Miscellaneous projects
I've undertaken a few projects outside my main research interests in alligators, eggs and termite mounds. There is a method to the madness - all have involved some interesting problem in heat or mass transfer between organisms and their environment. These have included:
- studies on trapdoor spiders in the Namib sand desert,
- how dwarf succulents manage to live in some of the harshest microenvironments in existence,
- whether black or white color of desert beetles has any affect on their body temperature,
- and why spittlebugs produce spit.
Shuttling of Namib dune trap door spiders (Seothyra henscheli): is it for thermoregulation?
Seothyra henscheli is a trapdoor spider that builds its web and trap in sand dunes in the Namib desert. It captures food with an hourglass-shaped sticky capture web that immobilizes ants foraging on the dunes. Quite often, prey is captured during the hottest parts of the day, when surface temperatures can exceed 70 degrees Celsius. This poses a quandary for the spider. It cannot tolerate the extreme temperatures at the surface. Yet it must go to the surface and undertake the lengthy task of untangling the stricken prey. If the spider chooses to leave the prey until surface temperatures become more tolerable, it runs the almost certain risk of losing its prey to passing birds who are always on the lookout for a meal.
Unlike many spiders, Seothyra henscheli retrieves its prey in a series of short visits to the surface. This looks like shuttling thermoregulation, where the spider makes a short foray into the hot environment of the surface, where it works until its body temperature approaches its maximum tolerable temperature. It then retreats to the cooler depths of its burrow, where it cools down until it is ready for the next foray to the top.
Joh Henschel, Yael Lubin and I examined this problem through a combination of transient-state modeling of body temperature, along with observations of vertical gradients in temperature and time sequences of shuttling during prey retrieval. We found that, in general, shuttling conferred no protection from high temperature. Specifically, if all the time spent retrieving the prey had been combined into one continuous visit to the surface, the spider would not have overheated. The shuttling behavior could not, therefore be for thermoregulation.
J S Turner, J B Henschel and Y D Lubin. 1993. Thermal constraints on prey-capture behavior of a burrowing spider in a hot environment. Behavioural Ecology and Sociobiology 33: 35-43. [pdf]
Body color and temperature in black vs white Namib desert beetles
The "black desert beetle paradox" was originally posed by John Cloudsley-Thompson, and refers to the seemingly anomalous abundance of black beetles in hot sunny environments. Why would beetles adopt a color that maximizes absorption of solar radiation when the environment is already quite hot. Would it not make more "sense" to be white?
The Namib tenebrionids beetles offer an interesting test of this paradox. Many, like Onymacris unguicularis (on the left), are black, but others, like Onymacris bicolor (on the right) have bright white elytra. Will these stark differences in color have any effect on body temperature? Previous studies had indicated that the dark beetles would indeed be driven to higher body temperatures than the white beetles would.
This did not make sense to Mandy Lombard and me, because the two beetles often shared the same habitat and were active at similar times of the day. So we decided to look into this by doing experiments that would let us tease out the contributions of radiation, air temperatrure and convection on body temperature. For example, we teased out the effects of convection by putting beetle carcasses (which could serve as operative temperature thermometers) into a wind tunnel with an intense light source above. We could tease out the effect of direct vs reflected radiation by suspending the carcass over either foil (direct + high reflected radiation) or a screen (direct radiation only).
In a nutshell, we found that body color only had a significant effect of body temperature when winds were very slow, less than one meter per second. At higher wind speeds, convection so strongly dominated the beetle's heat budget that radiation had virtually no effect. Mandy later found that both black and white beetles were active on the dune surfaces only when winds were brisk and color would not matter.
And those studies that did find an effect of body color on body temperature? They had been done under still air conditions, because winds "disrupted" the results!
J S Turner and A T Lombard. 1990. Body color and body temperature in white and black Namib desert beetles. Journal of Arid Environments 19: 303-315. [pdf]
Thermal ecology of embedded dwarf succulents
Southern Africa is host to a s
ucculent flora that is among the most diverse in the world. One particular type of succulent is expecially interesting from a thermal adaptation perspective: the embedded dwarf succulents, or "stone plants" of the Mesembryanthemaceae. These plants are reduced to two fleshy leaves, and virtually the entire plant is located underground. The only part of the plant exposed to the outside air is a flat depigmented surface on the leaf tip, known as the "window", through which light is admitted into the plant.
