WATER RELATIONS OF OTHER SPECIES 



131 



promptly after each reversal of air flow (B, fig. 78). The evidence 

 here too is clear that the rat frequents high humidities, and espe- 

 cially when it is in water deficit. 



How the difference of humidities is detected is not known, 

 except that the rat explores before settling in one position. Nor is 

 it known whether humidity is sensed as such or as an effective tem- 

 perature, as a rate of drying of nasal mucosa, as pain, or as touch. 

 I imagine that many environmental combinations attract and repel 

 a rat ; sometimes humidity or some correlative of it overrules other 

 factors in the preferences. Since water deficit reinforces the dis- 

 crimination, an ''operant" behavior is at stake. 



-30 -20 -10 



Total Water Load 



Preference for moister air (% of times tested) in relation to water 

 10 individuals. Two hundred tests are divided into 5 equal groups in 

 order of water load. Standard errors are indicated, and show that the 3 groups on 

 the left each differ significantly (P<.01) from 50 per cent preference. New data. 



Fig. 79. 

 deficit. Eat. 



It is clear, then, that rats frequent air of high humidity and 

 shun air of low humidity ; a behavior that in itself tends to preserve 

 water content. Deficit of water in the body augments the fre- 

 quenting of moist air, thereby economizing water in those physio- 

 logical states of water shortage where additional means of preser- 

 vation are appropriate. 



In brief, the rat's water exchanges are characterized by its par- 

 tition of losses, as compared with larger mammals. In turnover 

 about two-thirds of the loss is evaporative; even higher propor- 

 tions are evaporative in the losses at diverse deficits. But just as 



