The brown tree snake (Boiga irregularis), accidentally introduced on Guam shortly after World War 11, is the target of extensive efforts aimed at reducing populations and preventing their spread to other Pacific islands. Chemical attractants and repellents have been investigated, and this paper presents an overview of current knowledge. In particular, chemical cues that have strong effects in laboratory tests have had only modest (though significant) effects in field tests on Guam. Reasons for the different outcomes of laboratory and field studies are discussed along with recommendations for the redesign of laboratory experiments

Chiszar, David

Rodda, Gordon H.

Smith, Hobart M.

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
National Wildlife Research Center Repellents 
Conference 1995 
USDA National Wildlife Research Center 
Symposia 
August 1995 
Experiments on Chemical Control of Behavior in Brown Tree 
Snakes 
David Chiszar 
University of Colorado, Boulder, CO 
Gordon H. Rodda 
National Biological Service 
Hobart M. Smith 
University of Colorado, Boulder, CO 
Follow this and additional works at: https://digitalcommons.unl.edu/nwrcrepellants 
 Part of the Natural Resources Management and Policy Commons 
Chiszar, David ; Rodda, Gordon H.; and Smith, Hobart M., "Experiments on Chemical Control of Behavior in 
Brown Tree Snakes" (1995). National Wildlife Research Center Repellents Conference 1995. 8. 
https://digitalcommons.unl.edu/nwrcrepellants/8 
This Article is brought to you for free and open access by the USDA National Wildlife Research Center Symposia at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in National Wildlife Research 
Center Repellents Conference 1995 by an authorized administrator of DigitalCommons@University of Nebraska - 
Lincoln. 
11 
Experiments on Chemical Control of Behavior in 
Brown Tree Snakes 
David Chiszar, Department of Psychology, University of Colorado, Boulder, CO 
80309-0345 
Gordon H. Rodda, National Biological Service, Fort Collins, CO 80527 
Hobart M. Smith, Department of Environmental Population, and Organismic 
Biology, University of Colorado, Boulder, CO 80309 
ABSTRACT 
The brown tree snake (Boiga irregularis), accidentally introduced on Guam shortly after 
World War 11, is the target of extensive efforts aimed at reducing populations and preventing their 
spread to other Pacific islands. Chemical attractants and repellents have been investigated, and 
this paper presents an overview of current knowledge. In particular, chemical cues that have 
strong effects in laboratory tests have had only modest (though significant) effects in field tests 
on Guam. Reasons for the different outcomes of laboratory and field studies are discussed along 
with recommendations for the redesign of laboratory experiments. 
KEY WORDS 
attractants, brown tree snakes (Boiga irrenularis), captivity, field tests, foraging, 
Guam, habituation, laboratory tests, repellents, supersensitivity 
INTRODUCTION 
Brown tree snakes are rear-fanged colubrids, native to New Guinea and to nearby islands, as 
well as to northern Australia. This species was accidentally introduced on Guam shortly after 
World War 11, with a variety of primary and secondary consequences unfolding during subsequent 
decades (Fritts 1988, Rodda and Conry 1992). Half of Guam's bird species are now extinct, and 
most remaining species are in precarious condition, as are several species of mammals and lizards 
(Savidge 1987, Rodda and Fritts 1992). The snake, being venomous, also presents a medical 
problem, although only very young humans appear to be at risk (Fritts and Haddock. 1990, Vest 
et al. 1991, Weinstein et al. 1991). Numerous electrical outages are caused by brown tree snakes 
as these arboreal snakes climb power poles in search of avian or saurian prey (Fritts et al. 1987). 
Efforts to control brown tree snakes on Guam have emphasized the need for attractants to enhance 
success of trapping programs and for repellents to drive snakes out of sensitive areas, including 
critical habitat for endangered species and cargo-holding facilities from which stowaway snakes 
could be transported to other islands. 
122 ATTRACTANTS A N D  REPELLENTS FOR SNAKES 
We want to make three points during this presentation. The first is that we have data on 
attractants and repellents in brown tree snakes, so at least modest progress has been made in these 
directions. Second, our research paradigms have been borrowed, with only slight modification, 
from the basic, theoretically driven studies of chemical ecologists whose goal was to identify 
chemical cues (particularly those derived from prey) that are detected by snakes. Consequently, 
high sensitivity and ecological face validity are characteristic of these paradigms. At the same 
time they have another less desirable characteristic; they tend to focus on only the terminal aspects 
of feeding episodes, measuring the acts of attacking and devouring nearby prey rather than earlier 
components of feeding episodes such as foraging, stalking, or selecting locations where prey are 
likely to appear. This brings up our third point that attractants and repellents identified in standard 
laboratory tests have had smaller effects in the field than we had hoped, not because the laboratory 
tests were fallacious, but because chemical cues that determine terminal aspects of predation do 
not necessarily influence earlier components of predatory episodes. Implications of this point will 
be addressed in the discussion. 