Like the Namib dune spiders described above, stone plants like Lithops exist in the very harsh thermal environment of the soil surface. Various hypotheses exist in the literature for how these plants manage to live there. Among them are the idea that Lithops only lives in environments covered with light reflective pebbles that ameliorate the heating of the soil surface by the sun. Variations of window color are also a popular explanation.
Mike Picker and I decided to look into this question. We carried out various experimental manipulations of surface thermal properties of the plants and soil and measured their effects on the daily course of temperature in the plant. We did these experiments on two species: Lithops comptonii in the comparative cool Ceres Karoo near Cape Town, and Lithops gracilidelineata in the hot sunny Namib near Walvis Bay in Namibia.
As an example, we altered reflectivity of the surrounding soil by placing "collars" of aluminum foil around a plant, or painting the soil dark black. We also did things like remove the reflective gravel overburden to change thermal capacity. In a nutshell, we found no alteration of the surface absorptivity of either the plant or soil had any meaningful effect on plant temperature throughout the day. The only thing that did have an effect was window clarity: Plants with clear windows heated up more than did plants with relatively opaque windows. Computer models of heat exchange revealed that temperature of Lithops leaves is driven most strongly by how far into the plant solar radiation can penetrate. Clear windows obviously let more light deeper into the plant, heating it more.
This explains an interesting biogeographic pattern among the stone plants: clear-windowed plants (stippled) tend to be found in cool areas with relatively sparse insolation, which more opaque windows (black) are found in hot environments with comparatively high insolation.
J S Turner and M D Picker. 1993. Thermal ecology of a subterranean dwarf succulent from southern Africa (Lithops spp: Mesembryanthemaceae). Journal of Arid Environments 24: 361-385. [pdf]
What's the deal with spittlebug spit?
This was a fun summer project that I never took anywhere.
S
pittlebugs are xylem parasites of vascular plants. They tap into the xylem through a proboscis, suck out the xylem sap, and absorb the nutrients within. Xylem sap is very dilute, though, and they must process a lot of the liquid sap to obtain their nutrients. The excess liquid is then excreted from the anus. As the liquid passes through the anus, the spittle bug adds silk-like protein to it. As this emerges from the anus, the spittle bug folds air into the liquid with its legs, producing the characteristic spittle that envelops the insect.
There have always been various theories about what this spittle does. Some say it protects the bug from desiccation. Some say it keeps the bug cool. Some say it hides the bug from predators. Many of these don't hold up. For example, the spittle nest is highly conspicuous, not what one would expect if its purpose is to hide the bug from predators.
I decided to look into this. I found that spittle does indeed retard water loss from the bug. This is probably due to the silk-like proteins in the spittle acting as barriers to evaporation. However, it doesn't follow that the spittle is protection from desiccation. The bug produces the spittle, after all, because it has a large surplus of water from the xylem sap. The bug therefore has no more need to protect itself from desiccation than would, say, a frog in water. Nor would the spittle help keep the bug cool - retardation of water loss will also impede evaporative cooling of the spittle.
My pet theory is that the spittle is an accessory kidney. Xylem sap is rich in amino acids and very sparse in carbohydrates. Because amino acid metabolism produces ammonia as a waste product, this means that spittle bugs will face a serious problem of ammonia intoxication. Unlike most terrestrial animals, spittlebugs do not convert this ammonia waste into urea or uric acid, and therefore must eliminate it as ammonia. It's difficult to eliminate ammonia from liquid to air, though. One way to help it along is to flush out the ammonia into the excreta, and then greatly expand the surface area from which the ammonia can be volatilized.
J S Turner. 1994. Anomalous water loss rates from spittle nests of spittle bugs (Homoptera: Cercopidae). Comparative Biochemistry and Physiology. 107A: 679-683.
