Review Essay On Physical Adaptations Of Small Mammals In Desert Environments

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Introduction

Deserts known for their oven air temperature (38oC or higher), intense radiation from the sun, hot soil surface and the absence of surface water, it’s not surprising that even the average human can only last a couple of hours in these extreme conditions. In this review essay, we will be looking at the physiology of small desert mammals and how over time their behavioural and physiology has adapted so that they have been able to live in an environment with such scarce resources. First signs of life originated in aquatic habitats that secondarily invaded terrestrial habitats, this meant that species that evolved to live on to land had to go through extreme significant changes to their physiology in order to adapt and create coping mechanisms to thrive in these radical changes/differences in environment, differences such as a aquatic environment, a low temperature wet conditions to the likes of deserts that have a high temperature range and being able to deal with intense short wave radiations throughout the day. Due to low precipitation, the lack of water and high seasonal temperatures and presented with high heat and aridity makes for profound challenges to the species that inhabit its vast regions, especially mammals. Deserts such as the Savannas’ has a high level of net primary productivity compared to the biomass (dry mass organic matter) of vegetation at one time (Noy-Meir 1973) New south Wales desert has an average rainfall under an inch of rain, due to the droughts it experiences which are up to a size of California. Though deserts are generally known for heat stress which is accompanied by low atmospheric humidity, even the Polar Regions are classified as deserts, for its lack of natural resources and flora, mammals in these regions show an exaggeration of characteristic of metabolic heat generation which is presented with physical factors such as body fat or thick fur which provides for insulation.

Small Desert mammals/Rodents

Small desert mammals fall into the same needs as other mammals in their characteristics on which they need to survive. All mammals need to constantly maintain a core body temperature of 37oC – 38oC in order for them to maintain their regular bodily functions, like the need for maintain water and save energy resources. In a non desert environment mammals use primary thermoregulatory to keep their core body temperature higher than their surrounding environment in order to keep warm, in deserts these mammals function in a completely opposite manner, in the desert due to being presented with the high environment temperature that exceed a mammals normal body temperature, their use for thermoregulation relies on being able to effectively remove the excess heat that builds up in their bodies to stop them from overheating in these conditions (Louw and Seely 1982). Due to the lack of resources in these environments such as water which would be natural for mammals to use too cool down, thermoregulation is vital in their role for survival in environment.

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Desert rodents are a diverse group that, play an important role in desert communities, these creatures have been able to maintain their ecosystem, by modifying soil structures, consumers of plants, and prey for other species. Though due to their small build it has prevented them from travelling far distances to areas where there is widely spaced surface water, so in order for these desert rodents to survive they have had to adapt these extremely hot environments without drinking water.

Because a majority of desert rodents are nocturnal they occupy burrows to avoid the day time heat, this helps with thermoregulation, and allows for periods of cooling and allows for them to escape the dry humidity. The ideal temperature for desert rodents below ground level has been shown to be around 30oC during their summer months. In Mexico, there is a species of desert rodent called Merriam’s kangaroo rat (dipodomys merriami) (Walsberg.G 2000), which have been known to build burrows below the ground, their nest chambers are normally the deepest part of the burrow this is due to it being the coolest part of the burrow so that when they sleep they aren’t using up excess energy in thermoregulation to cool down and helps to prevent water loss from excess heat evaporation (sweating). In the Sonoran desert it is estimated that 93% of the rodents that inhabit it are nocturnal this behaviour is so that they can alternate between cool burrows during the day and the dry atmosphere of the deserts soil surface at night.

In the desert nocturnal temperature range falls by 15o C – 20oC below the maximum temperature during the day, because of this the daytime temperature can reach up to 40oC – 45oC that at nocturnal minima will drop as low as 25oC – 30oC. This comparison with other deserts makes it so the nocturnal minima of deserts that are located in higher altitudes and latitudes. Because of these harsh temperatures small desert mammals have adapted crucial features in order to survive around their physiology; such as how precisely small desert mammals are able to regulate their core body temperature to such a margin with tactics as torpor which is used to avoid water shortages that come with the dry seasons or energy expenditure, which is a massive cause of water gain and loss (Walsberg.G 2000). Although evolution has created solutions for all small mammals overall there are key similarities in the physiology of small desert mammals which have been developed.

