Folia Primatol 2006;77:27–49
Eye Morphology in Cathemeral Lemurids
and Other Mammals
E. Christopher Kirk
University of Texas at Austin, Austin, Tex. , USA
Vision ? Visual system ? Cornea ? Primate ? Anthropoids ? Activity pattern ?
Eulemur ? Hapalemur ? Evolutionary disequilibrium
The visual systems of cathemeral mammals are subject to selection pressures that
are not encountered by strictly diurnal or nocturnal species. In particular, the cathem-
eral eye and retina must be able to function eff ectively across a broad range of ambient
light intensities. This paper provides a review of the current state of knowledge regard-
ing the visual anatomy of cathemeral primates, and presents an analysis of the infl uence
of cathemerality on eye morphology in the genus Eulemur . Due to the mutual antago-
nism between most adaptations for increased visual acuity and sensitivity, cathemeral
lemurs are expected to resemble other cathemeral mammals in having eye morpholo-
gies that are intermediate between those of diurnal and nocturnal close relatives. How-
ever, if lemurs only recently adopted cathemeral activity patterns, then cathemeral le-
murids would be expected to demonstrate eye morphologies more comparable to
those of nocturnal strepsirrhines. Both predictions were tested through a comparative
study of relative cornea size in mammals. Intact eyes were collected from 147 specimens
of 55 primate species, and relative corneal dimensions were compared to measure-
ments taken from a large sample of non-primate mammals. These data reveal that the
fi ve extant species of the cathemeral genus Eulemur have relative cornea sizes interme-
diate between those of diurnal and nocturnal strepsirrhines. Moreover, all Eulemur spe-
cies have relative cornea sizes that are comparable to those of cathemeral non-primate
mammals and signifi cantly smaller than those of nocturnal mammals. These results sug-
gest that Eulemur species resemble other cathemeral mammals in having eyes that are
adapted to function under variable environmental light levels. These results also sug-
gest that cathemerality is a relatively ancient adaptation in Eulemur that was present in
the last common ancestor of the genus (ca. 8–12 MYA).
Copyright © 2006 S. Karger AG, Basel
E. Christopher Kirk
Department of Anthropology
1 University Station C3200, Austin, TX 78712 (USA)
Tel. +1 512 471 0056, Fax +1 512 471 6535
© 2006 S. Karger AG, Basel
Accessible online at:
Fax +41 61 306 12 34
Folia Primatol 2006;77:27–49
A Brief Review of Cathemerality: Distribution and Infl uences on
Although mammalian activity patterns are often described dichotomously as
either diurnal or nocturnal, a large number of mammal species are regularly active
under both diurnal and nocturnal conditions. Such species are most appropriately
considered to have a ‘cathemeral’ activity pattern [Tattersall, 1987; Curtis and Ras-
mussen, 2002]. Cathemeral species are found in each of the three major clades of
living mammals. Among monotremes, both the echidna Tachyglossus and the platy-
pus Ornithorhynchus are cathemeral [Augee and Gooden, 1993; Otley et al., 2000].
Although most marsupial species are strictly nocturnal, the honey possum Tarsipes ,
the dunnart Sminthopsis , and many wallabies and kangaroos (e.g., Macropus and
Setonix ) are cathemeral [Nowak, 1991; Arrese et al., 1999, 2002, 2003]. Addition-
ally, koalas (Phascolarctos cinereus) spend approximately 20 h per day resting, and
are commonly stated to be nocturnal [Lee et al., 1990]. However, nearly 25% of
koala activity occurs during the day [Mitchell, 1990], so the species is most appro-
priately described as cathemeral.
In contrast to marsupials, a large number of placental mammals are cathemeral,
including members of the Artiodactyla, Carnivora, Eulipotyphla, Lagomorpha, Ro-
dentia, Perissodactyla, Primates, Proboscidea, Tenrecomorpha, Tubulidentata, and
Xenarthra [Nowak, 1991]. In some of these orders, cathemeral activity occurs in all
(e.g., perissodactyls and proboscideans) or most (e.g., artiodactyls) species. In pri-
mates, however, cathemerality is relatively uncommon and occurs only in the genus
Eulemur and some populations or species of Hapalemur , Aotus , and possibly
Alouatta [Wright, 1996; Curtis and Rasmussen, 2002; Mutschler, 2002; Fernandez-
Periods of wakefulness and activity in cathemeral species may occur intermit-
tently during the day and night over a single 24-hour cycle [e.g., Overdorff, 1988].
