Monday, July 27, 2015

Your Body and Heat

"The thermometer nailed to a post reads 110 degrees F., but in the shade, with a breeze and almost no humidity, such a temperature is comfortable, even pleasant." -Edward Abbey

Devils Golf Course in Death Valley

I will have to respectfully disagree with  Mr. Abbey on this point. I've spent much time in the desert, and not once have I found 110 degrees to my liking under any circumstances. Perhaps an exaggeration on his part? As even animals that evolved here avoid those sorts of extremes at all costs.

I recently found myself  in this condition though. Hot desert sun baking me, bare rock beneath my feet radiating that heat back towards my body. Another point I will contest Abbey on is where he exactly expects one to find shade, in many parts of the hottest deserts there is none. In my case the shade of boulders existed ephemerally, disappearing and shifting as the sun danced across the sky. What is one to do when no amount of liquid can quench your thirst? When drenching yourself with provides only a moment's relief, and when the sun becomes your worst enemy? Scramble for shade. Get wet. Get indoors are the solutions which come to mind.

More importantly for our purposes, is the question of what is happening inside our bodies and cells when we (or any other animal) are tested with these extremes in temperature.

First lets focus on the brain. Our most important organ, the control center for our body. What happens to our cognitive abilities during heat stress? Well it turns out that it depends on a number of factors. In general, when you are challenged by heat, your cognitive performance will decrease. The degree to which it is negatively affected depends on many things including difficulty of the task, gender (females are perhaps less susceptible to cognitive decline during heat stress), and hydration (the better hydrated you are the less steep the decline when challenged with heat). This cognitive decline is notable because your ability to make smart decisions decreases when you are challenged with heat, potentially leading to mistakes which could make a bad situation worse.

What about in your muscles? As you work in hot conditions, your muscles become fatigued more quickly. You will be unable to exert as much force following long periods of exercise in hot conditions. This affect on muscles seems to be mainly seen during long periods of exertion, short term physical exertions are not influenced as much unless they come immediately following a period of prolonged exercise in hot conditions. 

What about at the cellular level? Well, the cellular response to heat stress is perhaps one of the most well preserved responses throughout evolution. Following exposure to high temperatures your cells will start producing a special type of "heat shock proteins". Organisms needed a way to deal with the consequences of stresses like heat, and these special proteins could be evolution's answer. These proteins are recruited to make sure that your cells don't succumb to the often deadly affects of heat. That much is clear, but more remains to be understood regarding their mechanism of action within a cell.

There may be some temporary benefits of being challenged by heat. Rabbits that were challenged by heat had hearts that were less susceptible to other environmental stresses 24 hours later. It seems that dealing with heat, may equip your body to deal with other environmental stressors within a short time frame. Though this advantage seems to disappear the longer an organism is removed from the heat.

Decreased brain function, muscles that fatigue more quickly, and cells functioning differently than they would otherwise. All are consequences of heat exposure. Stay safe, stay hydrated, and stay cool.


References and further reading:


Hancock, P. A., and Ioannis Vasmatzidis. "Effects of heat stress on cognitive performance: the current state of knowledge." International Journal of Hyperthermia 19.3 (2003): 355-372.

Gopinathan, P. M., G. Pichan, and V. M. Sharma. "Role of dehydration in heat stress-induced variations in mental performance." Archives of Environmental Health: An International Journal 43.1 (1988): 15-17.

Morimoto, Richard I. "Regulation of the heat shock transcriptional response: cross talk between a family of heat shock factors, molecular chaperones, and negative regulators." Genes & development 12.24 (1998): 3788-3796.

Currie, R. W., R. M. Tanguay, and J. G. Kingma. "Heat-shock response and limitation of tissue necrosis during occlusion/reperfusion in rabbit hearts."Circulation 87.3 (1993): 963-971.

Nybo, Lars, and Bodil Nielsen. "Hyperthermia and central fatigue during prolonged exercise in humans." Journal of applied physiology 91.3 (2001): 1055-1060.

Lindquist, Susan. "The heat-shock response." Annual review of biochemistry55.1 (1986): 1151-1191.

Wednesday, July 15, 2015

Water

"Nowhere is water so beautiful as the desert, for nowhere else is it so scarce." -Edward Abbey


Salt Creek in Death Valley National Park. Home to the endemic Salt Creek Pupfish


One of the greatest paradoxes of the desert is that its most limiting resource is simultaneously the most powerful force shaping the the landscape; geologically, biologically, and socially. When the rains do come here, they come furiously. Eroding clay, dirt, and rock. The meticulous planning of water resources makes it possible that people are able to live in the desert, and fights over water rights still often constitute the biggest political and legal battles in desert regions (follow the link to learn about an ongoing legal battle regarding water rights in Nevada).