Hetrothermy

Mammals that inhabit the desert are divided into two different groups which are those that are able to tolerate high temperatures for an extensive period of time and those that have a limited tolerance under these conditions. There are only 4 species of small desert mammals that spend their time in day light hours which are a species of squirrels such as the antelope squirrels (ammospermophilus harrisi) and the round- tail squirrel (spermophilus tereticadus). They display similar traits to that of larger mammals by exhibiting hetrothermy by keeping substaintial elevations of body temperature when exposed to high ambient temperatures, the antelope squirrels that are wild in the North American desert display short-term (less than an hour), major variations ranging from 4oC-5oC in their body temperature when they are active during hot summer season (Chappell and Bartholomew 1981). They are able to allow their body temperature to reach just higher than 40oC before having to retreat to their burrows so they are able to rapidly cool back down. This behavior cycle is useful as it stops the need for evaporation and water loss within the body; this is achieved by the animal by pressing its abdomen on the cool burrow floor using the cool surface area to lower their body temperature.

On the other hand desert mammals that have a limited tolerance, species such as the kangaroo rat like many desert rodents are only able to tolerate a limited exposure to the high temperatures of the desert environments; they typically have a resting body temperature of 37oC – 38oC when the environment temperature is below 35oC. Soranon desert averages only 3 hours per night where air temperature 3cm above ground fall to around 35oC, this limits the ability of to sustain exercise which only gives a small window where desert rodents and get around as only 5 hours of the night will be around 38oC. When the environment temperature is above 35oC their body temperature will begin to increase along with it, if body temperature hit 38oC- 40oC the animal will only be able to tolerate this for roughly an hour before it is at considerable risk of dying. Ord’s kangaroo rats (Dipodomys ordis) are able to exercise by running for periods of about 20 minutes at a given time; there body temperature can be regulated within a margin of 1.5oC -3oC above its resting level, though this is only able to be sustained for a small period of time before they are put at risk of being in with lethal levels. Extreme hetrothermy in desert mammals is used to help deal with environment temperatures that surpass that of the core body temperature, in a hot environment by using thermoregulation the animal will raise its own body temperature to lower the difference of the environmental temperature to reduce the difference between these two factors, by doing this it lowers the need for evaporation cooling and helps the animal sustain its body’s water levels, if the environment temperature is lower than the body temperature; then the body temperature will be lowered, this will help conserve energy by trying to keep the animal warm we can see this in behaviors such as hibernation where an animal will burn major levels of fat to keep its body functioning over the winter periods.

Water Balance

A common conception is that it’s believed rodents survive only on a diet of seeds and nuts to maintain their water levels without the actual need for drinking, though this theory does hold ground and is generally accepted throughout the scientific community, it’s still up for debate on whether this is the case. Hetermoyid rodents, particularly the kangaroo rat, have bore countless studies that show a dry diet, with no signs of drinking; this goes back to the first studies that brought into question how their physiology works in their favor, because low temperatures is such a rare case during summer in hot subtropical deserts. The maximum temperature that is known for mammals to maintain it water levels is only 26oC and the temperature that’s calculated for its metabolic water production that balances its evaporative loss on its diet in a kangaroo rat is in the range of 16oC-17oC. Kangaroo rats (Dipodomys) are able preserve their water levels by producing high concentrations of urine as waste (Walsberg.G 2000) and dry fecal matter, this allows for minimal water loss within its body. Evaporation is a major water loss from its respiratory tracks, they have a nasal counterflow system that reduces water loss that is caused by pulmonary evaporation, this is achieved when air is inhaled and flows over the membranes that cover the animal’s nasal turbinate bones. Due to the flood of dry air it evaporates water from the nasal mucosa, which in turn has a chilling effect (Walsberg.G 2000). During the action of exhaling occurs the warm air present in the lungs passes over the mucosa so the exhaled air is cooled down and condenses moisture which passes over the nasal mucosa that in turn allows for heat and water to be conserved by the animal.

