The Evolution of Territoriality in Butterflies
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The evolution of territoriality in butterflies Preface In The descent of man, and selection in relation to sex (1871) Charles Darwin devoted a chapter to butterflies and moths in order to build up a body of facts and cases to support the theory of evolution through sexual selection. He gave plenty of examples of differences in coloration between males and females, and explained this by intersexual selection via female choice, but he also reflected on intrasexual selection through male-male conflicts: “Although butterflies are such weak and fragile creatures, they are pugnacious, and an Emperor butterfly has been captured with the tips of its wings broken from a conflict with another male.” -Charles Darwin (1871) Darwin was not only one of the first to describe male-male contests in butterflies, but was also the first to give a possible explanation for the origin and maintenance of such peculiar behaviour. In fact, Darwin refers to the naturalist Cuthbert Collingwood who described frequent contests between butterfly males in Borneo, on a scientific voyage in 1866-1867. Collingwood (1868) expresses his frustration over the troubles to capture unharmed specimens as he writes: “Another source of disappointment arose from the fact that not infrequent, when one thought oneself fortunate in capturing a fine insect, after carefully disentangling it from the net, its wings turned out to be so torn and rubbed as to render it almost useless, except indeed as a decoy. This circumstance is due, I imagine, partly to their frequent battles with one another, in which they whirl round each other with the greatest rapidity, and appear to be incited by the greatest ferocity...” -Cuthbert Collingwood (1868) Ever since Darwin‟s and Collingwood‟s time, researchers and naturalists have recorded and studied interactions between males of butterflies. A noticeable part of their behaviour is also that males often are exceptionally faithful to specific patches, which they frequently guard from intruding males via aerial flight disputes, just like the ones described by Collingwood (1868). But despite numerous publications and excellent field studies over decades, there are a number of parts concerning the evolution of territorial behaviour in butterflies that have never been thoroughly investigated. There are also parts of territorial behaviour in butterflies that have received a lot of scientific attention, but that still puzzle researchers today. The aim of this thesis is to investigate the parts of territorial behaviour in butterflies that are still largely unknown, and thereby to shed new light on the questions that despite earlier studies, remain unanswered. 1 Territoriality and mating success Resources are not expected to be randomly distributed in the environment (Parker 1974) and selection will favour animals to search where the resources are concentrated. But what are the resources? In vertebrates the defended resources are sites associated with reproduction or areas that relate to both feeding and reproduction, i.e. a “home range” where both sexes maintain all activities, including reproduction. In insects, a vast majority of the defended resources relates to mating opportunities (Baker 1983). If receptive mates are patchily distributed in the environment it will be favourable to search in areas where the probability of encountering receptive mates is particularly high. In butterflies, the way mates find each other can be divided into two categories: “perching” and “patrolling” (Scott 1974; Wiklund 2003). Males of patrolling species spend most of their time on the wing and actively search for receptive females. This strategy implies continuous flight, stopping only for feeding or when weather conditions make it difficult to fly. In a patrolling species the male typically detects females while he is flying, whereupon the male approaches the female and courts her. In perching species, the male remains stationary and applies a sit-and-wait tactic, resting at some vantage point, waiting for females to fly by. Here the female is the active and mobile part and if the female flies into the visual field of a perching male, the male will take off from the perching site and investigate the female. The couple will eventually alight whereupon the male starts his courtship ritual. The areas used by males as perching sites are often well defined and may correlate to some resource utilised by females, such as larval host-plants (Baker 1972; Courtney & Parker 1985; Rosenberg & Enquist 1991; Lederhouse et al. 1992) or female food resources (Suzuki 1976; Fischer & Fiedler 2001). However, areas used as perching sites may also lack any obvious correlation to female resources and consist of some topographical or physical structure such as gullies (Cordero & Soberon 1990), elevations and hilltops (Shields 1967; Lederhouse 1982; Alcock 1987) or trees and bushes (Wickman 1985a). Whatever