Desert Mammals: Surviving Dehydration

Introduction

Desert mammals face the significant challenge of dehydration due to rapid water loss.
They generally lose more water than other terrestrial species.

Ways Mammals Lose Water

Respiratory Evaporative Water Loss: Water is lost through breathing.
Sweating: Mammals sweat to cool down, which causes water to evaporate and take heat away, but it also leads to water loss.
Urea Excretion: Nitrogenous waste is converted into urea, which is toxic; mammals must eliminate it but the exception is the camel.

Mammalian Adaptations for Water Conservation

Concentrated Urine: Mammals can produce highly concentrated urine to reabsorb as much water as possible.
Dry Feces: Producing very dry feces is another way to conserve water.
Highly Effective Kidneys: Kidneys with long loops of Henle are crucial for concentrating urine, utilizing a counter current multiplier system.

Kidney Structure and Function

Kidneys are made up of hundreds of thousands of nephrons arranged in parallel.
Nephrons are more complex in mammals than in reptiles or birds.
Filtration occurs in the nephron, followed by a series of convoluted tubules that end in the loop of Henle.
Blood supply around the loop of Henle helps concentrate the urine.
Ions are pumped across into the immediate area, drawing water out of the urine.
Urine contains ions, proteins, and urea, eventually going into a collecting duct to the bladder.

Osmolarity and Urine Concentration

Osmolarity is a measure of osmotic pressure; plasma has a value of about $280$ milliosmols.
Reptiles have urine concentration similar to plasma.
Birds show some ability to concentrate urine.
Mammals have a significant ability to concentrate urine, especially small mammals like the kangaroo rat and hopping mouse.
The hopping mouse can concentrate its urine up to $9,370$ milliosmols, whereas humans can only concentrate urine up to about $1,400$ milliosmols.

Water Balance Strategies

Strategies depend on body size; small mammals differ from larger mammals.
There is a close relationship between body temperature and water balance in larger mammals.

Small Mammals

Small mammals tend to use behavioral responses more than physiological specializations.
They avoid harsh conditions by living in burrows, which are cooler and more humid.
These animals can survive without access to standing water.
Kangaroo Rat: A rodent living in deserts (Mexico and Southwest United States), weighing about $30$ to $60$ grams.
- They are nocturnal and hop along the desert surface.
- They occupy large dens with multiple entrances and organized chambers for food storage and resting.
- They seal up the entrance of these burrows during the day to keep the conditions underground, nice and cool and humid.
- Can survive for prolonged periods without drinking water if fed on dry seeds (high carbohydrate diet).

Schmidt-Nielsen Experiment with Kangaroo Rats

Kangaroo rats weighing about $35$ grams were fed on barley seeds for four weeks.
Quantified water loss and gain.
90% of water gained through metabolism and 10% absorbed from dry seeds.
25% water lost in urine, a small amount in feces, and 70% through evaporation.
Recycling of water occurs as moisture from expired air is soaked up by stored seeds.
These animals have specialized kidneys with long loops of Henle for high water reabsorption.

Specialized Kidneys in Small Mammals

These kidneys include long loops of Henle.
Water reabsorption is among the highest in mammals.
They are able to concentrate their urine significantly more than humans (nearly ten times higher).

Counter Current Water Exchange in the Nose

Effective counter current water exchange helps conserve water.
Kangaroo rats can reduce the temperature of air leaving the nose down to about $27$ degrees Celsius, saving water.

Kangaroo Rat Water Balance Summary

Gaining metabolic water from dry seeds.
Free water absorbed by seeds.
Respiratory moisture in the nose.
Concentrating urine.
Producing dry fecal pellets.

Recent Studies on Kangaroo Rat Behavior

Studies in the 2000s examined animals in the wild.
In winter, kangaroo rats have a varied diet, including vegetation and insects.
They are more tolerant of high temperatures.
They hunker down in burrows and feed on dry seeds when conditions are severe.

Echidnas

Monotremes found in Australia and Papua New Guinea.
They blow bubbles of mucus on their snouts for temperature regulation.
Evaporation cools the snout, decreasing body temperature by $10$ degrees Celsius.
Has a blood supply just underneath the surface.

Implications of Water Loss in Echidnas

The evaporation involves water loss but serves a vital cooling function.

Larger Mammals

Ungulates and marsupials (like kangaroos) adapt by allowing body temperatures to increase during the day and decrease at night.
This is known as adaptive heterothermy.

Reasons for Adaptive Heterothermy

Large size results in a small surface area to volume ratio.
Fur makes it difficult to lose heat, reducing the need for water to cool down.
They reduce water in feces and produce concentrated urine.
They can survive long periods without water.

Water Saving Strategies in Large Mammals

They obtain most water by drinking and a small amount from food.
26% of water is gained via metabolism.
43% water lost during evaporation.
The rest in urine and feces.

Water Saving Strategies in Small Mammals

Evaporative water loss is even higher (70%).
Some water lost in urine.
If not given water, they can survive; water comes from dry seeds.

Camels

Camels are specialized for living in hot deserts and show adaptive heterothermy.

Body Temperature Regulation in Camels

They warm up to about $41$ degrees Celsius during the day and cool down to about $35$ degrees Celsius at night.
Depends on access to water; fluctuations more exaggerated when water is scarce.
It saves water that's not lost by evaporation.
It can reduce water loss quite considerably by five liters.

Water Loss Tolerance and Replenishment

Camels endure water loss, with blood volume staying the same but tissues becoming dehydrated.
They can survive for two weeks without drinking.
They are also known to store urea, which is toxic.

Rapid Rehydration

They can drink $200$ liters in three minutes (though this value is debated), equivalent to 400 pints of water.
They carry water in the intestine and gradually absorb it to prevent dilution of blood.

Nasal Counter Current Exchanges

Camels have broad nasal passages with high surface area and dry mucus for absorbing moisture.
60% of water from the lungs is retained in the nose.

Field Studies on Camels

The original camel was carried out on four camels that were corralled together.
Researchers have repeated these observations in the field.

Springbok Study

Researchers implanted transmitters in Springbok to monitor body temperature.
These transmitters were just looking at body temperature.
Springbok roaming in the wild did not show adaptive heterothermy.

Giraffe Spots

Giraffes have different patterns of spots; no two giraffes have the same pattern.
Larger spots may cause overheating due to dark color.
Network of capillaries underneath spots; giraffes with larger, darker spots may have problems with water balance.

Sand Gazelles

Size of goats, living in the Arabian Desert.
Heart and liver shrink during starvation and water deprivation.

Organ Size Reduction

Liver shrinks in size by 79%.
Reduces metabolic rate.
Saves water (reduced water loss by a third).
These mammals are undergoing really drastic changes for short periods of time.

Kangaroos

Some kangaroos live in deserts in Australia.
Main route of water loss: respiratory evaporative water loss.

Water Conservation Strategies in Kangaroos

Licking forearms to cool down via saliva evaporation (though attracting flies).
Concentrating urine.
Having slightly lower body temperatures and metabolic rates.

Red vs. Eastern Gray Kangaroos

Reds live in desert areas; eastern grays live in more moist areas.
Eastern grays drink twice as much as reds and produce less concentrated urine.
The Eastern gray kangaroo is invading the territory of the red kangaroo, due to access to water provided for cattle ranches, highlighting human intervention's impact.

Water Turnover and Body Mass

Endotherms have higher turnovers than ectotherms.
Water turnover in desert animals is lower than in temperate environments.

Water Balance Strategies

Summary of water balance strategies according to morphological, physiological, and behavioral mechanisms.