1. Biology 2672a: Comparative Animal Physiology
Water balance in terrestrial animals

2. Desiccation is the single biggest problem for life in terrestrial habitats
Only a few phyla have made it on land
Essentially aquatic
Nematoda
Tardigrada
Moderately successful
Onychophora
Mollusca
Highly successful
Arthropoda
Vertebrata

3. Water in and out Sweat/ excretion/ evaporation across skin Respiratory
Absorption
(not in mammals!)
Respiratory
Metabolic
Drinking
Water in food
Metabolic water in food
Faeces
Urine

4. Metabolic water from food
Difference in the amount of water used to process the food along different pathways
Table 26.3

5. How to survive in desiccating (ie terrestrial) environments?
Carry more water
Survive losing more water
Lose water more slowly

6. Carry more water

7. Carry more water II D flies survive for longer
Selection of Drosophila for desiccation resistance
D flies survive for longer
D flies carry more bulk water (mostly bound to glycogen)
Gibbs et al J. Exp. Biol. 200:

8. Carry more water III
Most terrestrial amphibians can store 20-50% of their body weight as urine in their bladder
As they desiccate…
plasma becomes hyperosmotic to urine
Bladder becomes permeable to water
Active pumping of NaCl out of bladder
Water is reabsorbed from bladder into plasma.

9. Survive losing more water
Start with the same amount of water, but tolerate losing a larger proportion of that water
Camels: tolerate 30-40% loss of body weight by dehydration
(most other mammals: %)
Desiccation-selected flies don’t tolerate any more water loss than controls

10. Extreme: Survive losing more water by anhydrobiosis!
Some small aquatic animals can tolerate the loss of >99% of their body water
Rotifers
Nematodes
Artemia (cysts)
Tardigrades
Also see Box 26.4

11. Anhydrobiosis
Polypedilum vanderplanki

12. Losing water slowly works!

13. Lose water more slowly Sweat/ excretion/ evaporation across skin
Respiratory
Metabolic
Faeces
Urine

14. Evaporative water loss (EWL)
Respiratory
Cutaneous

15. EWL varies between species
Fig

16. Minimising respiratory water loss I: Countercurrent heat exchange to retain moisture
Hot air in lungs holds lots of air when saturated
Air cooled in nasal passages, moisture condenses
Fig 27.14

17. Camels Have a countercurrent exchanger in their nose
reduce evaporative water loss
Allow their body temperature to rise during the day and fall at night
reduce water loss from panting or sweating

18. Minimising respiratory water loss II: Spiracles in arthropods
Fig

19. Minimising cuticular water loss
Waxy cuticle
All animals!
Fig
Fig

20. Reducing excretory water loss
Efficient kidneys
Get rid of a lot of salt and wastes per unit water
Mammals, birds, insects
See next lecture for mechanisms
Efficient re-absorption of water from gut
Dry Faeces

21. Water in and out Sweat/ excretion/ evaporation across skin Respiratory
Absorption
(not in mammals!)
Respiratory
Metabolic
Drinking
Water in food
Metabolic water in food
Faeces
Urine

22. Telling between the strategies
Carry more water
% original water
Lose water more slowly
Water loss survived
Survive losing more water
Time

23. Reading for Thursday Way to Pee!: Kidneys and urine formationPp