Re: nocturnal eyes question

[Mickey Rowe <rowe@psych.ucsb.edu>]

Christopher Taylor (ck.taylor@auckland.ac.nz) wrote:

> A recent study (can't track down the exact reference to it - I only
> glanced across it while looking for something else - someone help me
> please.... :S) looked at a close fossil relative to platyrrhines
> (New World monkeys) which has been considered nocturnal because of
> its large orbits. CAT scans of the braincase showed, however, that
> the olfactory bulbs in this beastie were fairly small, despite a
> sense of smell being usually well-developed in nocturnal
> mammals. The authors concluded that the animal was more likely to
> have been diurnal, and the large orbits were a relic from nocturnal
> ancestors (which it definitely had).

I don't know what that's about (would appreciate the reference if you
can dig it up), but anyhoo, the reference you probably want to see if
you're interested in this is:

Heesy, C.P., and Ross, C.F. (2001).  "Evolution of Activity Patterns
     and Chromatic Vision in Primates: Morphometrics, Genetics and
     Cladistics", _Journal of Human Evolution_, 40:111-149.

Keep in mind that primates (and mammals more generally) might not be
models for all animals, but Heesy and Ross -- relying primarily on
data from:

Kay, R. F. & Cartmill, M. (1977). "Cranial morphology and adaptations
     of Palaechthon nacimienti and other Paromomyidae (Plesiadapoidea,
     ? Primates), with a description of a new genus and
     species", _Journal of Human Evolution_, 6:19-35.

state:

    When orbit diameter is plotted against skull length nocturnal taxa
    are differentiated from diurnal taxa below a skull length of 75
    mm. [...]  Above this skull length activity pattern cannot be
    definitively reconstructed from relative orbit diameter. The
    reason for this is not clear. At smaller body sizes the orbit
    closely approximates the size of the eye [note that primates don't
    give you sclerotic rings as an indicator of eye size -- MPR ],
    whereas at larger body sizes the eye accounts for much less of the
    volume of the orbit (Schultz, 1940; Kay & Cartmill, 1977). Because
    orbit size becomes an increasingly inaccurate measure of eye size
    with increasing body size, activity pattern may not be reflected
    in relative orbit size because orbit size is not indicative of eye
    size. Another possibility is that orbit size is a sufficiently
    good estimator of eye size at large body size, but that eye size
    is not reflective of activity pattern at large body size
    (Cartmill, 1972; Ross, 1995). Regardless of the reasons for the
    breakdown of this relationship at larger skull sizes, relative
    orbit size can discriminate between nocturnal and diurnal primates
    at small skull sizes.

The Cartmill and Ross references are:

Cartmill, M. (1972). "Arboreal adaptations and the origin of the Order
     Primates". In (R. Tuttle, Ed.) _The Functional and Evolutionary
     Biology of Primates_, pp. 97-122. Chicago: Aldine.

and:

Ross, C. F. (1995). "Allometric and Functional Influences on Primate
     Orbit Orientation and the Origins of the Anthropoidea", _Journal
     of Human Evolution_ 29:201-227. 

More generally, bigger eyes can mean "better" vision in terms of
higher acuity (bigger eyes allow you to image the world onto a larger
area, so if receptor sizes and areal densities stay the same, then
bigger eyes allow you to see smaller objects at a given distance), or
higher sensitivity (larger eyes can collect more light which is
important when light levels are low and you want to collect as much
light as possible).  So the former definition of "better" would select
for larger eyes even in diurnal animals.  However, in this regard, you
can only make use of those larger eyes if you have more neurons to
detect and process the information, and that's a tradeoff that helps
to set eye size in all animals.  You don't need all the extra neurons
if you're only developing large eyes to help you see under low light
conditions.  The best way to see "well" in the dark is to add together
the outputs of a large number of small receptors.  That means that you
can improve sensitivity with a large eye and not a lot more neurons
(aside from the receptors).  The tradeoffs are different, so it pays
more to develop large eyes to improve sensitivity than to improve
acuity.

Bottom line is that larger eye size *does* generally correlate with
nocturnal activity patterns.  Even where that correlation breaks down
in primates with large skull sizes, it's still pretty good.  Although
the distributions of the ratio of orbit size to skull size for
nocturnal and diurnal primates overlap at large absolute skull size,
the overlap is incomplete.  Nocturnal animals still tend to have
larger ratios than do diurnal animals at any given skull size
(according to Heesy and Ross's figure anyways).

-- 
Mickey Rowe     (rowe@psych.ucsb.edu)