Sex, Age, and Population Density Affect Aggressive Behaviors in Island Lizards Promoting Cannibalism William E
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ethologyinternational journal of behavioural biology Ethology RESEARCH PAPER Sex, Age, and Population Density Affect Aggressive Behaviors in Island Lizards Promoting Cannibalism William E. Cooper Jr*, Ioannis Dimopoulos† & Panayiotis Pafilis† * Department of Biology, Indiana University Purdue University Fort Wayne, Fort Wayne, IN, USA † Section of Zoology and Marine Biology, Department of Biology, University of Athens, Athens, Greece Correspondence Abstract Panayiotis Pafilis, Zoology and Marine Biology, Department of Biology, University of Island populations may evolve distinct behavioral repertoires as a Athens, Panepistimioupolis, Ilissia 157-84, response to the conditions of insular life. Strong intraspecific competition Athens, Greece. is typical in insular lizards and may include cannibalism. In this study, we E-mail: ppafi[email protected] investigated sexual and age patterns of aggression in two populations of the Skyros wall lizard (Podarcis gaigeae), one from the main island of Sky- Received: July 2, 2014 ros (Aegean Sea, Greece) and another from the satellite islet Diavates. The Initial acceptance: July 26, 2014 latter is terrestrial predator-free biotope, hosting a dense population of Final acceptance: October 4, 2014 large-bodied lizards that have been reported to exert cannibalism. In (M. Herberstein) staged encounters, we examined the aggressive propensities of adult male doi: 10.1111/eth.12335 and female lizards against their age-peers and juveniles. Males from both populations were much more aggressive than females toward juveniles Keywords: intraspecific competition, and other adults. Males from Diavates were more frequently aggressive to insularity, aggressiveness, Lacertidae juveniles and other male lizards than males from Skyros. Diavates canni- bals also captured their targets at shorter latency. We ascribe this distinct behavioral pattern to the high population density. Infanticide and intra- male aggressiveness confer two great advantages to cannibals: food and elimination of future rivals. et al. 2001; Novosolov et al. 2013), triggering high Introduction intraspecific competition. This form of competition Island life deviates in many ways from mainland bio- includes aggression that frequently causes amputation logical norms (MacArthur & Wilson 1967). The Island of body parts such as toes (Vervust et al. 2009) or tails Syndrome (Adler & Levins 1994) and Island Rule (Jennings & Thompson 1999; Cooper et al. 2009b). (Van Valen 1973) describe some of the adaptations The most severe expression of intraspecific competition that animals have developed in response to insularity. is the killing and consumption of juvenile conspecifics In lizards, insular species and populations differ from (Jenssen et al. 1989; Elgar & Crespi 1992). their continental peers in a constellation of features Filial cannibalism depends upon a range of ecologi- such as body size, life history, digestion, thermal biol- cal conditions and population-level resource competi- ogy, and defense (Pafilis et al. 2007; Meiri et al. 2011; tion (Klug et al. 2006; Bonsall & Klug 2011). The Novosolov & Meiri 2013; Sagonas et al. 2013a; Coo- evolution of this strategy requires a trade-off between per et al. 2014). Behavior represents no exception. current and future reproductive success in which cur- Particular behaviors have been described from insular rent loss is minimized by the consumption of lower- populations, such as island tameness (Cooper et al. quality offspring (Manica 2004; Klug & Bonsall 2007). 2009a, 2014) and strong intraspecific competition Furthermore, infanticide may have a strong impact (Pafilis et al. 2008; Vervust et al. 2009). on community structure and trigger shifts in juve- Islands host depauperate communities that experi- niles’ biology (Melton 1982; Wagner & Wise 1996; ence lower predation risk and relaxed interspecific Keren-Rotem et al. 2006). Both sexes may engage in competition (Losos & Ricklefs 2009). As a result, insu- filial cannibalism (Bartlett 1987), only females (Kle- lar populations of lizards are particularly dense (Rodda mme et al. 2006; females are more prone to sexual Ethology 120 (2014) 1–10 © 2014 Blackwell Verlag GmbH 1 Aggression and Cannibalism in Island Lizards W. E. Cooper, I. Dimopoulos & P. Pafilis cannibalism, Pruitt et al. 2014) or, more frequently, Skyros Archipelago, where it occurs in almost all hab- only males (Mehlis et al. 2010). However, factors that itats (Valakos et al. 2008). The reproductive period lead to differences in infanticide among populations lasts from early Mar. until late Jul., and females lay within a single species are poorly understood. one to three eggs, although clutch size in Diavates In this