Investigating the Consistency of Mate-Locating Behavior in the Territorial Butterfly

P1: VENDOR Journal of Insect Behavior [joib] PP035-291465 January 15, 2001 16:6 Style file version Feb 08, 2000 Journal of Insect Behavior, Vol. 14, No. 1, 2001 Investigating the Consistency of Mate-Locating Behavior in the Territorial Butterfly Hypolimnas bolina (Lepidoptera: Nymphalidae) Darrell J. Kemp1 Accepted August 2, 2000; revised September 5, 2000 The study of butterfly behavior has afforded valuable insights into the evolution of alternative mating tactics. Two hypotheses derived from this area of research contend that (1) territoriality is only viable under low to moderate conspecific densities (due to the costs of site defence) and (2) perching may be employed only when thermal conditions constrain flight activity. These hypotheses were evaluated by investigating mate locating behavior in Hypolimnas bolina, a territorial species that is naturally subject to variation in population density and weather conditions. Male behavior was charted throughout the day during a period of high population density at an encounter site in tropical Australia. Perching was the primary tactic, although a small proportion of individuals patrolled nonaggressively in the afternoon. Population-level male behavior failed to support predictions drawn from either the “territory economics” or “thermal constraint” hypotheses. First, the proportion of perching males and the number of aggressive conspecific interactions (per male) increased with increasing male density at the site. Second, few males patrolled at the hottest, brightest time of day (approximately midday), and the diel distribution of perchers did not emulate the “U-shaped” distribution shown by the occurrence of dorsal basking behavior. These results show that perching in this species is not a suboptimal tactic employed when temperatures constrain flight activity but may represent the best method of locating receptive females. At this stage the reproductive significance of the observed patrolling behavior remains obscure. KEY WORDS: alternative mating tactics; sexual selection; intrasexual competition; behavioral plasticity; reproductive behavior; perching. 1School of Tropical Biology, James Cook University, P.O. Box 6811, Cairns, Queensland 4870, Australia. e-mail: [email protected]. 129 0892-7553/01/0100-0129$19.50/0 C 2001 Plenum Publishing Corporation P1: VENDOR Journal of Insect Behavior [joib] PP035-291465 January 15, 2001 16:6 Style file version Feb 08, 2000 130 Kemp INTRODUCTION Alternative male mating tactics are such a conspicuous component of ani- mal mating systems (reviews by Arak, 1984; Austad, 1984; Dominey, 1984; Gross, 1996) that this situation is now considered as the rule rather than the exception (Waltz and Wolf, 1984). In its simplest form, plasticity in mating behavior is expressed as the cooccurrence of territorial (aggressive) and nonterritorial or sneaky tactics (e.g., Arak, 1984; Gross, 1991; Higashi and Nomakuchi, 1997). Individual males of these species either may be morpho- logically “locked in” to specific tactics at certain life stages (e.g., Sinervo and Lively, 1996; Cook et al., 1997; Emlen, 1997) or may possess the flexibility to adopt either tactic (e.g., Davies, 1978; Alcock and Houston, 1987; Alcock, 1997). In all cases, the maintenance of alternative mating tactics within a population is believed to reflect underlying variation in either environmental conditions or competitive abilities (Austad, 1984, Hazel et al., 1990; Greeff, 1998). However, although a large body of empirical work has accumulated, the types and nature of environmental variation responsible for the evolution of behavioral plasticity are not completely understood. The study of butterfly behavior has provided a useful tool for document- ing the occurrence and ecological correlates of alternative mating tactics in animals (see Dennis, 1982; Wickman and Wiklund, 1983; Shreeve, 1984; Wickman, 1985, 1988; Alcock and O’Neill, 1986; Alcock, 1994; Hernández and Benson, 1998). Traditionally, the primary difference between individual mating tactics in this group has been related to mobility, that is, the extent to which males either perch or patrol to locate females. However, since perching behavior in butterflies is often accompanied by site defense (Dennis and Shreeve, 1988; Rutowski, 1991), species that exhibit behavioral plasticity invariably switch between territorial and nonterritorial behaviors (see examples as above). The correlates of these behavioral switches provide a framework for formulating adaptive hypotheses regarding the evolution of similar forms of plasticity in butterflies and animals generally. In most territorial systems, a proportion of males may be forced to adopt an alternative means of locating mates due to their inability to compete