Journal of Herpetology, Vol. 41, No. 3, pp. 354–359, 2007 Copyright 2007 Society for the Study of Amphibians and Reptiles Does the Lizard Platysaurus broadleyi Aggregate Because of Social Factors? 1 LUKE SCHUTZ,DEVI STUART-FOX, AND MARTIN J. WHITING School of Animal, Plant, and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa ABSTRACT.—Aggregations are a common feature of many species although for most taxa, the mechanisms underlying these aggregations are poorly understood. The Augrabies Flat Lizard (Platysaurus broadleyi)is a sexually dimorphic lizard that experiences intense conflict as a result of sexual selection. In the wild, P. broadleyi share communal crevices and aggregate in the presence of large insect plumes. We experimentally tested whether lizards aggregate as a result of social factors. We also tested whether aggregative behavior differed between the sexes and depends on density. We found no evidence that Augrabies Flat Lizards preferentially group for social reasons in the absence of resources or thermoregulatory benefits. This was true for both sexes and at both densities (two-lizard and four-lizard trials). Although social factors did not promote grouping, males sheltered alone significantly more often than expected by chance, suggesting that males actively avoid one another. Therefore, social factors may work to promote social isolation rather than aggregation, under certain circumstances. The formation of aggregations is a behavior form when there is mutual attraction between that has evolved in a number of different conspecifics because the presence of conspeci- lineages, including mammals, birds, fish, in- fics increases an individual’s fitness (Graves sects, and reptiles (e.g., Brown, 1986; Griffiths and Duvall, 1995). and Magurran, 1998; Grether and Switzer, 2000; The structure of social groups is often sex- McGuire et al., 2002; Jeanson et al., 2005; specific because of interactions within and Schradin and Pillay, 2005; Visagie et al., 2005). between the sexes (Wikelski et al., 1996; Persaud Recently, there has been increasing interest in and Galef, 2003; Pilastro et al., 2003; Dadda et al., reptilian aggregation behavior because it can 2005). For example, female eastern mosquitofish provide insight into the evolution of sociality experience high levels of harassment from males (e.g., Duffield and Bull, 2002; O’Connor and and form larger, more closely knit aggregations Shine, 2003; Shah et al., 2003; Chapple and as a result of male presence (Dadda et al., 2005; Keogh, 2006; Lancaster et al., 2006). Grouping Pilastro et al., 2003). Because levels of intraspe- behavior has independently evolved numerous cific competition and sexual conflict are often times in reptiles, but the mechanisms driving density dependent (Fitze et al., 2005; Le Galliard aggregation are poorly understood for the great et al., 2005), population density can influence the majority of these lineages (O’Connor and Shine, presence or nature of sex-specific grouping 2003). behavior. However, the effect of population Aggregations may be driven by either eco- density per se has seldom been tested. logical or social factors. Ecologically driven The Augrabies Flat Lizard, Platysaurus broad- aggregations occur when access to key re- leyi, presents a striking example of aggregation sources such as food, basking sites, oviposition behavior. These lizards form large aggregations sites, and mates is restricted because of their in the presence of plumes of black flies (their limited availability, their clumped spatial dis- primary prey) and fruiting Namaqua fig trees tribution, or their asynchronous availability (Greeff and Whiting, 2000) throughout the year. (Graves and Duvall, 1995). As a consequence In addition, individuals may refuge communal- of patchy or limited resources, the costs to an ly with over 100 lizards in a single crevice, and individual of trying to exclude the rest of the often these aggregations are strongly male population from a particular resource out- biased (MJW, unpubl. data). Small numbers of weighs the benefits, and aggregations will form females refuge in these large, male-biased, communal crevices, but where most females (Graves and Duvall, 1995). In contrast to refuge is unknown. One possibility is that ecological aggregations, social aggregations females may refuge in smaller groups with resident males that have crevices on their 1 Corresponding Author. E-mail: martin@gecko. territories. Therefore, social factors may play wits.ac.za important roles in aggregation behavior in this AGGREGATION IN FLAT LIZARDS 355 system. Platysaurus broadleyi is highly sexually nullify previous social interactions that might dimorphic (Branch and Whiting, 1997; Whiting influence a lizard’s propensity to aggregate. et al., 2006). Competition among males for However, we believe any effect of social history access to mates is intense (Whiting et al., 2003, would be negligible because lizards were pre- 2006), and females are subject to high levels