Predators are not so lucky in picking out a flightless insect from a marching mass.
Abstract
Mormon crickets and juvenile locusts form huge migratory bands — millions of individuals march in unison across the landscape1,2,3 and devastate vast agricultural areas, but little is known about why these bands form. Here we use radiotelemetry to show that band membership benefits these insects by greatly reducing the probability that they will become victims of predators. It is likely that migratory banding has evolved because it gives substantial protection to individuals within the group.
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Protection against predators through mechanisms such as enhanced early detection, predator confusion and dilution of risk (the ‘selfish herd’ effect) has been proposed to account for the evolution and maintenance of large aggregations in animals4,5. The anti-predator benefits of migratory-band formation have previously been assumed to be negligible3, but have never been quantified — largely because of the difficulty of tracking individuals within bands and the rarity of witnessing predation events1,3,4. We have used radiotelemetry to overcome these problems.
The mormon cricket, Anabrus simplex, is a flightless katydid6, native to western North America, which forms spectacular migratory bands. These can be more than 16 kilometres long and several kilometres wide, with each square metre containing dozens of insects that walk up to 2 km a day2,7 (Fig. 1). In a replicated mark–recapture experiment, we compared the survival of individual mormon crickets in naturally occurring migratory bands with that of individuals transplanted from the band to nearby sites; mormon-cricket bands had previously travelled through these sites, but they were empty at the time of the experiment. Individuals were located using small radiotransmitters8 (Fig. 2a), which enabled us to establish their fate with accuracy (for methods, see supplementary information).
Insects that had been translocated from the band suffered 50–60% mortality due to predation over just two days in three replicate experiments (Fig. 2b). By contrast, we observed no mortality among individuals in the band during the same period, including among those crickets moved to control for any transportation effect (Life Table Survival analysis, Gehan's generalized Wilcoxon test: P was 0.0118, 0.0047 and 0.0026 for three replicates, respectively; Fig. 2b).
Predation was evident from recovered radiotransmitters that had been partially chewed (Fig. 2a, inset) and often had body parts still attached; they were retrieved from trees and burrows, suggesting that birds and rodents are likely predators2. Two radiotransmitters were never recovered, presumably because of predator damage or removal from our detection range.
The evolution and maintenance of migratory-band formation requires the fitness benefits of group living to outweigh its costs4. Our results indicate that band formation confers a major anti-predator benefit, but what are the costs? Although difficult to quantify, intraspecific competition for resources is assumed to be a cost of group living and can account for group movement to exploit new resources1,3,4. Intraspecific competition in mormon-cricket bands mediates a food-stress-induced reversal of courtship roles at high population densities7,9. Mormon crickets are also notoriously cannibalistic and likely to attack immobile conspecifics2,7. Without discounting other hypotheses4, our results indicate that aggregation and constant movement protect band members from predators while reducing costs due to competition for resources and cannibalism. They also support a general anti-predator role for outbreaks such as those in periodical cicadas, where synchronized, mass emergence reduces predation on individuals by sating local predators10,11.
References
Uvarov, B. P. Grasshoppers and Locusts Vol. II (Centre for Overseas Pest Research, London, 1977).
MacVean, C. M. in Integrated Pest Management on Rangeland: A Shortgrass Prairie Perspective (ed. Capinera, J. L) 116–136 (Westview, Boulder, Colorado, 1987).
Dingle, H. Migration: The Biology of Life on the Move (Oxford Univ. Press, Oxford, 1996).
Krause, J. & Ruxton, G. D. Living in Groups (Oxford Univ. Press, Oxford, 2002).
Hamilton, W. D. J. Theor. Biol. 7, 1–52 (1971).
Sword, G. A. Anim. Behav. 69, 437–444 (2005).
Gwynne, D. T. Katydids and Bush-Crickets: Reproductive Behavior and Evolution of the Tettigoniidae (Cornell Univ. Press, Ithaca, New York, 2001).
Lorch, P. D. & Gwynne, D. T. Naturwissenschaften 87, 370–372 (2000).
Gwynne, D. T. Ecology 74, 1406–1413 (1993).
Karban, R. Ecology 63, 321–328 (1982).
Williams, K. S., Smith, K. G. & Stephen, F. M. Ecology 74, 1143–1152 (1993).
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Supplementary information
Supplementary Information
File contains additional methods and background information. (DOC 20 kb)
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Image of band of Mormon crickets. (JPG 154 kb)
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Sword, G., Lorch, P. & Gwynne, D. Migratory bands give crickets protection. Nature 433, 703 (2005). https://doi.org/10.1038/433703a
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DOI: https://doi.org/10.1038/433703a
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