METHODS 
Wilde (1938) introduced a technique for assessing snake response to chemical cues; and in 
recent years Arnold (1980, 1981), Burghardt (1970, 1980), Cooper (1989, 1990), and others have 
exploited this technique to study the effects of ontogenetic, genetic, and phylogenetic effects on 
chemoreception. In its simplest form, the method involves soaking cotton-tipped applicators with 
chemical extracts and then presenting the applicators to the lips of snakes for 60 sec, while tongue 
flicks and latency to bite the applicator are recorded. Cooper and Burghardt (1990) studied the 
efficiency of various dependent variables to compare snakes' responses to chemical cues derived 
from prey with other responses to various control solutions. The most recent review of the 
literature on reptile nasal and vomeronasal chemoreception was provided by Halpern (1992). 
We have used a slight variation of the applicator technique to study response of brown tree 
snakes to chemical cues. Because the brown tree snake is a large and aggressive snake, it is not 
possible to suspend an applicator in front of the snake for 60 sec without risking either an escape 
or an attack aimed at the investigator. Consequently, we simply touch the snake gently on the lips 
with the applicator and then close the cage, recording tongue flicks during the next 10 min. 
Chemicals that stimulate more tongue flicking than does water are said to produce positive 
responses, whereas chemicals followed by fewer tongue flicks than occur after application of water 
are said to produce negative responses. Finally, chemicals associated with rates of tongue flicking 
not different from rates seen with water are said to evoke no response. 
In our screening program, a positive response signals a potential attractant, provided that the 
same chemical applied to a euthanatized rodent either accelerates predatory attack or 
does not disrupt such an attack. A negative response signals a potential repellent, provided that 
the same chemical applied to a euthanatized rodent either disrupts predatory attack (usually by 
increasing its latency) or forestalls the attack entirely. Chemicals with these characteristics in 
laboratory tests are then subjected to field trials on Guam. 
REPTILES, CHAPTER 11 1 23 
OVERVIEW OF RESULTS 
Most chemicals tested in our laboratory have produced no response in brown tree snakes, but 
a few have been followed by significant positive responses. Examples of the latter include Russ 
Carmen's Triple Threat, Synthetic Fermented Egg, and Beaver Lure (Chiszar et al. 19%). These 
and numerous other lures for fur-bearing mammals are available from M & M Fur Company, Box 
15, Bridgewater, SD 57319. During our studies we also tested whole blood using cotton-tipped 
applicators, with the result that brown tree snakes exhibited strong positive responses to blood 
from several mammals, including humans (Chiszar et al. 1991, 19930). The substances that 
evoked positive responses in the applicator test also passed our second test in that their presence 
on mouse fur either accelerated predatory attacks or did not disrupt them. Consequently, these 
materials were tested as attractants in field trials on Guam. The result of the field tests was that 
significantly more brown tree snakes were trapped using these "attractive" materials as baits than 
when traps were unbaited or baited with odiferous but biologically irrelevant chemicals such as 
floral-based perfumes. Success was mitigated, however, by the fact that the trap grids also 
contained units baited with live mice, and these traps caught far more brown tree snakes than did 
any of the traps baited with chemical cues (Rodda and Chiszar 1993). 
The only substance that we examined in the applicator tests that might have repellent effects 
is synthetic monkey pheromone (SMP), supplied to us by U.S. Department of Agriculture's 
Denver Wildlife Research Center. This material elicited vigorous jaw rubbing, as snakes 
attempted to remove the chemical from their lips; and, when SMP was applied to rodent fur, 
predatory episodes were disrupted, with most snakes refusing to attack (Byall et al. 1993, Chiszar 
et al. 1996). No field tests have yet been conducted to determine if brown tree snakes or any 
other species will avoid areas containing SMP. It is noteworthy that we initially tested SMP as 
a potential attractant because to humans its odor resembled those of some of the mammal lures that 
produced positive responses in applicator tests. Hence, our identification of SMP as a possible 
repellent was a surprise, similar to surprises encountered by Clark (this volume). 
