Water Conservation and Economy Species Differences - Physiology

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Species Differences in the Ability To Conserve Water

Mammals and birds can produce urine both more and less concentrated than plasma in order to conserve or excrete water. The excretion is done without interfering with the levels of solute excretion. The ability to concentrate urine depends greatly on a species access to water. Animals with access to lots of water have less of a need and therefore have not evolved great mechanisms to concentrate urine for example the beaver. However animals who have limited access to water and say live in more arid environments have evolved very good mechanisms to reabsorb lots of water and produce a very concentrated urine. An example of this is the kangaroo rat which lives in a desert habitat.

Nephron Species Differences

The variation in nephron structure is pivotal to the species differences with regard to the ability to concentrate urine. Broadly there are two structures.

Juxtamedullary Nephrons

These nephrons have glomeruli in the cortex and loops of henle which descend into the medulla. These are the commonly described nephrons. Significant urine concentration is achieved thanks to the hyperosmolar medulla achieved via the counter current multiplier drawing water out of the collecting ducts.

Cortical Nephrons

These nephrons have glomeruli in the outer cortex and their loops of henle barely penetrate the medulla. They have very limited concentrating ability.

The length of the loop of henle is the determining factor with regard to how concentrated urine an animal can produce. In species which produce very concentrated urine almost if not all of their nephrons are juxtamedullary in type and they tend to have very long loops of henle. However animals such as beavers, where dehydration is not really an issue, have very short loops of henle and excrete dilute urine. However one surprise is that the kangaroo rat which can concentrate urine to the equivalent of a 20% saline solution actually has relatively shorter nephrons than some larger species with less ability to concentrate urine. This has been attributed to it having a much higher metabolic rate and thus more active transport meaning greater gradients can occur on any given length of its loop of henle.

The following table shows a comparison between the maximum concentrating abilities of the kidneys of various mammals. Data from Physiology of Domestic Animals - Sjaastad, Hove and Sand. For full text reference see the Reference Material section.

Species	Max Urine Osmolarity (mosmol/l)	Urine/Plasma Concentration Ratio
Beaver	520	2
Calf	500	2
Pig	1100	4
Humans	1200	4
Cow	1400	5
Sheep	3500	11
Horse	2000	7
Dog	2500	8
Cat	3000	10
Kangaroo Rat	5500	18

Species Differences in Water Economy

Animals which are native to desert environments such as camels and sheep tend to tolerate heat better than none familiar animals such as cows and dogs. This can be seen if you look at how body mass decreases per day without water in 40 degree heat. Camels and sheep drop 2 and 4% respectively where as cattle drop around 8%. Meaning the situation is life threatening for these animals within a few days. Animals tend to use the evaporative cooling of sweating in order to maintain body temperature. However small animals thanks to their large surface area to body mass would have to lose up to 30% of their body mass per hour for this to be sufficient so the majority of them seek shelter during the hottest parts of the day. This behavioural adaptation helps the kangaroo rat to survive almost entirely on metabolic water.

Camels

As these are large animals they immediately have the advantage of a relatively small surface area to body mass ratio meaning they receive less heat from their surroundings. On top of this they also actually lose less water than would be expected from sweating. This is because a camel is able to retain heat energy in its body with no adverse effects. If well hydrated it is common for an increase of 2 degrees over 12 hours to be seen however when a camel is severely dehydrated it can vary its body temperature by up to 7 degrees. It then loses this heat during the cooler night by radiation. This means when dehydrated the camel loses 6-7 litres of water less per 24 hour period than when it is well hydrated. The camel’s thick fur coat also helps by shielding the camel from some of the suns rays.

As well as these adaptations to reduce water loss by evaporation the camel is very frugal in its water wastage. It is able to produce very concentrated urine and loses very little water in its faeces. It is also able to tolerate water losses of up to 25-30% compared to most domestic species of around 10-12%.

These combined adaptations allow a camel to survive for 1 week in a desert environment with no water compared to 24 hours for a human.

Camels are also able to replenish the lost water quickly. For example a 500kg animal at full dehydration will drink ~140l in a few minutes of access to water. Although this massive intake does cause the blood to become hypo-osmolar it does not cause haemolysis because the camels erythrocytes are able to increase their water content a lot before bursting thanks to their flattened nature.

Sheep

Sheep reduce the heat loss required by panting thanks to a thick coat of wool which insulates and protects the animal underneath from the full heat of the day. This help conserve water. The ability to tolerate heat in this way is very breed specific. Merinos have a very well developed wool coat and thus cope very well however European breeds are not so good. Sheep are also able to minimise water loss in the urine and can when needed lose very little water in their faeces.

Sheep are able to tolerate water losses of up to 30% like camels and also drink very quickly when given access to water to try and replenish supply. However their erythrocytes are not adapted to cope with a sudden influx of water unlike camels but they do not need to be as the water is stored in the fore stomachs and absorbed at a steady rate.

Cattle

Although they have a smaller surface area to body mass ratio than sheep they lose water more quickly through the faeces and the urine when ambient temperature is high. They also have a far less developed coat. Indian Cattle (Bos Indicus) such as the Brahman and Zebu are better able to tolerate high temperatures than European cattle but only when water is readily available as their increased tolerance comes from an increased sweating ability which would be of no advantage if water was short. This dependence on water is obvious when you look at behaviour. Cattle will not willingly roam more than 1 day from water where as sheep and camels will move 3 and 6 days away respectively.