Water and its availability or lack thereof also has exceptional consequences for biotic communities throughout the desert. For perspective, it is estimated that about 0.4% of Arizona's surface area is a wetland, or riparian habitat. Nevertheless about 80% of vertebrate species in Arizona depend on riparian habitat at some point in their lives. 80% of species depend on 0.4% of total land area for the completion of their life cycles. Birds are especially dependent upon riparian habitat. Protection of a river, or stream can significantly increase bird biodiversity throughout an entire area.


Lost Palms Oasis, Joshua Tree National Park

Additionally, these riparian areas are important because the biological community in them is often quite different from the surrounding area. One need look no further than Ash Meadows National Wildlife Refuge to confirm this, this small area dotted with multiple springs in Nevada's Mojave Desert is home to the highest concentration of endemic species (species found nowhere else) in the World. The endemic species found at Ash Meadows include plants, snails, and some very special fish.

Wildlife that depends on these habitats are also disproportionately imperiled. Over one dozen freshwater fish species went extinct during the last century from desert regions of the United States and Mexico.  Extinction threatens 40% of desert pupfish species today. These numbers are higher when other groups that depend on these habitats, like amphibians, are included.

Water and its management will continue to shape the social, geological, and biological landscape in deserts going forward. Can we strike a balance between managing water for the well being of humans, and managing it in such way that no more distinct evolutionary lineages are lost forever?  Exceptionally unique species assemblages still exist where water reaches the surface, though much was lost, much still remains. Whether or not what is left will remain in the next century, depends largely on


References and further reading:

Sabo, John L., et al. "Riparian zones increase regional species richness by harboring different, not more, species." Ecology 86.1 (2005): 56-62.

Zaimes, George, et al. "Understanding Arizona's Riparian Areas." (2007).

Brown, James H., and C. Robert Feldmeth. "Evolution in constant and fluctuating environments: thermal tolerances of desert pupfish (Cyprinodon)."Evolution (1971): 390-398.

Williams, Jack E., et al. "Endangered aquatic ecosystems in North American deserts with a list of vanishing fishes of the region." Journal of the Arizona-Nevada Academy of Science (1985): 1-61.

Miller, Robert R., James D. Williams, and Jack E. Williams. "Extinctions of North American fishes during the past century." Fisheries 14.6 (1989): 22-38.

Thursday, July 9, 2015

Convergence

"It may be doubted whether any character can be named which is distinctive of a race and is constant." -Charles Darwin

The Sidewinder and Horned Vipers from Africa is a good example of convergent evolution.



Convergent evolution is a topic that should be of special interest to anyone who is interested in desert biology. Deserts are found world wide on multiple different continents in both the Southern and Northern Hemispheres. Of course, by their very nature these environments share many characteristics.

High temperatures is one characteristic that most deserts share, though there are exceptions and variability. Aridity is another characteristic that tends to typify desert environments. Convergence in evolutionary terms is the idea that similar environments will provide very similar selective pressures and select for similar adaptive suites or survival strategies.

In other words two species that are totally unrelated can come to look very similar if the selective pressures are similar enough. Some scientists even hypothesize that biological communities themselves can converge. In other words, two distinct geographic areas both have species that have very similar interactions with each other.

One of the most well known examples of convergent evolution is the "side winding " method of locomotion desert vipers. The deserts of the Southwest has Crotalus cerastes, and the North African deserts have Cerastes cerastes. Both of these vipers use this special form of locomotion, and horn like protrusions above their eyes, but they differ in their abilities to burrow under sandy substrates (Cerastes cerastes being the stronger burrower).

Cacti of North America and Succulents of Africa also appear very similar at first glance, though there are some important differences. North American Horned Lizards and Thorny Devils of Australia are another prime example of potential convergence. Fringes on lizards feet also evolve to be very similar in non-related species if the substrate in those two places is similar. The Fringe Toed Lizards have elongated scales on their feet which may help them move more easily in sandy environments.

The idea of convergent evolution gets to the very basis of  theoretical ecology and evolutionary biology. What causes plant and animal communities to interact the way that they do? Is community structure determined principally by environmental conditions (species and communities evolving to match their environments), or does evolutionary history and existing traits matter more ( a species lives in an area because it is already has the tools to do so, with selection refining advantageous traits and quelling bad ones)? What role do random chance and unpredictable events play? The idea of convergent evolution and convergence in community structure is a very interesting topic in science, and I recommend those who are interested to do more reading. As I only touched on some of the basics here.