of viously housed in small groups of only a few sexual harassment (MJW, pers. obs.). Given the individuals. These tanks were separated by different selective pressures facing each sex, opaque dividers to prevent visual contact males and females may have evolved different among individuals. Lizards were provided with patterns of aggregative behavior. We conducted fresh, folded newspaper for refuge, fed vitamin- experiments to examine whether social factors supplemented cat food and mealworms three are driving the aggregative behavior in P. times per week, and provided with water ad broadleyi and whether grouping behavior differs libitum. Lizards were individually numbered between the sexes. Specifically, we tested using a nontoxic xylene-free marker pen. whether lizards preferentially share refugia Experimental Design.—Experiments were per- when temperature is controlled and when formed in a heated room, maintained between resources (food and shelter) are not limited. In 32.5 and 33.5uC. The suspected preferred body addition, we tested whether aggregative behav- temperature of P. broadley is 35uC (D. Bauwens, ior differs between the sexes and whether it unpubl. data). Although the lizards were main- depends on density. tained at slightly lower room temperatures, P. broadleyi are active in the field at even lower temperatures and grouping for thermoregula- MATERIALS AND METHODS tory reasons can likely be ruled out. A 12:12 Study Population.—Platysaurus broadleyi is (light:dark) diel cycle was used to mimic the a relatively small cordylid lizard (64–84 mm natural photoperiod. The room contained six adult snout–vent length, SVL) restricted to 120 3 90 3 75 cm enclosures lined with a thin rocky outcrops in the Gordonia-Kenhardt dis- layer of plastic and containing shelters consist- tricts of Northern Cape Province, South Africa ing of a 20 3 20 cm terracotta tile raised 3 cm in (Branch and Whiting, 1997). We used captive the front and sloping down to ground level at lizards from Augrabies Falls National Park the back. The number of shelters in each (28u359S, 20u209E), hereafter referred to as enclosure corresponded to the number of Augrabies. The area is xeric, and the habitat lizards for a given trial (i.e., one shelter per consists of continuous granite sheets and lizard). The shelters were arranged equidistant boulders flanking the Orange River, sparsely from each other and the sides of the enclosures. dotted with Namaqua fig trees (Ficus cordata). This experimental design controlled for all Lizards shelter communally in crevices at night environmental variables that could promote and during the hottest parts of the day, with grouping, allowing us to investigate whether over 100 lizards having been observed in social factors are involved in P. broadleyi’s a single crevice (MJW, unpubl. data). Lizards spatial behavior. travel from crevices (normally ,100 m per day) We ran 20 trials for each of six treatments: (1) to feeding areas (where there are plumes of two males; (2) four males; (3) two females; (4) black flies or a fruiting fig tree) or, in the case of four females; (5) one male, one female; and (6) resident males, to a territory that they vigor- one male, three females. To ensure that differ- ously defend (Branch and Whiting, 1997; Whit- ences between treatments were not caused by ing et al., 2006). Although males are preferen- the order in which they were conducted, all six tially territorial, the high density of lizards, treatments were run concurrently. Lizards were particularly in preferred areas near the river, randomly allocated in the first set of trials and results in many males adopting a ‘‘floater’’ used in multiple trials (mean 6 SD 5 6.15 6 tactic. Because females must travel through the 1.20, range 3–8) but never in the same combi- territories of multiple males on their way to and nation. Thus, no lizard encountered the same from feeding areas and regularly encounter individual more than once in any of the trials floater males, levels of sexual harassment are across all six treatments. To minimize stress, high (MJW, pers. obs.). lizards were not used on consecutive days (i.e., Lizard Husbandry.—For experiments, we used they were given at least one day of rest between a captive colony of 23 adult males and 30 adult trials). females, which had been in captivity for 3– Trials were conducted from 1300–0800 the 4 years. Twelve weeks prior to the experiments, following morning. At the start of each trial, all the lizards were placed in separate, 30 3 30 lizards were released one by one into their 3 30 cm glass tanks, in rooms maintained at respective treatments, and the shelter (i.e., the 29uC on a 12:12 (light:dark) diel cycle. We tile) they selected was noted. The following cannot be sure that 12 weeks was sufficient to morning, their final shelter site selection was 356 L. SCHUTZ ET AL. TABLE 1. To test for aggregative behavior in the lizard Platysaurus broadleyi, we ran 20 trials for each of six treatments (listed in text).