188 
Regal 
raised by parietalectomy in Anolis caroUnen- 
sis (Roth and Ralph, 1976) and in Sceloporus 
magister (Engbretson and Hutchison, 1976). 
In part, this is evidence of involvement of 
the parietal eye in the control of circadian 
rhythms. The observations may help to ex- 
plain the greater exposure to sunlight re- 
ported by Stebbins and Eakin (1958). They 
had noted an extended exposure period for 
S. occidentalis. 
Palenschat (1964) reported that parietal- 
ectomized slow worm lizards (Anguis frag- 
ilis) tended to desynchronize from a light 
cycle, showed considerably more activity than 
controls under equivalent illumination; the 
cycle’s frequency increased. Further, he re- 
ported that, in normal but not in parie- 
talectomized lizards, violet light causes an 
increase in activity, suggesting that the 
stimulatory effect of violet light is mediated 
by the parietal eye. [Green light had the op- 
posite effect and stimulated experimentals 
but not controls.] Moehn (1974) investi- 
gated the effect of ultraviolet light on social 
behavior in a number of iguanid and agamid 
lizard species. Increases in agonistic be- 
havior were reported in all species following 
UV treatment. The effects were possibly 
mediated via the parietal eye. 
A rhythm of locomotor activity can be 
synchronized with a fluorescent light cycle 
after parietalectomy (Underwood and Men- 
aker, 1970; Underwood, 1973). This finding 
does not rule out the participation of the 
parietal eye in normal behavior (Underwood, 
1973). 
The reductions or loss of the parietal eye 
appears correlated with vertical perching 
habits of some species of lizards. This find- 
ing suggests that the function of the eye is 
related to information about the spectral 
quality of the zenith sky and not simply to 
sunlight. At the same time, it should be 
pointed out that lizards with the parietal eye 
also tend to be restricted to low latitudes 
(Gundy, 1974; Gundy et al., 1975), where 
stre.ss from dessication and thermal fluctua- 
tions is reduced. 
The light from the zenith can, in theory, 
give information about time of day since the 
“blueness” of the sky varies with the angle 
of the sun to the horizon (Rozenberg, 1966; 
McFarland and Munz, 1975). The blue sky 
consists of highly polarized short wave 
lengths of light with a maximum intensity 
at a 90° angle to the sun. Although atmos- 
pheric conditions introduce some variability, 
the ratio of short to middle wave lengths of 
light at the zenith (and the characteristics 
of polarization) could provide a guide to the 
time of day. Eder et al. (1975) claim that 
the anatomy of the eye would allow the dis- 
crimination of polarized light. 
Thus, one role of the eye could be time 
determination in the synchronization of 
rhythms or measurement of day length, when 
daily temperature is unpredictable. 
The spectral composition of the zenith 
might also be useful to basking lizards under 
relatively stable climatic regimens. Ecto- 
therms thermoregulate using a number of 
microclimatic heat sources and sinks that 
are changing from hour to hour. Anticipation 
of these changes could allow a finer adjust- 
ment to conditions than would trial and 
error. The available “climate space” may 
vary systematically throughout the year. For 
example, in the spring, activity temperatures 
may best be maintained in the middle of the 
day; in summer, the middle of the day may 
become too hot, and activity shows a bimodal 
pattern with morning and evening peaks 
(Heath, 1965). One way to anticipate chang- 
ing conditions would be to link activity with 
the spectral quality of the zenith. Thus, for 
most basking lizards in hot weather, the 
“bluer” zenith of morning and evening would 
be stimulatory and the “greener,” less polar- 
ized zenith of midday inhibitory. Both early 
and late in the year, with cooler tempera- 
tures, the “bluer” zenith might be inhibitory 
and the “greener” zenith of midday might be 
stimulatory. 
This point of discussion is not to advocate 
a particular hypothesis but to illustrate that 
the eye may have a variety of effects on 
behavior and physiology. The reporting of 
damage to the eye during surgery, lighting 
conditions, etc., may all contribute to repro- 
ducibility of studies. 
