Thursday, August 27, 2015

Kangaroo Rats

"He that has satisfied his thirst turns his back on the well." -Baltazar Gracian

A Kangaroo Rat, note the large hind limbs.

Kangaroo Rats are an interesting little desert rodent, with a variety of adaptations to desert living.  Perhaps the most readily observable is their morphology (body shape) and their method of locomotion. These animals have large, powerful hind limbs. Just like the Collared Lizards we previously discussed Kangaroo Rats never miss leg day at the gym. They exploit bipedal motion just like the Collared Lizard. Although rather than bipedal running the Kangaroo Rats tend to hop, like their Aussie namesakes. They leap about 1-2 feet per hop when they are in a hurry. No small feat for a small animal such as this. These powerful hind limbs also allow them display impressive vertical leaps when frightened or threatened by a predator. One kangaroo rat was observed to jump 5 feet horizontally and over 2 feet vertically. Based on their body size this is the equivalent of a six foot human leaping 60 feet on the long jump while simultaneously reaching 24 feet vertically. Now that's a leap!

Kangaroo Rats also have some pretty awesome adaptations to live in desert environments. They have very low metabolic rates and a low thermal conductance compared to a closely related species found in wetter environments. This helps them thermoregulate and minimize water loss. This might indicate that their adaptations to desert life happened relative late in their evolutionary history. Which makes sense because North American deserts haven't been around for very long relatively speaking.

Perhaps most amazingly is that the Kangaroo Rats REFUSE to drink, even when water is available to them. Seems counter-intuitive considering they live in some of the driest areas in the world. How can Kangaroo Rats do this without succumbing to their harsh environment?

Kangaroo rats gather dry seeds and then bring them back to their burrow to store and consume them later. Their burrows are much more humid than the outside environment, and that atmospheric water moves into the stored seeds. When the Kangaroo Rats eat the seeds they are full of water from their humid burrows. The behavior and physiology of the Kangaroo Rat complement each other in such way that help it easily survive in the desert, even without drinking a drop of water in their lives.

References and further reading:

Schmidt‐Nielsen, Bodil, and Knut Schmidt‐Nielsen. "A complete account of the water metabolism in kangaroo rats and an experimental verification." Journal of Cellular and Comparative Physiology 38.2 (1951): 165-181.

Bartholomew, George A., and Herbert H. Caswell. "Locomotion in kangaroo rats and its adaptive significance." Journal of Mammalogy (1951): 155-169.

McNab, Brian K. "Climatic adaptation in the energetics of heteromyid rodents."Comparative Biochemistry and Physiology Part A: Physiology 62.4 (1979): 813-820.

Wednesday, August 12, 2015

Divergence and Speciation

"There is grandeur in this view of life, with its several powers, being originally breathed into a few forms or into one... from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." -Charles Darwin, On the Origin of Species

A Diamondback Rattlesnake, 13 different species of Rattlesnake inhabit Arizona. Why so many? What forces led to the diversification and speciation of Rattlesnakes in the Southwest?

The question of how and why species diverge and differentiate is one of biggest questions in evolutionary biology. Basic understanding of speciation and divergence in many way's starts to address life's big questions, providing insight into why their is such a diversity of life on earth, and what forces led to the evolution of humans and humanity. 

First we must ask the question, "What is a species?". This question has been subject to much debate over the years. Most biologists subscribe to the "biological species concept" which defines a species as a group of potentially interbreeding individuals. Even this widely accepted view can be problematic at times, for instance what constitutes a species in groups of asexually reproducing organisms such as bacteria or Whiptail Lizards? Is geographic isolation enough e.g. if two lizards are separated by a canyon, or 500 miles can we consider them separate species simply because they are technically "reproductively isolated" due to geology or geography? You can see that defining a species can be problematic, but nevertheless the concept of species is an important tool for biologists. For our purposes we will consider reproductive isolation from other groups of organisms to be paramount in the discussion of speciation. How does reproductive isolation arise between two populations? 

There area many possibilities that can lead to the separation of one species into two (or many), below we will discuss a few which probably have played major roles in Desert regions and systems.

1.) Geologic barriers inhibiting or stopping gene flow
The Desert regions of the United States are famous for their geologic heterogeneity (variability). A high mountain range, an extremely low and hot valley, a Grand Canyon all have the potential to make interbreeding physically impossible between groups of otherwise closely related individuals. If this separation of the two groups is prolonged genetic and morphological differences can accumulate at random (due to genetic drift) and due to similar or differing selective forces (natural or sexual selection). 

Abert's Squirrels and Kaibab squirrels are separated by the Grand Canyon. The Colorado River eroding the canyon and the uplift of the Colorado Plateau made interbreeding of the squirrels on the South Rim and North Rim of the canyon impossible. Mutations and genetic differences accumulated in the North Rim and South Rim squirrels. Now these two squirrels look quite different. 

