Comparative Aggression in Sceloporus Virgatus, S
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Anim. Behav ., 1975, 23, 279-286 COMPARATIVE AGGRESSION IN SCELOPORUS VIRGATUS, S. UNDULATUS CONSOBRINUS, AND S. U. TRISTICHUS (SAURIA : IGUANIDAE) BY MARIAN B. VINEGAR Museum of Zoology, University of Michigan, Ann Arbor, Michigan* Abstract. Aggressive behaviour in both sexes of Sceloporus virgatus, S. undulatus consobrinus, and S. u. tristichus were compared by introducing a conspecific to a resident lizard in the field . Males of S. u. consobrinus exhibited high intensity aggression more commonly than did males of the other forms . Of the females, S. u. tristichus showed the greatest propensity for aggressive behaviour ; S. u . consobrinus showed the least . In males, degree of aggression was correlated positively with degree of sexual dichromatism. The high degree of aggression in male S. u. consobrinus may be selectively advantageous because of (1) low density increasing space between females, (2) low density reducing selection against a relatively high frequency of violent fighting, and (3) high predation favouring lizards that do not leave their sites upon intrusion of another male . High aggression in female S. u. tristichus may be selectively advantageous because of (1) higher density leading to reduced food supply and fewer egg- laying sites, and (2) lower predation reducing selection against mobility and fighting in females . Natural selection should favour a particular smaller than females . Sceloporus u. consobrinus level of aggression between conspecific organisms is the largest of the three forms ; S. virgatus is of the same sex within a population . Aggression the smallest . is related closely to the ability to compete for Males of S. undulatus have a blue patch on mates, food, space, or other resources. An each side of the throat and belly . Females of animal that is not aggressive enough may lose S. u. consobrinus have blue throat patches but in competition for these resources. An animal no blue belly patches . Females of S. u. tristichus, that is too aggressive, however, may not spend however, have blue belly patches that tend to enough energy in food-getting, mating, predator be lighter than those of the males, as well as avoidance, or other adaptive activities, and blue throat patches . Both S. virgatus males and would be at a disadvantage relative to a less females have no belly patches ; males and non- aggressive animal. The level of aggression that productive females have blue throat patches . is most adaptive should be related to such During the breeding season, however, the blue features as the population density, survivorship, throat patches of reproductive females are sex ratio, food supply, physical characteristics surrounded or replaced by orange. of the environment, and possibly sexual di- morphism. Study Areas This study determines aggressive behaviour All forms were studied where they seemed in three closely related forms of the lizard genus most abundant in south-eastern . Arizona and Sceloporus that differed in habitat, population south-western New Mexico . Sceloporus virgatus structure, and sexual dichromatism. Differences was studied in two areas in Cave Creek Canyon, in level of aggression are related to differences Cochise County, Arizona. One area by a road in environmental biology, Therefore, insight into was vulnerable to disturbance. The elevation the evolution of aggressive behaviour is gained was 1646 m, and this area was 0 .40 km away by comparing selective pressures on these lizards. from the Southwestern Research Station . The other area was about 0 .40 km away from a road, Methods and was rarely disturbed by people . The elevation Animals was 1798 m and this area was 2 .90 km from the Sceloporus virgatus, S. undulatus consobrinus, Southwestern Research Station. Both areas were and S. u. tristichus, all less than 80 mm in snout- along a tributary (North Fork) of Cave Creek . vent length, were compared . Males tend to be The creek ran intermittently, and the creek bed *Present address : Department of Environmental Health, contained, and was surrounded by, boulders . Kettering Laboratory, University of Cincinnati, Medical The principal tree species in both areas were Center, Cincinnati, Ohio 45267 . ponderosa pine (Pinus ponderosa), Apache pine 279 210 ANIMAL BEHAVIOUR, 23, 2 (Pinus engelmannii), alligator juniper (Juniperus painting their tails with model aeroplane paint . deppeana), Arizona white oak (Quercus arizonica) Various combinations of colours and symbols and sycamores (Platanus wrightii). The uneven were used for individual recognition . Mark- nature of the terrain made precise measurements recapture methods were used to determine impossible, but each area was approximately growth, population structure, and survivorship one hectare. Other lizard species present, in of lizards on study areas . The proportions of apparent order of abundance, were Urosaurus lizards with broken or regenerated tails on study ornatus, Cnemidophorus sp., Sceloporus jarrovi