ANIMAL BEHAVIOUR, 2000, 59, 861–870 doi:10.1006/anbe.1999.1397, available online at http://www.idealibrary.com on Is there an intraspecific role for density-dependent colour change in the desert locust? GREGORY A. SWORD & STEPHEN J. SIMPSON Department of Zoology and University Museum of Natural History, University of Oxford (Received 29 July 1999; initial acceptance 23 September 1999; final acceptance 7 December 1999; MS. number: A8555) Attempts to uncover the adaptive significance of density-dependent colour polyphenism in the desert locust, Schistocerca gregaria (Orthoptera: Acrididae), have been unsuccessful. Desert locust juveniles can change colour as part of a phenotypically plastic response to changes in local population density known as phase polyphenism. They are typically cryptic in colour at low rearing density (solitarious phase), but become conspicuous at high density (gregarious phase). Recent evidence indicates that this colour change functions interspecifically as an aposematic signal. Other recent evidence, however, suggests that previous attempts to demonstrate an intraspecific function of gregarious coloration in mediating group interactions among locusts may have been confounded by the effects of multiple sensory cues. We reinvestigated the intraspecific function of density-dependent colour polyphenism and specifically controlled for potentially confounding olfactory and tactile cues. We found no effect of gregarious phase (yellow and black) coloration as either a gregarizing stimulus to behaviourally solitarious locusts or as a visual aggregation stimulus behaviourally to gregarious locusts. We did, however, find that nonmoving solitarious phase (green) coloration significantly increased the activity levels of behaviourally gregarious locusts. We cannot explain this result and its biological relevance remains unknown. In the absence of support for the intraspecific visual cue hypothesis, we favour an aposematic perspective on the function of density-dependent colour polyphenism in the desert locust. The aposematic perspective parsimoniously accounts for density-dependent changes in both colour and behaviour.  2000 The Association for the Study of Animal Behaviour Many organisms display phenotypic plasticity in colora- tion (e.g. Fuzeau-Braesch 1985). Prior to the explosion of investigations into the role of phenotypic plasticity in evolution (see Schlichting & Pigliucci 1998) few studies were concerned with the evolutionary significance of these changes (Booth 1990). Locust species display an extreme form of phenotypic plasticity known as phase polymorphism (polyphenism). In these species, changes in local population density result in changes in a broad array of physiological, behavioural and morphological traits including coloration (Uvarov 1966; Pener 1991; Pener & Yerushalmi 1998). Owing to its agroeconomic importance, phase polyphenism in the desert locust, Schistocerca gregaria (Orthoptera: Acrididae), has been under investigation for nearly 75 years. Despite this, evidence in favour of an adaptive function for density- dependent colour change has only recently emerged (Sword et al. 2000). Desert locust juveniles are cryptic and typically green when reared at low population density (solitarious phase, or solitaria), but when reared at high density they become black with a contrasting yellow or orange background colour (gregarious phase, or gregaria; Stower 1959). Early on in the study of this polyphenism, the obvious con- spicuousness of gregarious phase coloration led investi- gators to suspect that it functioned interspecifically as aposematic coloration (Key 1957; Kennedy 1962). Repeated observations of desert locust consumption in both the field and the laboratory failed to provide any support for the aposematic hypothesis, however, and it was largely discarded (Gillett & Gonta 1978). In the absence of an interspecific aposematic function, investigators suspected that the colour patterns func- tioned as intraspecific visual cues. Because density- dependent changes in colour and behaviour are correlated in locusts, colour change was hypothesized to mediate density-dependent behavioural interactions among locusts, specifically nymphal aggregation (Ellis & Pearce 1962; Gillett 1973) and group cohesion during nymphal movement over land (marching behaviour) (Gillett & Gonta 1978). Although visual stimuli are Correspondence: G. Sword, Section of Integrative Biology and Brackenridge Field Laboratory, University of Texas, Austin, TX 78712, U.S.A. (email: gsword@uts.cc.utexas.edu). S. J. Simpson is at the Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K. 0003–3472/00/040861+10 $35.00/0  2000 The Association for the Study of Animal Behaviour 861