OIKOS 43: 291-300. Copenhagen 1984 Body size, prey size and herbivory in the Galapagos lava lizard, lkopidurus Dolph Schluter Schluter, D. 1984. Body size, prey size and herbivory in the Galapagos lava lizard, Tropidurus. - Oikos 43: 291-300. Body size and diet were studied in Tropidurus in relation to food supply, to evaluate the hypothesis that body size is adapted to efficient exploitation of available foods. The results suggest that size is adapted to a particular degree of herbivory (i.e. a mixture of plant and animal food) rather than arthropod prey size. Body size and prey size were only weakly associated in T. pacificus, and comparison with other studies suggests this results from a large mean adult body size relative to the most common prey sizes. Balanced-diet effects on prey choice were not detected. The total quantity of animal food in the diet was not related to body size. Tropidurus size on different islands was unrelated to mean prey size available. However, degree of herbivory usually increased with increasing body size in T. pacificus. Mean body size of this population is approximately the size expected for an iguanid exhibiting its degree of herbivory. Tropidurus body size on different Galapagos islands is inversely related to arthropod abundance, and some data suggest that degree of herbivory varies in correspondence. The results support the hypothesis that body size is adapted to food, though the possible importance of other factors needs to be investigated. D. Schluter, Div. of Biol. Sci., Univ. of Michigan, Ann Arbor, MI 48109, USA (present: Dept of Zoology, Univ. of British Columbia, 6270 University Boulevard, Vancouver, B. C., Canada V6T 2A9). onpaemmi pams~T~JBn cocTaB ITUIIVI y Tropiduriis B sa~umniOT sanaca m, am npoaepn runmesbl o TCM, YTO paw T- a.mmmBaH am ~a~6onee SX#R?KTIIBHOM ncnm30~mnccTynHoZ1 mqf. PesymTam no~a3arm, TO p?n~ep -Barn K menem Yacm~oU@i~c&rmi (~.e. aeai ~~CTUT~~HORM WOT- EJ& rmwi), mopee, sm K pawpy xepm us wcna memcToHorwr. Pas~epTerra u pa3w~xepmrrurub -60 CBFI~~H~Jy T. pacificus, n -Heme c pesym~a~a- MU wyrm nccnqoBaM no3~me~rrpeWIanaraT6, YTO TO - cnencTBMe ~pyxHb!% cpe9Hux -3MeWB B3- CC6& B -Hew C W3E- ~ H C T B ~ C&NM Xem. -Hue c~~JI~HCH~~B~HHO~OparZuOHa Ha &p XepTB He YCTaHO- MeH. KOJW4eCTBO XUfEOTHOZ1 ITUIIVI B mOHe He CBRSaHO C pd3MepZM Tens. Pamsp Tropidurus Ha pas= cap~axHe c~ffsamc cpzuwmi pa3~epminm- Tyn= XepTB. W~KO,CTWeHb paCTUTEXbHOFTIWCClu mH0B03mCTaeT C YBeTI1?- semm pa3~epo~~enay T. pacificus. CposHue pasew Terra y TOR n0nymw-m npmep~ocmTseTcmym cm.m- paswpa~nryaH, c coomeTcTsyxu& cTenem:o @nTcQarmi. Pamew T- Tropidurus Ha pas= Rmanaroccrn ocwaax &HO np3IIOmOH&Ti%Hbl WIeHUCTOHOrWX, U HeKOTOw DHWnOKa3bIBaKW, 'IT0 CT3ieHb paCTHTEXbHOFTIWOCTH BapbSipYeT C03TBeTCTBeHHO. Pe3YJBTaTbl nOKL'BepUl?.- IMrmoTesy o cmeTcmnn pas~epo~Tena COCT~BY m, XOTR BO~MOH(H~~3~a- sem ,rpyrux @KTO~BH~V~IOTCR B ELJEH~usmew. Accepted 15 November 1983 0 OlKOS OIKOS 43:3 (1984) Finally, I discuss other possible explanations for body 1. Introduction size in Tropidurus. Theoretical models of foraging, and their success in laboratory situations, have revived interest in quantita- 2. Methods tive field studies of diets (e.g., Smith et al. 1978, Doble and Eggers 1978, Craig 1978, Schluter 1981, 1982a, The Pinta study was conducted in two 1-ha sites on the Stamps et al. 1981, Sherry and McDade 1982). This is south slope in 1979. The first site, located in the arid chiefly because the models hold promise that bewilder- zone (Wiggins and Porter 1971) was relatively sparsely ing variation in diets in nature may be predicted by a vegetated. Bursera graveolens, Castela galapageia, small number of factors, particularly the profitability Chamaesyce punctulata and Rynchosia minima were the and abundance of foods (e.g., MacArthur and Pianka most common plant species. The substrate was largely 1966, Schoener 1971). However, there is a second bene- bare rock, and soil and litter was uncommon and pat- fit to be derived from such models: since food profit- chy. This contrasts with the second site located in the ability is a function of predator size and shape, it may be upper transition zone, a grassland-type habitat just be- possible to predict predator morphology from consid- low the humid forest (Wiggins and Porter 1971). erations of feeding efficiency and food abundance Ground cover was dense, dominated by Paspalum gala- (Schoener 1969, Belovsky 1978, Case 1979a,b, Schluter pageia and Rynchosia minima. Soil litter was abundant, and Grant 1984). and bare rock virtually absent. Tree cover, chiefly Zan- I investigated the diet of the lava lizard Tropidurus thoxylum fagara, was sparse. These two sites are the pacificus on Isla Pinta Galapagos, to test the hypothesis same as sites 1 and 5 in Schluter (1982b) where they are that