Oecologia (Berlin) (1986) 70:587-591 Oecologia 9 Springer-Verlag 1986 Lizard herbivory in a sand dune environment: the diet of Angolosaurusskoogi R.D. Pietruszka ~, S.A. Hanrahan 2, D. Mitchell 3, and M.K. Seely 1 1 Desert Ecological Research Unit, P.O. Box 1592, Swakopmund, 9000, SWA/Namibia 2 Department of Zoology, University of the Witwatersrand, Johannesburg 2001, South Africa 3 Department of Physiology, University of the Witwatersrand Medical School, Johannesburg 2193, South Africa Summary. Early observations of the feeding habits of Ango- losaurus skoogi (Andersson), a 50-120 g lizard inhabiting the almost vegetationless dunes of the northern Namib Des- ert, indicated that this species was herbivorous. Less than two percent of living lizard species are predominantly her- bivorous and these species tend to have a high body mass (> 100 g). We therefore analysed fecal pellets of A. skoogi to obtain dietary details of this relatively rare lizard. Eight- een food taxa were identified: 81 percent by mass was com- posed of vegetable matter, of which 56 percent was an en- demic, perennial cucurbit Acanthosicyos horrida. Both spa- tial and temporal variation in dietary composition were ap- parent. Ready access to water in the form of A. horrida is postulated to have allowed Angolosaurus to occupy this extreme habitat and to forage on components of the wind- blown detritus, a food source that would probably not be usable in other circumstances. Key words: Sand dune - Lizard - Herbivory Fecal analysis - Angolosaurus Lizard diets often are catholic, but generally limited to in- sects and vertebrates (Mayhew 1968); few species are her- bivorous. Reviewing the literature, Pough (1973) found only about 2 per cent of the 2,500 living lizard species to be predominantly herbivorous, and those tended to have high body mass (>100g). Several hypotheses have ap- peared in the literature concerning the adaptive significance of carnivory or herbivory in lizards. Szarski (1962) posited that herbivorous lizards would be subjected to greater pre- dation pressure because they required a longer time to for- age. In contrast, Dawson (1965) suggested that the higher preferred temperatures of herbivorous lizards allowed them to feed on plant material when potential predators could not be active. The prohibitive energy cost which would be experienced by large lizards in capturing living prey was advanced by Wilson and Lee (1974) and Ruppert (1980) as the basis of herbivory in large lizards (but see Pough and Andrews 1985). Pough (1973) suggested that the ener- getic demands of the larger species cannot be met from a diet of insects and few alternative prey are available. Other factors militate against herbivory in smaller lizards. Ostrom (1963) proposed that few herbivorous lizards exist because Offprint request to: R.D. Pietruszka of structural limitations of the lizard jaw. Sokol (1965) sug- gested that small lizards do not have the physical strength to reduce the particle size of vegetation sufficiently to allow for ingestion. In desert environments lizards are among the most abundant and conspicuous vertebrates. Angolosaurus skoogi (Andersson) is a chthonic inhabitant of the northern Namib Desert of southern Angola (Fitzsimons 1953) and Namibia (Steyn 1963). It is one of only two cordylid (--Cordylidae + Gerrhosauridae) species known to be herbivorous (Pough 1973). Angolosaurus is the largest Namib Desert dune liz- ard, but its size (adult range: 50-120 g) puts it on the lower limit postulated for herbivory. Despite being herbivorous, it lives in the vegetationless slipface habitat of the Namib sand dunes (Hamilton and Coetzee 1969). Details of the ecology of Angolosaurus are unknown. Currently, the total knowledge concerning diet is confined to the anecdotal account of Steyn (1963) based on an exami- nation of 12 specimens. A more thorough examination of its diet in relation to habitat may help elucidate how ecolog- ical specialization may promote herbivory in lizards despite small size (sensu Pough 1973). We have therefore investi- gated the diet of Angolosaurus skoogi using the technique of fecal analysis and assessed the degree of spatial and tem- poral variation in dietary composition. Analysis of fecal pellets (scats) has been used previously to acquire dietary information for a number of reptile spe- cies (e.g., Agama agama, Harris 1964; A. stellio, Mienis 1981 ; Physignathus lesueurii, Hardy and Hardy 1977; Saur- omalus obesus, Hansen 1974; Sphenodon punctatus, Walls 1981). It may be the preferred method with rare or endan- gered species where it is "unwise or unfeasible" to kill ani- mals for stomach analysis and where a comprehensive "population diet" is the objective (Hansen 1974), a situa- tion which prevails for Angolosaurus. Angolosaurus scats are easily obtained from dune slipfaces, and it is possible to find study areas virtually devoid of scats from other species. Study area The study area was located on the south bank of the usually dry bed of the Unjab River, approximately 15 km inland on the eastern (inland) side of the northern Namib dunes (20o09 ' S, 13~ ' E). Here the dunes are composed mainly of barchanoid ridges 3-10 m high with connected concentric