People are often surprised to learn that some butterflies are territorial. This may be due to the common view of butterflies as being carefree, fluttery creatures. Yet some male butterflies, including some in the genus Vanessa, are capable of maintaining and aggressively defending territories through purposive behaviors such as patrolling flights and complex interactions in flight with intruders of their own species (Bitzer and Shaw 1979).
One reason that territorial behavior was slow to be recognized and studied in insects was that physical aggression in defense of resources is often less overt than it is for birds and mammals (Baker, 1983). Butterflies especially might seem at first thought to have little capacity for physical aggression. Thus Scott (1974) and Suzuki (1976) insisted that what seemed to be a
How might we know whether a butterfly is truly being territorial?
Scott's critical interpretation of what many observers in the 1970s and 1980s were calling "territorial behavior" of butterflies touched off a controversy over whether any butterfly species were actually territorial, and over how one might determine that a butterfly was actually defending an area around its perch site, rather than simply flying up at moving objects passing overhead.
In response to such questions, Baker (1983) argued that an animal can defend a resource without once showing overt physical aggression. If we first observe that whenever an individual occupies an area, every intruder entering that area moves on rather than remaining, then this suggests the animal is excluding other individuals from the area. Suppose we then remove the occupant and find that the area is invariably occupied quickly by one of the first intruders to arrive. These two observations alone are enough for us to conclude that a territorial system is operating, he maintains, even if we never saw contact between occupant and intruder. Such removal experiments, according to Baker (1983), have been one of the primary means of demonstrating territoriality in insects (Baker, 1972; Davies, 1978).
A stronger argument for true territoriality among butterflies is to show that the distribution of outcomes of interactions between occupants and intruders follow patterns which game models predict should occur if the interactions are true territorial contests. These predictions are as follows: 1) larger and/or stronger butterflies should be more likely to win contests and to occupy and accumulate in territories; 2) the greater the differences in size and strength between contestants, the shorter the contest and the more quickly the winner is determined (Enquist and Leimar, 1983), and 3) given animals of equal size, occupants should win more contests than intruders (Maynard Smith and Parker, 1976; Leimar and Enquist, 1984).
Such evidence for true territoriality has been found for several
butterfly species. Wickman (1985) showed that resident Coenonympha
pamphilus males were larger on average than intruders. Rosenberg
and Enquist (1991) demonstrated that larger Limenitis
weidemeyerii males were more successful in both defending and
capturing territories, and, when size differences were taken into
account, occupants were more successful than intruders. One effect of
such competition is that larger males should tend to accumulate in
territories over time; this is, in fact, what Rosenberg and Enquist
(1991) and Bitzer and Shaw (in preparation) have found.
Are Scott's (1974) criteria necessary for territoriality?
Noble's (1939) definition of territoriality as a "defended area"
has been restated by Brown (1975) as "...a fixed area from which
intruders are excluded by some combination of advertisement...,
threat, and attack." This definition does not include Scott's (1974)
requirement of territory occupation for "several days." Another
complication that arises with Scott's definition is that the number
of days that V. atalanta males return to a particular site may
depend on whether or not they are migrating. Red Admirals of the
spring generation may migrate northward many miles each day, and as a
result, will set up a territory in a new location each afternoon. On
the other hand, summer generation butterflies, which appear to be
much more stationary, may often return to the same territory on
successive afternoons (Bitzer, in preparation). Yet the behavior of
butterflies of both generations appears similar. Should we then
conclude that spring generation individuals are not territorial,
whereas summer generation butterflies are territorial, simply because
of their degree of day-to-day movement?
Additional Complicating Factors: Might Weather Conditions Also be Influential?
For Red Admirals, temperature-induced asymmetry between conditions
experienced by occupants and intruders might explain part of the
advantage of being an occupant. Since temperature differences between
the warmer perching substrate and the cooler air flight level are
greater on cooler days (sunlit ground ca. 20°C warmer than the
air) and less on warmer days (ground ca. 5°C warmer), this might
partly explain why occupants are somewhat more likely to retain
territories on cooler days (Bitzer and Shaw, in preparation). If
temperature differences explained a substantial part of the advantage
of occupancy, however, we would expect this advantage to diminish
greatly on overcast days as well as later in the territorial period
on sunny and partly cloudy days as the sun drops lower in the sky and
substrate temperature decreases. Some experimental procedures, such
as providing an artificially-warmed perching substrate such as a
light-colored heating pad to see whether interaction durations or
occupants' probability of winning might increase, might also provide
some additional answers to this question.
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Baker, R.R. (1983). Insect territoriality. Annual Review of Entomology 18: 65-89.
Bitzer, R.J. and Shaw, K.C. (1979). Territorial behavior of the Red Admiral, Vanessa atalanta (L.) (Lepidoptera: Nymphalidae). Journal of Research on the Lepidoptera 18: 36-49.
Bitzer, R.J. and Shaw, K.C. (in preparation). Territorial interactions of the Red Admiral, Vanessa atalanta:. Larger butterflies, chasers, and occupants usually win.
Brown, J.L. 1975. The Evolution of Behavior. W.W. Norton and Company, Inc., New York.
Davies, N.B. (1978). Territorial defence in the Speckled Wood Butterfly, Pararge aegeria: the resident always wins. Animal Behaviour 26: 138-147.
Enquist, M. and Leimar, O. (1983). Evolution of fighting behaviour: decision rules and assessment of relative strength. Journal of Theoretical Biology 102: 387-410.
Leimar, O. and Enquist, M. (1984). The effect of asymmetries in owner-intruder interactions. Journal of Theoretical Biology 111: 475-491.
Maynard Smith, J. and Parker, G. (1976). The logic of asymmetric contests. Animal Behaviour 24: 159-175.
Noble, G.K. 1939. The role of dominance in the social life of birds. Auk 56: 263-273.
Rosenberg, R.H. and Enquist, M. (1991). Contest behaviour in Weidemeyer's Admiral Butterfly Limenitis weidemeyerii (Nymphalidae): the effect of size and residency. Animal Behaviour 42: 805-811.
Scott, J.A. (1974). Mate-locating behavior of butterflies. American Midland Naturalist 91: 103-117.
Suzuki, Y. (1976). So-called territorial behavior of the small copper, Lycaena phlaeas daimio Seitz (Lepidoptera: Lycaenidae). Kontyû Tokyo 44: 193-204.
Wickman, P.-O. (1985b). Territorial defence and mating success in males of the small heath butterfly, Coenonympha pamphilus (L.) (Lepidoptera: Satyridae). Animal Behaviour 33: 1162-1168.
Wickman, P.-O. (1988). Dynamics of mate-searching behavior in a hilltopping butterfly, Lasiommata megera (L.): the effects of weather and male density. Zoological Journal of the Linnaean Society 93: 357-377.
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