This type of divergence has been common in aquatic desert organisms as stream flow has changed and pleistocene lakes receded leaving many aquatic organisms to be isolated from each other in distinct drainages or springs separated from other groups by miles of barren desert.

2.) Ecological Speciation
Australian deserts have a large diversity of lizards, but no obvious geologic barriers currently exist or have been known to be present in the desert regions there. How then have lizards diverge there despite a lack of a physical barrier? 

The answer likely lies in the diversity of habitats that exist in deserts there. Rocky habitats, sand plains, and shrub-acacia deserts are a few of the habitats found here. As lizard populations adapt to localized conditions they can often become obligates to a certain habitat type. In other words the barrier that exists between lizard species is ecological rather than geologic. Lizards under differing selective pressures have become separated here because selection has forced them down quite different evolutionary roads.

This same diversity of habitats is present in the American Desert regions: bajada and arroyo habitats, creosote flats, sand dunes, rocky habitats, and wetland habitats all exist here. Many of these habitats do have one or two lizard species typical of that habitat. So perhaps ecological separation has also shaped lizard evolution here as well.

3.) Hybridization
Ahh.... the Whiptail Lizards of the Desert Southwest. Ubiquitous throughout the desert and sometimes uninteresting in their appearance. These are not Gila Monsters (big, colorful, and venomous) or Collared Lizards (bright blue and eat lizards). I have at times been guilty of glancing at a lizard darting in a jerky motion through the bushes and thinking "It is JUST a Whiptail". 

However, when it comes to their reproduction, evolution, and classification it is hard to think of a more magnificent lizard. Again, what constitutes a species when we are talking about individuals which reproduce asexually? 

In this group of lizards there are multiple species which reproduce asexually and multiple species which reproduce sexually. A closer look at their evolutionary relationships proves even more confusing. Often times the closest relatives of an asexual Whiptail are a group of sexual Whiptails. In other words asexual reproduction has arisen AND persisted multiple times in Whiptail Lizards.

The likely cause of rapid speciation in this group of lizards is likely hybridization. One species of Whiptail mates with another species of Whiptail, and the daughter is a fully asexual NEW species! This means that the separation or speciation takes place in a single generation; speciation that is spontaneous! You see evolution can be quite rapid, it doesn't have to take thousands or millions of years. We can test it experimentally and observe it in our lifetimes. In the case of Whiptail Lizards a single mating event can sometimes spur an entirely new reproductively isolated species of lizards. 

I can't help but end in the same way I began, with a wonderful quote from Darwin's masterpiece which I think provides a simple observation regarding speciation and evolution, "....from so simple a beginning.... ENDLESS forms most beautiful and wonderful".  

A Fence or Spiny Lizard, genus Sceloporus, this genus of lizard is incredibly diverse.

References and further reading:

Knott, Jeffrey R., et al. "Reconstructing late Pliocene to middle Pleistocene Death Valley lakes and river systems as a test of pupfish (Cyprinodontidae) dispersal hypotheses." Geological Society of America Special Papers 439 (2008): 1-26.

Nosil, Patrik, Luke J. Harmon, and Ole Seehausen. "Ecological explanations for (incomplete) speciation." Trends in Ecology & Evolution 24.3 (2009): 145-156.

Santucci, Vincent L. "Historical perspectives on biodiversity and geodiversity."Geodiversity & Geoconservation 22.3 (2005): 29-34.

Blackwelder, Eliot. "Lake Manly: an extinct lake of Death Valley." Geographical Review (1933): 464-471.

Pianka, Eric R. "Zoogeography and speciation of Australian desert lizards: an ecological perspective." Copeia (1972): 127-145.

Cole, Charles J., et al. "Laboratory hybridization among North American whiptail lizards, including Aspidoscelis inornata arizonae× A. tigris marmorata (Squamata: Teiidae), ancestors of unisexual clones in nature." American Museum Novitates 3698 (2010): 1-43.

Goldman, E. A. "The Colorado River as a barrier in mammalian distribution."Journal of mammalogy 18.4 (1937): 427-435.

Tuesday, August 4, 2015

Arthropods in the Desert. A physiological paradox?

"The Spider's touch, how exquisitely fine! Feels at each thread, and lives along the line." -Alexander Pope

A tarantula in the Sonoran Desert.

When outsiders think of the desert, many immediately picture a landscape covered with all sorts of intimidating looking arthropods (which includes insects and arachnids among other things). This vision wouldn't be at all wrong. Arthropods are the most abundant and diverse organisms in most places, and deserts prove to be no exceptions.