the perching site may be, it is always expected to have a high flow of receptive females passing by. Males of perching species are often exceptionally faithful to the perching sites and will attempt to exclude other males from the area. Consequently, males of perching species are often defined as territorial. The territories are thought to serve as a rendezvous place where the sexes meet and are expected to be located in areas where the probability to encounter females is particularly high. Consequently, territory residency is assumed to be correlated with high mating success. However, there are few studies that give empirical support for such a prediction (although see Wickman 1985b), and the consensus that territories are used as rendezvous sites is largely based on circumstantial evidence. In paper I we wanted to empirically test the hypothesis that territory residency in butterflies is correlated to high mating success. This would shed new light on the evolution of territoriality in butterflies, since higher mating success for territory owners implies a greater reproductive output and selection for such behaviour. We used the speckled wood butterfly (Pararge aegeria) as a model species (figure 1a). P. aegeria is one of the most frequently used species for studies in butterflies. Over the last decades it has become something of a model species in butterfly research and used in studies on behaviour (e.g. Davies 1978; Wickman & Wiklund 1983; Shreeve 1984, 1986, 1987; Van Dyck et al. 1997a,b; Stutt & Willmer 1998; Kemp & Wiklund 2004; Kemp et al. 2006a,b), life history traits (e.g. Gotthard et al. 1994; Berger et al. 2008; Gotthard & Berger 2010) and climate change effects on the distribution of animals (e.g. Hill et al. 1999). Males of P. aegeria use a perching mate locating strategy and are found perching in large sunspots on the 2 forest floor (figure 1b, Davies 1978; Wickman & Wiklund 1983). If a flying object enters the sunspot area, occupied by a P. aegeria male, the resident male immediately takes off and pursues the intruder to investigate what the intruding object might be. If the intruder is a conspecific female, a flight pursuit follows. However, the nature of the flight pursuit is highly dependent on the mating status of the female. If the female is already mated the flight pursuit is ended after just a few seconds, often caused by the female doing a vertical drop into the vegetation. If the female is receptive the flight pursuit will be significantly longer (Bergman, M. unpublished). The female will conduct a rapid flight and the male will follow at a close distance behind. The couple will eventually alight in the vegetation, whereupon the male starts a courtship ritual. But if another male enters the sunspot area, the territory resident will take off and the two will engage in a flight contest (figure 1c), where the winner gets sole ownership of the sunspot and the loser leaves the area and has to search for a new suitable sunspot (Davies 1978; Wickman &Wiklund 1983). (a) (b) (c) Figure 1: (a) A male of the speckled wood butterfly (Pararge aegeria) on an oak leaf. (b) A typical habitat for P. aegeria - a deciduous forest in Ransvik, Kullaberg. (c) Two P. aegeria males in a territorial contest over the residency of a large sunspot. Photo (a) and (b) by Christer Wiklund. Photo (c) by Martin Bergman. 3 There are surely several reasons why studies on mating success in butterflies are few, but it often comes down to the difficulties to observe natural matings in field. Typically females mate very soon after eclosion and a majority of the females in the wild are already mated (Wiklund & Fagerström 1977; Wiklund 2003). Studies on mating success require individual observation of unmated receptive females and, consequently, these studies require the release of laboratory-reared females. Another difficulty with studies of natural mating in butterflies is that in perching species mating does not necessarily occur in the territories (e.g. Bitzer & Shaw 1983; Alcock & Gwynne 1988; Brown & Alcock 1990). When a perching male detects a passing female the male flies up and investigates the passing female. The female response for this is usually a rapid flight, with the male in close association to the female. This can be a way for non-receptive females to avoid male courtship but can also be a way to test the male‟s ability. However, non-receptive females might also respond to the inspecting male by doing a vertical drop into the vegetation (Wiklund pers comm). In some species, such as Polygonia c- album, Aglais urticae, Inachis io and Vanessa atalanta, the courtship phase is excessively prolonged; the male follows the female for hours before mating is initiated (Wiklund pers comm). Consequently, matings occur somewhere away from the perching sites and this makes it virtually impossible to locate butterflies in copula. Thirdly, when a female and a male have alighted and copulate they are usually very cryptic and hard to find in nature.