The ecology of island and mainland populations of Anolis lemurinus

In Honduras, populations of the lizard Anolis lemurinus can be found on the mainland and on the Cayos Cochinos and Bay Islands, where they have been isolated for roughly 10,000 years.  For my master's thesis, I am comparing the ecology of mainland and Cayos Cochinos populations. 

On the mainland, A. lemurinus is generally found in extreme-shade situtations, usually in primary forest.  On Cayo Mayor and Cayo Menor, the two largest islands of the Cayos Cochinos, the abiotic and biotic environments appear to be quite different, and I am examining the morphological, ecological, and physiological correlates of these differences in environment.

From June through August 2008, I measured 15 morphometric variables, 13 habitat variables, field-active body temperatures, upper thermal tolerances, and evaporative water loss rates for over 150 individuals in the Cayos Cochinos.  This dataset is beginning to reveal differences in sex ratios, abundance, morphology and microhabitat use among island populations.  Cayo Mayor males, for example, appear to be smaller and in worse body condition than Cayo Menor males.  Interestingly, despite their smaller body size, Cayo Mayor males also appear to have larger dewlaps!  I suspect that these results might have something to do with differences in population density between islands, but more definite conclusions regarding this pattern are pending further data acquisition.

Additionally, museum specimens collected from mainland populations and previously published physiological data are being used to reveal differences in morphology and physiology between mainland and island lizards.

Anolis lemurinus from Cayo Mayor

Sonora semiannulata

The thermal biology of Sonora semiannulata

Due to the temperature dependence of biochemical reactions, every physiological function operates within an optimal temperature range.  Ectothermic animals must therefore behaviorally thermoregulate to maintain maximum physiological performance. Although the environmental temperatures needed to achieve internal body temperatures that provide for maximum performance may be available somewhere within an organism's habitat, the benefits associated with locating them must outweigh the costs incurred from exposure to predators and decreased energy allocation to other behavioral tasks.  As a result, there are often physiological consequences to these behavioral decisions, and the ability of ectotherms to maximize physiological performance (through behavioral thermoregulation) is predicted to differ depending on ecological context. 

Numerous studies have examined the thermal biology of diurnally active, terrestrial and arboreal ectotherms.  Few, however, have looked at how fossorial or semi-fossorial ectotherms utilize their thermal environment despite the fact that unique constraints on their thermoregulatory behavior are likely present.

In collaboration with Christian Cox (University of Texas-Arlington), we are attempting to characterize the thermal biology of Sonora semiannulata, a small, semi-fossorial snake from the deserts of the southwest US and northern Mexico, by comparing field active body temperatures with operative temperature distributions ("null" distribution of available temperatures acquired with copper temperature models randomly placed under rocks that may serve as retreat sites) and the species' preferred temperature range (developed by exposing snakes to laboratory thermal   gradients).

Thermal indices (e.g. accuracy of thermoregulation, thermoregulatory precision, quality of the thermal environment, and effectiveness of thermoregulation) will be used in conjunction with additional environmental parameters (such as the dimensions of rocks used as retreat sites, retreat site moisture content, and invertebrate and vertebrate abundance) to examine constraints on thermoregulatory behavior, and to parcel out the relative importance of thermal physiology in determining retreat site use in S. semiannulata.

Insular dwarfism in tarantulas

Several years ago, an amateur tarantula enthusiast from the UK collected several  specimens of what he thought was Ornithoctonus aureotibialis from the island of Koh Samui, in the Bay of Thailand.  O. aureotibialis is a large, terrestrial, burrowing tarantula that is common on mainland Thailand.  Because these particular specimens were small, the collector assumed they were juveniles.  He was quite surprised, then, when several days later the "juvenile" tarantulas laid viable egg sacks!  It turns out they were sexually mature adults, and less than half the size of their mainland counterparts.  Nevertheless, in every other way they appeared to be morphologically identical to the mainland species.  The collector realized that he had accidentally discovered a dwarfed population of O. aureotibialis.  After several successive trips to the island, it has been confirmed that the entire Koh Samui population is dwarfed.

Chris Hamilton, a fellow graduate student here at the University of Texas-Arlington (an acquaintance of the aforementioned collector) and I, were fascinated by this bizarre case of insular dwarfism when we first received word about it.  As far as we were aware, this was the only known population of insular dwarf arachnid in the world, and one of the only clear-cut (i.e. with an extant sister species living on the mainland) cases of insular dwarfism in an invertebrate.  Examples of insular dwarf invertebrates are rare partly because one of the dominant explanations for dwarfism----the tendency for food resources to be limiting on islands----is unlikely to be applicable to a generalist, insectivorous arachnid.  Additionally, this case appears to violate the "island rule," which states that large species tend to decrease in size after island colonization, while small species tend to increase in size.  The island rule is widely regarded as one of the most generally applicable "rules" in ecology, with very few known exceptions.

So how can a population of dwarf tarantulas, which are most likely not experiencing a prey limited environment, exist?

One hypothesis is based on two principles, the first relating to island organisms in general, and the second to tarantulas specifically:   

1)  Island populations are often found at much higher densities then their mainland counterparts.

2)  Male tarantulas allocate virtually all of their energy to growth, maintenance, and storage until they go through a terminal molt, after which they leave their burrows and disperse in search of females.

Higher densities might therefore reduce the distance male tarantulas would have to disperse to locate females.  It follows that selection for males who don't waist time and energy accumulating fat reserves might result in decreased body size at sexual maturity, and as energy is diverted from the growth of  somatic tissues to the growth of reproductive tissues, we might also expect sexual maturity to be achieved at an earlier age.  So if we were to find that the island population of O. aureotibialis is at higher densities than mainland populations, this might explain why we see dwarfed males.  But what about females?  If females don't disperse, why are they also small?  And there are other critical questions that need to be considered.  These include:

How many, how large, and how often are females producing offspring relative to mainland populations?

How sexually dimorphic are these tarantulas relative to mainland populations? 

What is the role of density-independent factors such as frequency of disturbance, soil characteristics, and the thermal environment? 

Are there effects from predation or interspecific competition?

Chris Hamilton and I are presently designing a project aimed at answering these questions.  After developing a molecular phylogeny to determine relative divergence times of the populations, we will quantify population densities, growth rates, and male dispersal distances, and then move on to the examination of other potential selective forces operating differentially in the two environments.  We hope to eventually understand the selective pressures and conditions that have lead to the evolution of the world's only known insular dwarf spider.

Ornithoctonus aureotibialis from Koh Samui

Calliophis nigrescens, "yellow phase"

Calliophis nigrescens, "melanistic phase"

Geographic variation in the Indian coral snake, Calliophis nigrescens

Calliophis nigrescens, a species of coral snake (Elapidae) from the Indian subcontinent, is distributed across a wide geographic range, and displays three distinct color morphs.  These three color morphs, yellow, red, and melanistic, appear to fall out onto three distinct (although somewhat overlapping) geographic regions.

In collaboration with Dr. Eric Smith from the University of Texas at Arlington, we are analyzing several morphological characters from nearly all of the known museum specimens of this species.  We are attempting to determine whether C. nigrescens morphology differs between geographic region, whether other morphological characters co-vary with color morph, and whether these geographic "races," in fact represent distinct species.