A product called Snake-a-Way@ is currently marketed as a snake repellent. It had no effects 
in tests reported by McCoid et al. (1993) 
DISCUSSION 
While our laboratory experiments on attractants have produced insights into the chemical 
control of snake predatory behavior (e.g., Chiszar et al. 1993b), it is disturbing that substances 
eliciting strong positive responses in the laboratory had only modest effects in the field. Although 
we have not tested putative repellents in the field, Nishimura and Kamura (1993) did so with habu 
(Trimeresums Jlavoviridis). Substances that had repellent effects in laboratory tests had 
insignificant effects in the field in that snakes were not stimulated to leave their refugia (crevices 
in rock walls) when the areas were heavily sprayed with the presumed repellent. Accordingly, 
we urge that attention be given to the reasons why laboratory and field tests have produced such 
discrepant outcomes. 
124 ATTRACTANTS AND REPELLENTS FOR SNAKES 
Two classes of phenomena are probably responsible for these differences: 
(1) Because the generally impoverished and unstimulating nature of most captive 
environments cause normally important but episodic events, such as presentation of 
prey, to take on even greater significance than would be the case in nature, snakes and 
other animals long maintained in captivity become supersensitive or superresponsive 
to certain cues, especially those arising from prey (Hediger 1964, Sharpless 1975). 
Laboratory paradigms for study of responses to chemical cues have focused, perhaps 
inadvertently, on aspects of behavior that are strongly influenced by captivity-induced 
supersensitivity, especially the terminal aspects of predatory episodes. This focus on 
terminal aspects of predation has probably increased our estimates of the magnitude of 
response to chemical cues relative to estimates that would be obtained with wild 
snakes. 
(2) Response of brown tree snakes to attractants and repellents in the field depend upon 
the ability of these stimuli to influence searching behaviors, precisely those aspects of 
predation that have been ignored in laboratory studies, which have measured only 
attacking and ingestive behaviors. If initial aspects of searching are not strongly 
influenced by the same stimuli that modulate terminal attack and swallowing 
components, then use of these stimuli to attract foragers to traps is not likely to be 
successful. For example, it appears that the brown tree snake relies heavily upon 
vision during initial searching behavior and that chemical senses become involved only 
later as predatory attack becomes imminent. Hence, a truly effective artificial 
attractant probably must have both visual and chemical elements. 
Laboratory research on repellents is especially likely to be affected by the habituation effects 
that are inevitable aspects of captivity. While long-term captives become strongly responsive to 
preyderived cues, the snakes become unresponsive to previously threatening stimuli associated 
with humans and with the laboratory environment (Chiszar et al. 1995). Hence, captive snakes 
might easily be induced to leave a refuge by a mildly noxious odor, whereas a wild snake would 
not do so because the countervailing forces keeping the animal in its refuge are stronger than are 
the mildly aversive ones added to the refuge by the investigator. Interactions of this sort are 
probably common, greatly complicating the task of discovering repellents that would be effective 
in the field. 
Our view is that laboratory studies of attractants must be redesigned to measure initial, 
secondary, and other aspects of predation as well as its terminal components, attack and ingestion. 
We must create empirically verifiable models of the sequence in which sensory modalities 
participate in the guidance of foraging activities, and development of attractants must be informed 
by these models. Laboratory studies on repellents also must focus on searching snakes as well as 
hiding snakes. After all, even if we drive all resident snakes out of refugia in sensitive areas, this 
victory will remain incomplete if new foragers enter because cues which influence inactive, hiding 
snakes have little effect on actively foraging immigrants. At the same time, it is necessary to 
REPTILES, CHAPTER 11 125 
consider the habituated nature of long-term captive snakes when we evaluate repellent effects. 
Perhaps the best way to do this is to replicate studies of promising repellents with freshly caught 
snakes to verify that they behave as do long-term captives. Thus, while discrepancies between 
laboratory and field data have led to some disappointment in the study of snake attractants and 
repellents, we believe that the problems are not insurmountable and that they can be addressed by 
developing new, more complete experimental designs. In many respects, our ideas are convergent 
with those of Clark (this volume), who has developed new test procedures to measure movement 
patterns such as those that occur during foraging and refuge seeking. 
LITERATURE CITED 
Arnold, S. J. 1980. The microevolution of feeding behavior. Pages 409-453 in A. C. Kamil 
and T. D. Sargent, eds. Foraging behavior: ecological, ethological and psychological 
approaches. Garland STPM Press, NY. 
. 1981. Behavioral variation in natural populations. I. Phenotypic, genetic and 
environmental correlations between ckmoreceptive responses to prey in the garter snake, 
Thinnophis elegans. Evolution 35:489-509. 