Alternatively, cathemeral species may shift seasonally over a yearly cycle between
periods of predominantly diurnal and predominantly nocturnal activity [e.g., Ras-
mussen, 1999]. Regardless of the type of cathemeral activity that a species exhibits,
the consequences for visual ecology are the same: cathemeral species must be able to
maintain functionality of the visual system across a wide range of ambient light in-
tensities. For example, in a single 24-hour period during a waxing moon, a cathem-
eral species may be active in an environment illuminated successively by moonlight,
starlight alone (after moonset), and sunlight. The requirement of maintaining visual
functionality under these highly variable lighting conditions does not represent a
trivial problem – light intensities in full sunlight are 100 billion times greater than
light intensities on an overcast moonless night [Dusenbery, 1992]. How then do the
visual systems of cathemeral species cope with these differences?
Visual Adaptations for Cathemerality
Walls  was one of the fi rst authors to clearly identify a set of visual features
that could be construed as representing an adaptation for cathemeral activity. Ac-
cording to Walls, a cathemerally adapted eye is one that demonstrates an anatomical
compromise between the functional demands of vision under nocturnal and diurnal
conditions. Such morphologies are most readily apparent when the visual systems of
cathemeral species are contrasted with those of nocturnal and diurnal species.
Cathemeral Eye Morphology
Folia Primatol 2006;77:27–49
Nocturnal species typically exhibit a number of features of the visual system
that represent adaptations for increased visual sensitivity (the ability to detect weak
or dim stimuli). Sensitivity-enhancing adaptations include increasing the size and
curvature of the cornea and lens to collect more light and form a brighter retinal im-
age, increasing the ratio of photoreceptor cells to retinal ganglion cells (retinal sum-
mation), and increasing the proportion of rods relative to cones [Detwiler, 1939,
1940, 1941; Walls, 1942; Prince, 1956; Duke-Elder, 1958; Tansley, 1965]. The ratio
of rods and cones in the retina is a functionally signifi cant feature because rods are
inherently more sensitive than cones and retinal space is fi nite. Accordingly, visual
sensitivity increases when more of the limited retinal area is devoted to rods than
Diurnal species, by contrast, typically have visual systems that maximize vi-
sual acuity (the ability to detect fi ne spatial details) at the expense of sensitivity. Ad-
aptations for enhanced acuity in diurnal species include reducing the size and cur-
vature of the cornea and lens to form a larger retinal image, decreasing retinal sum-
mation (equivalent to increasing the number of independent sampling units in the
retina), and increasing the proportion of cones relative to rods [Detwiler, 1939, 1940,
1941; Walls, 1942; Prince, 1956; Duke-Elder, 1958; Tansley, 1965].
In these three respects (eye morphology, retinal summation, and rod:cone ratio),
nocturnal and diurnal visual adaptations represent two extremes on a continuum.
Accordingly, cathemeral species can function visually across a broader range of am-
bient light levels simply by occupying the morphological ‘middle ground’. For ex-
ample, the eyes of cathemeral species generally have corneas and lenses that are nei-
ther as large and curved as those of nocturnal species nor as small and fl attened as
those of diurnal species [cf. fi g. 71 in Walls, 1942]. Similarly, cathemeral species
typically have retinas with an intermediate degree of retinal summation and a more
equable proportion of rods and cones than is seen in strictly nocturnal or diurnal
species [Ahnelt and Kolb, 2000; Kay and Kirk, 2000; Kirk and Kay, 2004].