with superior males (Dawkins, 1980). This “best of a bad job” tactic has been observed in butterflies (e.g., Davies, 1978; Hernández and Benson, 1998), although the determinants of competitive superiority are not always clear (Austad et al., 1979; Stutt and Willmer, 1998). Outside of this prospect, the primary ecological correlates of conditional behavior of male butterflies are conspecific male density (Dennis, 1982; Alcock and O’Neill, 1986) and weather (Wickman, 1985, 1988). Males of several butterfly species freely switch between territorial perching and nonterritorial patrolling behavior P1: VENDOR Journal of Insect Behavior [joib] PP035-291465 January 15, 2001 16:6 Style file version Feb 08, 2000 Behavioral Consistency in H. bolina 131 depending on the density of conspecific males at the encounter site (Dennis, 1982; Alcock and O’Neill, 1986). This type of behavioral switch has generally been interpreted in terms of the economics of territory defense, with rising cost of defense eventually outweighing the reproductive benefits of this activity (Parker, 1978; Thornhill and Alcock, 1983; Alcock and O’Neill, 1986; Rutowski, 1991). Alternatively, male butterflies may switch between perching and patrolling depending on the prevailing thermal conditions (e.g., Dennis, 1982, 1987; Wickman and Wiklund, 1983; Shreeve, 1984; Wickman, 1985, 1988). Wickman (1985) suggested that perching behavior is the most ef- ficient method of mate location at suboptimal temperatures for flight, evolved by many temperate butterfly species due to the restrictions of low temperatures on extended flight activity. However, the generality of this hypothesis is unclear, and therefore, the role of thermal constraints as an influence on the evolution of butterfly mate locating tactics is not fully understood. Here I aim to broaden our general understanding of behavioral plasticity in butterflies by investigating the consistency of male mate-locating behavior in the tropical species Hypolimnas bolina (L.) (Nymphalidae). Males of this species are noted for their territorial male mate-locating behavior (McCubbin, 1971; Rutowski, 1992). This behavior (described in detail later) is comparatively very similar to that of other butterfly species that de- fend perching sites to maximize their encounters with receptive females (see Davies, 1978; Wickman and Wiklund, 1983; Rosenberg and Enquist, 1991; Lederhouse et al., 1992; Lederhouse, 1993; Hernández and Benson, 1998). Moreover, throughout tropical Australia, male H. bolina are potentially subject to regimes of ecological variation (outlined specifically below) similar to those of some territorial temperate species that exhibit alternative mating tactics [e.g., Coenonympha pamphilus (Wickman, 1985) and Strymon melinus (Alcock and O’Neill, 1986)]. This species therefore provides an ideal tropical candidate for testing predictions regarding the occurrence of behavioral plasticity in butterflies. Mate-locating male H. bolina are subject to considerable daily and sea- sonal variations in the two variables that correlate primarily with behavioral plasticity in temperate species—population density and weather conditions. First, although normally present at low densities throughout north Queensland (see Rutowski, 1992; Kemp, 1998), numbers of this species at particular sites can increase markedly at times during the wet season [im- mediately after extended rainy or overcast weather patterns (D. J. Kemp, unpublished data)]. This population variation is qualitatively similar to that described for S. melinus, a predominantly territorial species that possesses a density-related switch to patrolling behavior (Alcock and O’Neill, 1986). The “territory economics” hypothesis predicts that a greater proportion of male P1: VENDOR Journal of Insect Behavior [joib] PP035-291465 January 15, 2001 16:6 Style file version Feb 08, 2000 132 Kemp H. bolina should either patrol (Alcock and O’Neill, 1986; Rutowski, 1991) or continue to perch nonaggressively (Alcock, 1985) when the male density at the encounter site reaches extreme levels (Emlen and Oring, 1977). Second, because males are active at encounter sites for an extended daily period [from 0800 to 1700 h (Rutowski, 1992)], and temperature/solar radiation changes considerably during this time, mate-locating males are potentially subject to a broad range of thermal conditions. If territoriality in this species is adopted at suboptimal temperatures, then this behavior should be limited to those times of day when males are thermally constrained. The “thermal constraint” hypothesis predicts that a greater proportion of males should patrol in the middle of the day when temperatures are least limiting and that the diel distribution of perching behavior should follow a “U” shape (as demonstrated

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