study, we aimed to shed light on the factors may reach five eggs (Pafilis et al. 2011). Podarcis gai- that promote cannibalism in small island communi- geae eats primarily terrestrial arthropods, mainly ties. To this end, we assessed the cannibalistic behav- insects (Adamopoulou et al. 1999). ior in different populations of the endemic Skyros Our study focused on a population from the main wall lizard (Podarcis gaigeae; Aegean Sea, Greece). island of Skyros (area 207 km2, location Palamari, N Twenty-one islets surround the main island of Skyros 38°5704400,E24°3002600) and the giant’s population and one of them, Diavates, harbors a lizard population from the satellite islet Diavates (area 0.019 km2,N that differs from the others in color morph (Rune- 38°4702000,E24°3004700). Diavates has several impor- mark et al. 2010), thermoregulation strategy (Sagonas tant ecological particularities. It is a terrestrial preda- et al. 2013b), reproductive performance (Pafilis et al. tor-free islet that also has minimal interspecific 2011), and ectoparasite load (Pafilis et al. 2013). The competition (Pafilis et al. 2009a, 2011). There is only most striking difference although lies in body size: liz- one other terrestrial vertebrate there, the Kotschy’s ards from Diavates are 40% longer, and almost 300% gecko (Mediodactylus kotshyi), which does not compete heavier than those in other populations (Pafilis et al. with lacertids for food or space (Valakos & Vlachop- 2009a). Previous studies, based on stomach content anos 1989). The vegetation is lush, composed of analysis, provided solid evidence of cannibalism in nitrophilous species, contrary to the typical phrygana Diavates: adult male P. gaigeae consumed body parts and maquis vegetation of the other islets (Bohling (tails and feet) of conspecifics and even entire juve- et al. 2002; Snogerup & Snogerup 2004). This dense niles (Adamopoulou et al. 1999; Pafilis et al. 2009a). herbaceous vegetation is supported by marine subsi- Namely, only a low 1.2% of Skyros stomachs con- dies that reach the islet through a thriving colony of tained conspecifics’ parts while the equivalent per- the yellow-legged gull (Larus michahellis) (Pafilis et al. centage for Diavates was 21.42%. Although some of 2011). The increased primary productivity of Diavates the special features of the particular Diavates popula- leads to a richer invertebrate fauna (Pafilis et al. tion have been attributed to intense intraspecific com- 2013) that fuels the densest population recorded in petition (Pafilis et al. 2009a, 2011; Sagonas et al. the eastern Mediterranean: 850 individuals per hect- 2013b), no direct evaluation of cannibalism has been are (Palamari population harbors 185 lizards per hect- made. are, Pafilis et al. 2009a). To quantify the extent of cannibalistic propensities, In two consecutive field seasons (late Jun. 2012 and we staged encounters in the laboratory and focused 2013), we captured by noose 175 lizards (68 males, 41 on two populations, one from Diavates and another females and 7 juveniles from Skyros and 29 males, 25 from Skyros. We formulated three hypotheses. First, females and 5 juveniles from Diavates). We aimed we expected that assaults on juveniles would be more only for non-gravid females (tested for gravidity with frequent by male lizards than females as males are lar- palpation). Lizards were transferred to the facilities ger and more frequently cannibalistic in interspecific of the Biology Department at the University of studies. Second, we anticipated that Diavates lizards Athens. Lizards were housed in glass terraria would attack juveniles more frequently than their (20 9 25 9 15 cm) at a constant temperature (25°C) Skyros kin because their larger size makes cannibal- and controlled photoperiod (12 h light: 12 h dark). ism easier. Third, we predicted that, due to the more One incandescent heat lamp (60 W) in each terrar- intense intraspecific competition on Diavates, adult ium provided 8 h of light per day and permitted bask- lizards would behave more aggressively toward their ing. Lizards were fed mealworms three times weekly age-peers than those from Skyros. and had access to water ad libitum. Food was withheld from all animals for the 48 h preceding the experi- mental trials. Materials and Methods Study System Behavioral Tests The Skyros wall lizard is a small-bodied lacertid with We recorded lizard behavior in staged encounters that snout-to-vent length (SVL) approximately 60 mm were held in an arena of 150 cm2 with a digital cam- and body mass ca. 6.6 g. The species is endemic to the era (Canon FS200) that was hidden behind a folding 2 Ethology 120 (2014) 1–10 © 2014 Blackwell Verlag GmbH W. E. Cooper, I. Dimopoulos & P. Pafilis Aggression and Cannibalism in Island Lizards screen. In each trial, we first released the lizard nov and Levene’s tests, respectively. When variances