Burghardt, G. M. 1970. Chemical perception in reptiles. Pages 241-308 in J. W .  Johnston, D. 
G. Moulton, and A. Turk, eds. Advances in chemoreception. I. Communication by chemical 
signals. Appleton-Century-Crofts, NY. 
-. 1980. Behavioral and stimulus correlates of vomeronasal functioning in reptiles: 
feeding, grouping, sex, and tongue use. Pages 275-301 in D. Muller-Schwarze and 
R. M. Silverstein, eds. Chemical signals of vertebrates and aquatic invertebrates. Plenum Press, 
NY. 
Byall, C.,  H. M. Smith, and D. Chiszar. 1993. Response of brown tree Snakes (Boiga 
irregularis) to Synthetic Monkey Pheromone. J. Colo. Wyo. Acad. Sci. 25:28. 
Chiszar, D., T. M. Dunn, and H. M. Smith. 1993a. Response of brown tree snakes (Boiga 
irregularis) to human blood. J. Chem. Ecol. 19: 91-96. 
,- , and -. In Press. Pages xx in Rodda, G. H., Y. Sawai, D. Chiszar, and 
H. Tanaka, eds. Problem snake management: habu and brown tree snake examples. Cornell 
University Press, Ithaca, NY. 
-, K. Fox, and H. M. Smith. 1991. Stimulus control of predatory attack in brown tree 
snakes. IV. Effects of mammalian blood. Behav. Neural Biol. 57: 167-169. 
126 ATTRACTANTS AND REPELLENTS FOR SNAKES 
, G. Hobika, and H. M. Smith. 19936. Prairie rattlesnakes (Crotalus viridis) respond to 
rodent blood with chemosensory searching. Brain Behav. Evol. 4 1 :229-233. 
, G. H. Rodda, and R. A. Odurn. 1995. Two problems in ophidian conservation biology: 
an explosive exotic on Guam and an endangered endemic on Aruba. Paper presented at the 
meeting of the Animal Behavior Society, University of Nebraska, Lincoln, July 9-13. 
Cooper, W. E., Jr. 1989. Absence of prey odor discrimination by iguanid and agamid lizards 
in applicator tests. Copeia 472-478. 
. 1990. Prey odor detection by teiid and lacertid lizards and the relationship of prey odor 
detection to foraging mode in lizard families. Copeia 237-242. 
, and G. M. Burghardt. 1990. A comparative analysis of scoring methods for chemical 
discrimination of prey by squamate reptiles. J. Chem. Ecol. 16:45-65. 
Fritts, T. H. 1988. The brown tree snake, Boiga irregularis, a threat to Pacific islands. U.S. 
Fish Wildl. Serv. Biol. Rep. 88, Washington, DC. 36 pp. 
, and R. L. Haddock. 1990. Risks to infants on Guam from bites of the brown tree snake 
(Boiga irregularis). Amer. J. Trop. Med. Hyg. 42: 607-61 1. 
, N. J .  Scott, Jr., and J. A. Savidge. 1987. Activity of the arboreal brown tree snake 
(Boiga irregularis) on Guam as determined by electrical outages. The Snake 19:51-58. 
Halpern, M. 1992. Nasal chemical senses in reptiles: structure and function. Pages 423-523 
in C. Gans and D. Crews, eds. Biology of the Reptilia, 18, Physiology E, Hormones, Brain and 
Behavior. Univ. Chicago Press, IL. 
Hediger, H. 1964. Wild animals in captivity. Butterworths Sci. Publications, Dover, NY. 
McCoid, M. J., E. W. Campbell, 111, and B. C. Alokoa. 1993. Efficacy of a chemical repellent 
for the brown tree snake (Boiga irregularis). The Snake 25: 115-1 19. 
Nishimura, M., and T. Kamura. 1993. Tests of repellents of habu, Trimeresums flavoviridis. 
9. Tests to eject habu from stone walls. Reports of ecological researchers to diminish bites of 
habu (Trimeresums flavoviridis) in Okinawa Prefecture 16: 2 1-24. 
Rodda, G. H., and D. Chiszar. 1993. Are cotton swab presentations a sufficient model for 
ophidian prey recognition? Poster presented at the meeting of the Amer. Soc. Ichthyologists and 
Herpetologists, Austin, TX, 27 May-2 June (see pp. 265-266 of the Program and Abstracts). 
REPTILES, CHAPTER 11 127 
, and P. J. Conry. 1992. Origins and population growth of the brown tree snake, Boiga 
irregularis, on Guam. Pac. Sci. 46:46-57. 