Visual Anatomy of Cathemeral Primates
Unfortunately, very little is known about the visual adaptations of cathemeral
primates. There have been few publications regarding cathemeral behavior in howl-
er monkeys (Alouatta) , and it is not clear to what extent cathemeral activity charac-
terizes members of this predominantly diurnal genus [Dahl and Hemingway, 1988].
Not surprisingly, Alouatta resembles all other diurnal anthropoids in having a retina
with a well-developed all-cone fovea [Franco et al., 2000]. Alouatta further resembles
diurnal catarrhines in exhibiting routine trichromatic color vision [Jacobs et al.,
1996]. Information regarding the visual system of bamboo lemurs (Hapalemur) is
restricted to Pariente’s [1976, 1979] brief description of the diurnal species H. gri-
seus . The visual anatomy of the only bamboo lemur taxa known with certainty to be
cathemeral (H. g. alaotrensis and H. simus) [Tan, 2000; Mutschler, 2002] has never
been studied. Although the visual anatomy of owl monkeys (Aotus) has been the
subject of a substantial number of investigations [Ogden, 1994; Lima et al., 1996;
Silveira, 2004], none have attempted to examine differences between nocturnal spe-
cies (e.g., A. trivirgatus ) and the cathemeral species A. azarai [Wright, 1996; Fernan-
It is also not known to what extent recent habitat disturbances or range expan-
sions have contributed to the adoption of a cathemeral activity pattern in H. g. alao-
Folia Primatol 2006;77:27–49
trensis and A. azarai . H. g. alaotrensis represents a relict population that is under
intense ecological pressure due to habitat conversion [Mutschler, 1998]. Similarly,
cathemerality in A. azarai has been clearly linked to ecological release in the form of
reduced interspecifi c competition and raptor predation relative to other Aotus spe-
cies [Wright, 1996; Fernandez-Duque, 2003]. If either H. g. alaotrensis or A. azarai
has only recently adopted a cathemeral activity pattern, then directional selection
may not have operated over a period long enough to cause these groups to differ ap-
preciably in visual anatomy from close phyletic relatives [cf. van Schaik and Kap-
In contrast to the variable occurrence of cathemerality in Hapalemur , Aotus ,
and Alouatta , the genus Eulemur represents an ecologically diverse radiation of 5
species and 11 subspecies [Garbutt, 1999] that are exclusively cathemeral. Patterns
of cathemerality are relatively well studied in Eulemur [Curtis and Rasmussen,
2002; Kappeler and Erkert, 2003], and the potential adaptive signifi cance of cat-
hemerality in this clade has been the subject of intense debate [Engqvist and Rich-
ard, 1991; van Schaik and Kappeler, 1996; Curtis et al., 1999; Donati et al., 1999;
Wright, 1999; Kappeler and Erkert, 2003]. However, nearly everything that is known
about the visual anatomy of the various Eulemur species is derived from older anal-
yses that were conducted prior to the development of many contemporary methods
for visualization and quantifi cation of retinal features (e.g., photoreceptor immuno-
labeling [Wikler and Rakic, 1990]). As a result, existing descriptions of retinal anat-
omy in Eulemur are predominantly qualitative and provide only a limited basis for
comparison with other taxa [Wolin and Massopust, 1970].
For example, Kolmer  noted that both E. fulvus (= ‘ Lemur rufi frons ’) and
E. macaco have retinas that exhibit little central specialization and contain both rods
and cones. Rohen and Castenholz  also reported the absence of a domed cen-
tral retinal thickening, or area centralis, in E. fulvus , but could not verify the presence
of cones. Similarly, Woollard  described the retina of E. macaco (= ‘ Lemur
niger ’) as lacking both central specializations and cones. However, prior disagree-
ment over the presence or absence of cones in Eulemur primarily refl ects the limita-
tions of conventional light microscopy as a method for describing retinal anatomy.
