations. Further intermolt times were not included in the analysis because high mortality interfered with design balance. This animal undergoes five nymph instars before attaining adulthood.
There was a 1.29x increase in length and 1.27x increase in width between molts (±1.2% depending on population), lower than the maximum allowed by Dyar’s Rule, which posits a maximal 1.4x increase in size between instars.
I hypothesize that the differences in body size between populations living under different thermal regimes could be due to three main factors: 1. A physiological response to colder environments and selection for a particular optimal body size for a given mean annual temperature. 2. Selection for smaller size and faster development in warmer environments to optimize the number of generations possible during the growth season, balanced by the stored-resource requirements of overwintering adults. 3. Selection for larger egg-size and first-instar nymph size in colder habitats to favor increased energy content of early offspring, as a hedge against fluctuating conditions; this is balanced in the warmer habitats by selection for small egg-size that allows females to fit more eggs onto the males’ hemelytra, the surface area of which limits the fecundity of this species. The results of this study suggest all these factors may be in play to some degree, but the strongest evidence supports a physiological and life-history response in body size based strictly on local thermal regime. |
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