body size is adapted to food. Tropidurus was of described in more detail. interest for several reasons. First, the genus is wide- Diets were studied in both February and November. spread in the Galapagos and has differentiated mor- The first month corresponded to the early wet season, a phologically among islands (Van Denburgh and Slevin season of heavy but infrequent rains, sunny skies, and 1913, Carpenter 1970). Hence body size on any particu- warm temperatures (Wiggins and Porter 1971, Grant lar island (e.g. Pinta) may be an evolutionary response and Boag 1980). In November, the late dry season, to local food conditions. Second, individual T. pacificus temperatures were cooler and skies were generally feed on a wide variety of food types, especially plant cloudy until 1-2 PM daily. Arthropod abundance and parts and arthropods. Both the size of arthropod prey activity, and flower and fruit production was greatest in consumed by lizards, and the degree of herbivory ex- the wet season. hibited, are expected to be related to body size Fifteen T. pacificus were collected over several days (Schoener and Gorman 1968, Schoener 1969, Pough from each site in each month. Collections were made at 1973, Case 1979a,b). Finally, there is at most one Tropi- approximately the same time of day, in mid-afternoon. durus species on any Galapagos island, and it is the only About half of each sample were males, and half were small diurnal terrestrial lizard there. This permits an females. Each lizard was weighed to the nearest 0.1 g investigation into causes of Tropidurus size uncompli- with a spring balance, and snout-vent length was meas- cated by the presence of closely related competitor spe- ured to 1 mm with a ruler. Measurements are sum- cies. marized in Tab. 1. I begin this paper by describing the diet of adult T. Guts were removed and stored in alcohol. Stomach pacificus in terms of the types of prey taken, the sizes of contents were later identified and measured in the labo- arthropods consumed, and the extent of herbivory. I ratory. Plant and animal portions were separated, dried then examine arthropod prey size and degree of her- and weighed. While total lengths of available prey were bivory as alternate explanations for body size. through measured in the field (see later), it was not possible to tests of predictions with field data. If body size is adap- determine the original length of arthropod prey in the ted to efficient exploitation of arthropod prey sizes, gut. Instead, width of the head capsule was used as a then 1) a correlation is expected between body size and measure of prey size in the diet. The hindgut was also prey size among individual T. pacificus; and 2) dif- dissected and head widths of all arthropods were meas- ferences among islands in Tropidurus size should be ured, excluding ants. Ants were very common in the associated with island differences in available prey sizes. Alternatively if body size is adapted to a mixed diet of plant and animal foods, then 1) degree of herbivory Tab. 1. Mean snout-vent length (rnrn) and body mass (g) for 60 among T. pacificus individuals should be related to Troprdurus pacificus collected. Standard deviations are in body size; 2) mean body size of T. pacificus should be parentheses. intermediate between strictly carnivorous and strictly herbivorous lizards (Pough 1973); and 3) differences Sex SVL Mass N among islands in Tropidurus body size should be asso- ciated with island differences in the supply of plant and/ Male 96.4 (6.1) 24.4 (5.5) 3 1 Female 80.3 (5.8) 15.1 (3.4) 29 or animal foods. 292 OIKOS 43 3 (19x4) diet, but the vast majority were uniform in size (0.3-0.6 other plants were also taken. Overall, averaging across mm head capsule width). Since their abundance tended sites and months, about 63% of the diet by weight was to overwhelm statistical characteristics of diet, ants are plant material. considered separately. The diversity of food items in the diet results from a Food supply was assessed in the two Pinta sites in wide range of foraging positions and methods. Many both months. To better estimate food characteristics for prey are snatched from the ground when discovered the whole island, four other sites were also sampled, in from a stationary position in classic sit-and-wait be- different habitats (Schluter 1982b). Food was also havior (Pianka 1966). However the lizards also actively sampled in different years in sites on four additional search for prey. Subsurface neuroptera and beetle lar- islands where Tropidurus is present (Schluter and Grant vae are often obtained through "digging": the head and 1984): San Salvador (wet season 1978, dry season 1981; forelegs are used to shove aside litter and soil (see also 3 sites), Marchena (wet and dry seasons 1979; 2 sites), Carpenter 1970).