Did extinct lemurs exhibit low visual acuity

On the basis of the relatively small size of their orbits, giant extinct lemurs have been reconstructed as having diurnal activity cycles (Gingerich and Martin, 1981; Jungers et al., 2002; Martin, 1990; Walker, 1967). No orbits are preserved for the giant aye-aye, Daubentonia robusta, but one might infer nocturnality based on its detailed similarity to the nocturnal living aye-aye (Godfrey et al., 1997b). Because there are no large-bodied, nocturnal anthropoids to which we can make direct comparisons, the accuracy of such inferences remains unknown. The largest extinct lemur, Archaeoindris fontoynontii, has an orbit area within the range of gorillas, whereas most other subfossil lemurs have orbits considerably smaller than size-matched diurnal anthropoids (Table 5). Archaeolemur, Pachylemur, Babakotia, Mesopropithecus, and Palaeopropithecus have orbits roughly the absolute size of much smaller, diurnal living lemurs. If we accept the conventional wisdom that virtually all the extinct lemurs were indeed diurnal, that does not imply that their visual acuity was also comparable to living, size-matched anthropoids (Kirk et al., 2002). It also seems highly unlikely that, as a group, they possessed trichromatic color vision.

Kay and Kirk (2000) offer osteological metrics that are correlated with the degree of retinal summation and inferred visual acuity. These indices are based on the relative size of the optic foramen or optic canal. If orbit size is used as a surrogate for eyeball size, and since eyeball size is proportional to retinal area, one can construct a ratio of optic foramen area to orbit area as an "index of summation," the Optic Foramen Index (OFI); lower indices imply higher retinal summation. Nocturnal primates tend to sacrifice visual acuity in order to maximize sensitivity, and they are characterized by low indices. When compared to diurnal anthropoids, strepsirhines as a clade have low indices and reduced acuity regardless of their activity cycle. The nearly ubiquitous strepsirhine tapetum (only Eulemur macaco macaco is reported to have lost it; Pariente, 1979), a light-reflecting membrane within the eye, is consistent with this reduced visual acuity. Kay and Kirk (2000) also noted a confounding allometric trend within each major clade of primates such that the OFI also decreases with increasing body/skull size. We have employed a slightly modified version of their OFI. We measure area of the optic foramen or canal directly by digitizing cross sections of molds (Coltene President Plus Jet); we use the smaller of the vertical or horizontal diameter of the orbit to calculate area as a circle. The ratio of the two areas is multiplied by 100 to create our version of the OFI (Table 5).

The aforementioned allometric trend is evident within strepsirhines when the large subfossils are compared to living lemurs. Large-bodied taxa such as Archaeoindris (~160 kg), Hadropithecus (~35 kg), and Megaladapis (~ 45-90 kg) have the lowest OFIs. Within clades of extinct lemurs, the pattern is striking: among sloth lemurs (family Palaeopropithecidae), Palaeopropithecus (~ 40-45 kg) and especially Archaeoindris have values lower than Mesopropithecus (~10-15 kg) and Babakotia (~20 kg). Among monkey lemurs (family Archaeolemuridae), Hadropithecus has values lower than Archaeolemur (~18-25 kg). Among koala

Table 5. Orbit size and the Optic Foramen Index—an osteological estimate of retinal summation in giant lemurs and selected living primates

Species

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