However, arthropods surviving in the desert may be of at least some surprise at least from a physiological perspective. In fact, arthropod diversity is lower in extremely dry areas, compared to areas which have higher humidity. Recall from the recent post about water in the desert that for some groups of organisms wetlands increase species richness in deserts not by supporting communities that are more diverse than arid communities, but by supporting unique communities which could not exist in more arid areas. For arthropods in the desert, wetter areas tend to support a higher number of species in addition to supporting taxa which could not live in dryer areas.

Deserts test arthropods in a number of ways, so their survival here should be more of a surprise than an expectation.

Test 1: Arthropods in many cases cannot exploit evaporative cooling unlike many desert organisms.

Dogs, humans, sheep, and camels are all animals which can exploit evaporative water loss as a mechanism for regulating body temperature. Recently it was shown that Gila Monsters use evaporative cooling through their cloaca to regulate temperature. The larger your size, the better you are able to exploit this mechanism for thermoregulation. Insects and other arthropods have been thought for many years to be too small to be able to thermoregulate in this fashion. It would require more water than they are able to expend for many species of small insect. Being unable to exploit this process could be a huge disadvantage for some desert arthropods. Though recently some have argued that the body size limit for cooling in this manner is smaller than previously thought, and there is evidence that desert cicadas do use evaporative cooling. It is still difficult for many arthropods to use evaporation to maintain body temperature due to their small size.

Instead of exploiting evaporative cooling many arthropods thermoregulate behaviorally by moving between shade and sun during the course of the day or entering and exiting burrows.

Test 2: High respiratory and cuticular(surface) water loss.

Insects and arachnids have respiratory organs which are quite different from our own. Insects breathe through spiracles, tiny pores on their body which allow oxygen to diffuse in. When these spiracles are open insects are incredibly sensitive to water loss to their environment.

In wet environments many arthropods exhibit very high rates of water loss. Desert insects exhibit water loss that can be as little as 1/5 of the rate of loss in related species found in wet environments. Desert arthropods often have waxy surfaces and lipids on their exoskeleton  which prevent water loss.  Scorpions exhibit much lower rate of water loss during the summer than they do in the winter because as temperature increases they ramp up production of these protective coatings.

Next time you are thirsty try holding your breath, this is what some insects do, perhaps in order to minimize respiratory water loss.
Some insects have also evolved "discontinuous gas exchange". Instead of keeping their spiracles (respiratory organs) open all the time they open and close them in a cyclic manner. Keeping spiracles closed does seem to reduce water loss and minimizing water loss could be one adaptive advantage of discontinuous gas exchange in insects. Though how and why discontinuous gas exchange evolved in insects is not completely resolved.

Test 3: Prolonged high temperatures and low thermal tolerances.

Extreme heat is a test for every desert organism, and arthropods are no exception. The average summer high temperatures in many hot deserts is higher than the critical thermal maximum for many arthropod species we consider synonymous with deserts, including the American Cockroach. In fact for 4 months of the year desert high temperatures are at or above the critical maximum for this species. Many other insect species have critical thermal maxima that are near or the same as the American Cockroach. No wonder desert dwellers find cockroaches and other insects so often have taken refuge in their air conditioned homes in the summer months.

Arachnids are however more tolerant of very high temperatures. The sun spider (Solpugidae), which looks like it evolved in a nightmare, has a critical thermal maximum of 50 degrees Celsius (122 F). Good luck killing that horror.

So next time you're in the desert and happen upon an unnerving arthropod don't be so quick to judge. That these small animals can survive at all in the deserts, despite the plethora of challenges they are presented with (just by virtue of being an arthropod) deserves our respect and perhaps our admiration.

Arachnids are already the stuff of nightmares for many. They are remarkably well adapted to high temperatures compared to other arthropods.

References and further reading

Lighton, John RB. "Discontinuous gas exchange in insects." Annual review of entomology 41.1 (1996): 309-324.

Schmidt-Nielsen, Knut, and Bodil Schmidt-Nielsen. "Water metabolism of desert mammals." 

Prange, Henry D. "Evaporative cooling in insects." Journal of Insect Physiology42.5 (1996): 493-499.

Tigar, Barbara J., and Patrick E. Osborne. "Patterns of arthropod abundance and diversity in an Arabian desert." Ecography 20.6 (1997): 550-558.

Edney, E. B. "Water balance in desert arthropods." Science 156.3778 (1967): 1059-1066.

DeNardo, Dale F., Tricia E. Zubal, and Ty CM Hoffman. "Cloacal evaporative cooling: a previously undescribed means of increasing evaporative water loss at higher temperatures in a desert ectotherm, the Gila monster Heloderma suspectum." Journal of experimental biology 207.6 (2004): 945-953.

CLOUDSLEY‐THOMPSON, J. L. "Lethal temperatures of some desert arthropods and the mechanism of heat death." Entomologia experimentalis et applicata 5.4 (1962): 270-280.