For kangaroo rats (dipodomys) and other small desert mammals in order to be able to produce high concentration urine, they have specially adapted to possess long loops of henle, in their kidneys they have effectors organs which help with the maintenance of salt and their excretion products. The tissue of their kidneys is made of nephrons, these are thin walled tubles, these are ducts that’s role is to collect and transfer urine to the ureter, by going through the papilla (Ojeda, R. A., Tabeni, S. and Corbalán, V. 2011). There are two types of nephron as we can see below (figure 1) The nephron has a part called the bowman capsule which acts as a cup and surrounds a “cluster” of capillaries also known as glomerulus, the blood goes through the medulla to undergo “super” filtration, if we follow the route of the bowman capsule it leads through the proximal convoluted tubule, this leads to the loop of henle. The medulla role is to help with the fileation of sodium, salt and water, at the end of the proximal convoluted tuble there is the medullary collecting duct which is to reabsorb the water before releasing the high concentration of urine. (figure 2)

Figure1 – Two typical nephrons. The one on the right is long-looped and the one on the left is short-looped. The long-looped nephron is paralleled by a loop formed by the blood capillary. A capillary network surrounds the short-looped nephron.

Figure two – A partial view of the nephron that shows the proximal convoluted tubule, these are the limbs of the loop of Henle, the distal convoluted tubule and collecting ducts.

Reproduction

Though those in the small desert rodent family showed signs of being able to be reproductive all year round, it was apparent that mating was mainly seasonal. It’s suggested that 50% of males with the merriums kangaroo rats (dipodomys merriami) remained in a physically fit condition where breeding was possible, apart from the month of October where because of the season would experience major loss in its body mass. In contrast the female population it was suggested that only 37% of their population would show signs of reproduction, especially in months in the middle of winter and summer season (Zeng, Z., & Brown, J. 1987). Their main reproduction would be around march, most mammals will time their reproductively around this time as in spring there is a increase in rainfall, due to the presence of rain some females in the species will experience lactation before hand as it’s a sign for abundant supply of food and the resources that are available comes with a higher success rate for the survival of the next generation.

Conclusion

Small desert mammals are faced with many challenges from their small statue, having to deal with extreme temperatures, the lack of resources such as lack of water and food supplies in certain times of the years, but because of this they have adapted and evolved over time to survive in the conditions very effectively to their genetic structure and their behavioral traits such as surviving underground to escape the heat of the long days and yearly, I think that in present years to come with the cause of global warming and the increase of day to day temperature it could cause even more challenges in these animals lives as deserts begin to grow bigger and bigger taking over grassland, which could present a challenge for their survival or the lack of rainfall meaning reproductive seasons becoming out of sync. More research is required in seeing how far these creatures will fair in the coming years and how they will further adapt to these changes.

Bibliography

  1. Chappell, M. A. and Bartholomew, G. A. (1981) “Activity and Thermoregulation of the Antelope Ground Squirrel Ammospermophilus Leucurus in Winter and Summer,” Physiological Zoology, 54(2), pp. 215–223.
  2. Louw, G. and Seely, M. (1982) Ecology of desert organisms. Essex, UK: Longman Group Limited (Tropical ecology series).
  3. Noy-Meir, I. (1973) “Desert Ecosystems: Environment and Producers,” Annual Review of Ecology and Systematics, 4, pp. 25–51.
  4. Ojeda, R. A., Tabeni, S. and Corbalán, V. (2011) “Mammals of the Monte Desert: From Regional to Local Assemblages,” Journal of Mammalogy, 92(6), pp. 1236–1244. doi: 10.1644/10-MAMM-S-352.1.
  5. WALSBERG, G. L. E. N. N. E. (2000) “Small Mammals in Hot Deserts: Some Generalizations Revisited,” BioScience, 50(2), pp. 109–120.
  6. Zeng, Z., & Brown, J. (1987). Population Ecology of a Desert Rodent: Dipodomys Merriami in the Chihuahuan Desert. Ecology, 68(5), 1328-1340.

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