areas were noted as a rough indicator of pred- (less abundant in the upper study area than in ation pressure (Pianka 1970 ; Tinkle & Ballinger the lower), Sceloporus clarki, and Phrynosoma 1972). Information concerning clutch size and douglassi. Gerrhonotus kingi and Eumeces obso- number of clutches per season was obtained letus were not seen on the study areas but were from lizards sampled in the vicinity of the study seen in similar habitats nearby. areas. A more detailed description of methods Sceloporus undulatus consobrinus was studied used in determining population biology of these in a desert-grassland 16 .89 km north of Lords- lizards will be presented elsewhere, (Vinegar, burg, Hidalgo County, New Mexico, at an in preparation). elevation of 1219 m . The dominant vegetation Seasonal differences in natural intra- and included yucca (Yucca elata), Mormon-tea intersexual interactions within all these forms and mesquite (Prosopis sp.). (Ephreda sp.), were noted in all 4 years (1969 to 1972) . Tests The S. u. consobrinus were usually found in the to determine aggression of males and females yuccas. The area was bordered by an arroyo were performed in 1969 to 1971 . In these tests, along which grew . desert willow (Chilopsis an intruding lizard was tied by a 75 to 110 mm linearis) and by New Mexico Highway 464 . string to a stick. The intruder was thus pre- This area was 4 .25 ha. Other lizard species vented from running away while still leaving present included Cnemidophorus tigris, Phryno- some freedom of movement . The tethered lizard soma cornutum, Crotaphytus wislizenii, and was introduced then into the vicinity of a free Holbrookia maculata. adult lizard of known sex that usually had been Sceloporus u. tristichus was studied in the marked previously . The habits and size of the Pinos Altos mountains 1 .61 km north of Pinos resident, therefore, usually were known. Male- Altos, Grant County, New Mexico, in the Gila male, male-female, and female-female en- National Forest at an elevation of 2057 m . The counters were staged . The behaviour patterns rocky study area was along intermittent Bear of both the intruder and resident when the Creek. The dominant vegetation included pon- intruder was tethered were compared with those derosa pine, alligator juniper, and willow of lizards that naturally encountered each other (Salix sp.). The area was 1 .5 ha. Other lizard and with those in which the intruder was species present included Urosaurus ornatus, introduced simply by releasing the lizard to Sceloporus poinsetti, Cnemidophorus sp., determine the effects of tethering. Phrynosoma douglassi, and Gerrhonotus kingi. Both Bear Creek and Cave Creek are usually dry during May and June, usually very dry Results months in south-eastern Arizona and south- In general, the timing and behaviour involved western New Mexico . Creeks flow in July and in the life histories of these forms of lizard August after the start of the rainy season, usually are as follows : mating occurs in the spring ; at the beginning of July. Creek beds sometimes mating usually involves a display by males are subject to flash floods . toward females in which the male bobs his head rapidly up and down (`courtship nods', Carpen- Methods of Observation ter 1962 ; `jiggling', Ruibal 1967 ; `shuddering', Data were obtained from I May to 1 August Ferguson 1970) . If receptive, the female is quiet in 1969, 1 May to 15 September in 1970 and and the male mates with her . If not, she avoids 1971, and 1 May to 15 August in 1972 . The him by moving away, by nipping him, or by S. virgatus study areas were visited at least hopping stiffly on all four legs with her back twice each week, and the S. undulatus areas were arched ('sidlehopping', Carpenter 1962) . Females visited once or twice each week for 3 to 8 hr . lay their eggs by mid-summer, and hatchlings Lizards on study areas were marked perma- appear in mid-summer to early fall, probably nently by toe-clipping (Tinkle 1967), and by depending on when the eggs are laid, how long VINEGAR : AGGRESSION IN SCELOPORUS LIZARDS 2si they are retained in the body of the female, to 1971 and 1971 to 1972 are combined here . and temperature. Survivorship data for the two S. virgatus study Sceloporus virgatus females lay one clutch per areas were not significantly different and are year, with an average clutch size of 9 .5 eggs. combined in Table I. A 1 : 1 sex ratio at the Sceloporus u . consobrinus females lay at least egg stage was assumed . Details of the population four clutches per year, with an average clutch biology will be presented elsewhere (Vinegar, size of 9 .9 eggs. Sceloporus u. tristichus lay at in preparation) . least two clutches per year, with an average The higher incidence of broken or regen- clutch size of 7 .2 eggs. erated tails and the lower survivorship found in The sex ratio among S. virgatus yearlings females of S. u. consobrinus suggest that they (lizards in their first season following the are more vulnerable to predation than females season of hatchling) generally favoured females of S.