Modern opsin-labeling methods have demonstrated the presence of cones that are
morphologically similar to rods in nocturnal species previously characterized as hav-
ing ‘all-rod’ retinas (e.g., Aotus and Otolemur [Wikler and Rakic, 1990], Tarsius
[Hendrickson et al., 2000]). Similarly, various diurnal species long thought to have
‘all-cone’ retinas have also recently been shown to have rods that are morphologi-
cally similar to cones (e.g., Tupaia [Müller and Peichl, 1989]; Spermophilus [Kryger
et al., 1998]). These analyses suggest that all mammals have duplex retinas contain-
ing both rods and cones, and that functional differences between taxa are primarily
the result of variation in the relative proportion of rods and cones in the retina [Ah-
nelt and Kolb, 2000].
Confi rmation of the presence of functional cones in Eulemur has been provided
by both electroretinographic fl icker photometry [Jacobs and Deegan, 1993] and op-
sin sequencing [Tan and Li, 1999; Kawamura and Kubotera, 2004]. In a preliminary
immunohistological description of the photoreceptor mosaic of E. fulvus , Peichl et
al.  reported peak cone densities of 15,500 cones/mm 2 and peak rod densities
of 330,000–430,000 rods/mm 2 . These peak photoreceptor densities for E. fulvus fall
between those for Microcebus murinus (nocturnal; 11,000 cones/mm 2 ; 400,000–
Cathemeral Eye Morphology
Folia Primatol 2006;77:27–49
sirrhines, it is immediately apparent that there is a great disparity in eye morphol-
ogy between the two primate suborders [Kirk, 2004]. If diurnal anthropoids exhibit
the morphological adaptations ‘expected’ for any diurnal primate [cf. van Schaik and
Kappeler, 1996], then diurnal and cathemeral strepsirrhines have clearly have fallen
short of this mark. However, this argument is not supported by the fact that diurnal
strepsirrhines have eye morphologies that are comparable to those of other diurnal
mammals ( fi g. 6 and 7 ) [Kirk, 2004]. Diurnal anthropoids, by comparison, are high-
ly derived relative to most other diurnal mammals in having very small relative cor-
nea sizes ( fi g. 6 and 7 ) [Ross, 2000; Kirk, 2004]. Accordingly, cathemeral lemurs
should not necessarily be expected to converge on the condition seen in diurnal an-
thropoids if they are presently in a state of adaptive fl ux.
While not completely ruling out the possibility that some extant lemuriform
species are in a state of evolutionary disequilibrium due to recent and ongoing eco-
logical disturbances, the fi ndings of this analysis and others [Kay and Kirk, 2000;
Heesy and Ross, 2001; Kirk, 2004] more clearly delineate the positions that are
available to advocates or detractors of the EDH with regard to cathemerality in Eu-
lemur . On one hand, cathemerality may be a relatively ancient adaptation that was
present in the last common ancestor of extant Eulemur species, which lived approx-
imately 8–12 million years ago according to molecular estimates (Yoder and Yang,
2004). This hypothesis is supported by the fact that the visual systems of Eulemur
species are not known to differ in any functionally signifi cant way from those of ex-
tant cathemeral non-primate mammals. On the other hand, the fi ve extant species
of Eulemur may have independently acquired a cathemeral activity pattern within
the last 2000 years and subsequently evolved eye morphologies comparable to extant
cathemeral non-primate mammals. While this latter hypothesis is certainly plausi-
ble, it must be regarded as highly speculative until additional evidence favoring evo-
lutionary diseqilibrium in Eulemur can be brought to light. At least with regard to
the visual system, application of Occam’s razor favors the conclusion that Eulemur
species are in a state of evolutionary equilibrium rather than disequilibrium.
Special thanks are due to Deborah Curtis, Giuseppe Donati and Michele Rasmussen for or-
ganizing the 2004 IPS symposium on cathemerality. Access to eye specimens was generously pro-
vided by Nate Dominy, Bill Hylander, Bill Jungers, Pierre Lemelin, Callum Ross, Chet Sherwood,
Russ Tuttle, Chris Vinyard, and the staff of the Duke University Primate Center. This paper was
greatly improved due to the thoughtful comments of Nate Dominy, Becca Lewis, and 2 anonymous
reviewers. Financial support for this research was provided by the Leakey Foundation, the Duke
University Arts and Sciences Research Council, and the Duke University Primate Center.
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