Chromatophores and Color Change 



715 



er^^^ has pointed out the possibiHty of an alternative interpretation, that the 

 pigment cells in these forms may show direct responses when their state is 

 not determined by a dominating mechanism. 



In summary we appear to find among the reptiles, as with fishes, varying 

 degrees of evolution of chromatophore control from systems involving a 

 probably primitive hormonal mechanism of coordination of the chromato- 

 phores, through those in which both nerves and hormones cooperate, to those 

 largely dominated by direct nervous innervation. 



FUNCTIONAL SIGNIFICANCE OF CHROMATOPHORES 

 AND COLOR CHANGE 



Since the responses of the chromatophores are predominantly responses 

 to color or shade of background, one is led to the hypothesis that the color 

 changes contribute significantly to the obliterative coloration of the ani- 

 mal for protection or aggression and hence increase its chances for sur- 

 vival. One of the few experimental demonstrations that physiological color 



•Species of Fishes 



Fig. 275 



ninutes' after c/ionfe of backgrvund 



Fig. 276 



Fig. 275. Percentage choice of a black over a white background for each of eight species 

 of fishes adapted to black and white backgrounds. From Brown and Thompson.'*^ 



Fig. 276. The rate of change in percentage choice of a black background following 

 transfer of Ericymha from a black to a white background and vice versa. From Brown 

 and Thompson.^ 



changes do actually increase chances for survival, a view often questioned, 

 is that of Sumner,^*^^ in which he found that fishes given time to change 

 their coloration were seized in smaller percentages by a predatory bird than 

 were unadapted ones. It has also been clearly demonstrated that fishes 

 which are black-adapted tend to select black backgrounds, when given a 

 choice of black or white, more commonly than do white-adapted individu- 

 3ls.35. no {^pjg 275), and that the rate of change of choice with change of 

 background in Ericymha is approximately the same as the change in skin 