References and further reading:

Luke, Claudia. "Convergent evolution of lizard toe fringes." Biological Journal of the Linnean Society 27.1 (1986): 1-16.

Melville, Jane, Luke J. Harmon, and Jonathan B. Losos. "Intercontinental community convergence of ecology and morphology in desert lizards."Proceedings of the Royal Society of London B: Biological Sciences 273.1586 (2006): 557-563.

Mares, Michael A. "Desert rodents, seed consumption, and convergence."BioScience (1993): 372-379.

Samuels, Corey L., and James A. Drake. "Divergent perspectives on community convergence." Trends in Ecology & Evolution 12.11 (1997): 427-432.

Alvarado-C├írdenas, Leonardo O., et al. "To converge or not to converge in environmental space: testing for similar environments between analogous succulent plants of North America and Africa." Annals of botany 111.6 (2013): 1125-1138.

Mosauer, Walter. "Adaptive convergence in the sand reptiles of the Sahara and of California: a study in structure and behavior." Copeia 1932.2 (1932): 72-78.

Thursday, July 2, 2015

Color Patterns


"Colors must fit together as pieces in a puzzle or cogs in a wheel." -Hans Hoffman

Why do dark color patterns exist in deserts?
This Tarantula Hawk has a dark body and orange wings, which could be a form of aposematic coloration (warning colors).



Conventional wisdom tells us that animals that live in the desert should tend to be light colored. Reflecting heat should be a beneficial trait in a hot environment. If you go walking in the desert in a black t-shirt in July it will take a few seconds before you start to question the decision.

In fact there are a great deal of organisms in the desert that do display light coloration. Plants are great examples here. Brittle bush and other plants often have light green leaves in order to reflect the sun's intense rays. Rattlesnakes and Zebra Tail Lizards often have very light coloration. Desert Iguanas and Tenebrionid Beetles are able to change colors depending on environmental conditions. Desert Iguanas tend to have darker colors when their body temperature is low (heat up quick), and become lighter and lighter as their temperature increases, likely to reflect heat away from its body. Tenebrionid Beetles on the other hand change colors as a response to humidity (lighter in drier conditions, darker in humid conditions).

These Sunrays have light leaves which reflect intense sunlight.

However, there are also a great deal of organisms that display coloration which would seem to be detrimental in these conditions. Ravens, Chuckwallas, Turkey Vultures, all have extremely dark coloration AND all of these creatures also tend to be active during the hot daytime hours. The outer feathers of a Raven can heat to greater than 176 degrees Fahrenheit (80 C) during a hot day. What benefit is this are dark color patterns to these creatures? Why don't all desert organisms display patterns which tend to reflect rather than absorb light and heat?

The answer lies in the multiple selective pressures that are shaping the evolution of these creatures. Color patterns are extremely important and serve multiple purposes. Hiding from predators, warning predators and competitors, attracting a mate, and thermoregulation to name a few. It is tough to imagine that extreme heat wouldn't be the strongest selective force acting on color patterns, but it simply may not be for all organisms. Many animals have behavioral and physiological adaptations to help them deal with the harsh conditions, while color patterns may serve other purposes.

Though, with dark color patterns being as common as they are in the desert it is difficult to believe that there is absolutely no thermal benefit. Lets return to the case of the Raven. 176 degrees on its dark outer plumage, yes. But only 104 degrees on its skin (ha only!). There is some evidence that if an animal has a thick layer of dark plumage or fur that the heat becomes trapped at the surface and doesn't reach the skin as effectively. Light fur on the other hand is easily penetrated by the suns rays. Can this provide some explanation for why dark color patterns can be quite common in the hottest places on earth? Is a change in color pattern mechanistically more difficult to evolve than a change in behavior or physiology? Is behavior simply more important when it comes to thermoregulation?

References and further reading:

Marder, Jacob. "Body temperature regulation in the brown-necked raven (Corvus corax ruficollis)—II. Thermal changes in the plumage of ravens exposed to solar radiation." Comparative Biochemistry and Physiology Part A: Physiology 45.2 (1973): 431-440.

Beckman, W. A., John W. Mitchell, and Warren Paul Porter. "Thermal model for prediction of a desert iguana’s daily and seasonal behavior." Journal of Heat Transfer 95.2 (1973): 257-262.

Norris, Kenneth S., and Charles H. Lowe. "An analysis of background color-matching in amphibians and reptiles." Ecology (1964): 565-580.

Sowell, J. Desert Ecology: An Introduction To Life In the Arid Southwest. 2001.