WATER 



185 



5. Burrowing and nocturnal habits 



6. Humidity control 



7. Migration 



8. Obtaining water from food and from 

 metabolism 



1. Impervious Integument 



Only those animals that have a relatively 

 or completely impermeable body covering 

 can invade the drier habitats. Reptiles, 

 birds, mammals, and many insects have 

 such an integument. Some mammals, nota- 

 bly men, apes, and horses, lose much water 

 (and salt) through sweat glands in heat 

 regulation. Most rodents and some rumi- 

 nants—antelopes, for example— nearly or 

 completely lack sweat glands. Moist-skinned 

 animals, certain mites, soft-bodied insects, 

 earthworms, and amphibians are terrestrial 

 hygrocoles restricted to swamps, stream 

 margins, moist soil, and other similarly 

 damp places, or they must be able to retire 

 readily to such niches. These are frequently 

 crepuscular, nocturnal, or shade-living crea- 

 tures. The dry-skinned insects, reptiles, 

 birds, and the nonsweating mammals are 

 adapted to live in drier habitats, but even 

 among them further adaptations are needed 

 before the comparatively dry regions can 

 be successfully occupied. 



2. Internal Lungs or Tracheal System 



The mode of respiration also is important. 

 The scaly body covering of a fish may be 

 practically impermeable to water, and ex- 

 changes may be limited to gills and gut. 

 Some few fishes, like the mudskipper, Per- 

 iophthalmus, can venture out of water into 

 moist ad-aqueous habitats. Crustacea, with 

 their gills covered by a water-retaining 

 carapace, carry with them a liquid environ- 

 ment for their gills. Though more terrestrial 

 than the mud-skipping fishes, land-dwelling 

 crayfishes burrow to water, and the more 

 terrestrial land crabs are not successful 

 invaders of dry habitats far from waters. In- 

 ternal lungs, whether in pulmonate snails, 

 land isopods, spiders, or higher vertebrates, 

 together with the internal tracheal system 

 of insects, are water-saving. Much water is 

 lost in breathing, even by animals equipped 

 with internal lungs. The loss in insects and 

 in many gastropods is less than might be 

 expected, since in these animals the exter- 

 nal openings close under excessively dry 

 conditions. 



3. Dry Excretions 



A further water-saving device is the ex- 

 cretion of concentrated, relatively dry nitro- 

 genous and fecal waste material. Again, as 

 in the osmotic relations of aquatic animals, 

 we are reminded that the organs secreting 

 nitrogenous wastes, whether malpighian 

 tubules of insects or kidneys of vertebrates, 

 have important ecological relations. Even 

 the land mammals least dependent on water 

 conservation concentrate their urine by 

 active transfer of water into the blood 

 sti'eam against the osmotic gradient. Water- 

 saving insects, reptiles, and birds dispose 

 of their nitrogenous wastes as solid uric 

 acid; the ostrich, though a bird of dry re- 

 gions, is an exception and secretes hquid 

 urea. The deposition of dry feces is com- 

 mon among water-saving animals; the dry 

 fecal deposits of rodents and antelope con- 

 trast strikingly with the more liquid feces 

 of cattle. Insects, reptiles, and birds typi- 

 cally deposit fairly dry feces. 



4. Suspended Animation 



Some animals with simpler organization, 

 especially the bryocoles— such as tardi- 

 grades, rotifers, and nematode worms— can 

 retain their vitality in long-continued drying 

 in direct sunlight and regain activity when 

 water is again available. Desert snails also 

 are resistant to drying (p. 20). These ani- 

 mals are not completely desiccated, al- 

 though they approach that condition. Other 

 animals aestivate during droughts, and are 

 active only in the moister seasons of the 

 year. 



5. Burrowing and Nocturnal Habits and 

 Modes of Humidity Control 



A simple form of aestivation is closely 

 associated with burrowing down to con- 

 tinually moist earth and there remaining 

 dormant until the rains come. Various types 

 of frogs and toads have this habit, as do 

 some aquatic forms that live in lakes, ponds, 

 or streams that often become dry— the 

 African lungfish, for example. Still other 

 animals die oflF, leaving resistant eggs that 

 are often protected by impervious egg 

 cases. Related desiccation-resistant devices 

 include the gemmules of fresh-water spon- 

 ges and the statoblasts of bryozoans. Noc- 

 turnal animals are exposed to lowered 

 temperature and lower relative humidity, 

 conditions that tend to reduce the rate of 