, and T. H. Fritts. 1992. The impact of the introduction of the colubrid snake Boiga 
irregularis on Guam's lizards. J. Herpetol. 26: 166-174. 
Savidge, J. A. 1987. Extinction of an island forest avifauna by an introduced snake. Ecology 
68 : 660-668. 
Sharpless, S. K. 1975. Supersensitivity-like phenomena in the central nervous system. Fed 
Proc . 34: 1990- 1997. 
Vest, D. K., S. P. Mackessy , and K. V. Kardong. 1991. The unique Duvernoy ' s secretion of 
the brown tree snake (Boiga irregularis). Toxicon 29: 532-535. 
Weinstein, S. A., D. Chiszar, R. C. Bell, and L. A. Smith. 1991. Lethal potency and 
fractionation of Duvernoy's secretion from the brown tree snake, Boiga irregularis. Toxicon 
29: 40 1 -407. 
Wilde, W. S. 1938. The role of Jacobson's organ in the feeding reaction of the common garter 
snake, Thamnophis simlis sirtalis (Linn.). J. Exp. Zool. 77:445-465. 


Experiments on Chemical Control of Behavior in Brown Tree Snakes

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln National Wildlife Research Center Repellents Conference 1995 USDA National Wildlife Research Center Symposia August 1995 Experiments on Chemical Control of Behavior in Brown Tree Snakes David Chiszar University of Colorado, Boulder, CO Gordon H. Rodda National Biological Service Hobart M. Smith University of Colorado, Boulder, CO Follow this and additional works at: https://digitalcommons.unl.edu/nwrcrepellants  Part of the Natural Resources Management and Policy Commons Chiszar, David ; Rodda, Gordon H.; and Smith, Hobart M., "Experiments on Chemical Control of Behavior in Brown Tree Snakes" (1995). National Wildlife Research Center Repellents Conference 1995. 8. https://digitalcommons.unl.edu/nwrcrepellants/8 This Article is brought to you for free and open access by the USDA National Wildlife Research Center Symposia at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in National Wildlife Research Center Repellents Conference 1995 by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 11 Experiments on Chemical Control of Behavior in Brown Tree Snakes David Chiszar, Department of Psychology, University of Colorado, Boulder, CO 80309-0345 Gordon H. Rodda, National Biological Service, Fort Collins, CO 80527 Hobart M. Smith, Department of Environmental Population, and Organismic Biology, University of Colorado, Boulder, CO 80309 ABSTRACT The brown tree snake (Boiga irregularis), accidentally introduced on Guam shortly after World War 11, is the target of extensive efforts aimed at reducing populations and preventing their spread to other Pacific islands. Chemical attractants and repellents have been investigated, and this paper presents an overview of current knowledge. In particular, chemical cues that have strong effects in laboratory tests have had only modest (though significant) effects in field tests on Guam. Reasons for the different outcomes of laboratory and field studies are discussed along with recommendations for the redesign of laboratory experiments. KEY WORDS attractants, brown tree snakes (Boiga irrenularis), captivity, field tests, foraging, Guam, habituation, laboratory tests, repellents, supersensitivity INTRODUCTION Brown tree snakes are rear-fanged colubrids, native to New Guinea and to nearby islands, as well as to northern Australia. This species was accidentally introduced on Guam shortly after World War 11, with a variety of primary and secondary consequences unfolding during subsequent decades (Fritts 1988, Rodda and Conry 1992). Half of Guam's bird species are now extinct, and most remaining species are in precarious condition, as are several species of mammals and lizards (Savidge 1987, Rodda and Fritts 1992). The snake, being venomous, also presents a medical problem, although only very young humans appear to be at risk (Fritts and Haddock. 1990, Vest et al. 1991, Weinstein et al. 1991). Numerous electrical outages are caused by brown tree snakes as these arboreal snakes climb power poles in search of avian or saurian prey (Fritts et al. 1987). Efforts to control brown tree snakes on Guam have emphasized the need for attractants to enhance success of trapping programs and for repellents to drive snakes out of sensitive areas, including critical habitat for endangered species and cargo-holding facilities from which stowaway snakes could be transported to other islands. 122 ATTRACTANTS A N D  REPELLENTS FOR SNAKES We want to make three points during this presentation. The first is that we have data on attractants and repellents in brown tree snakes, so at least modest progress has been made in these directions. Second, our research paradigms have been borrowed, with only slight modification, from the basic, theoretically driven studies of chemical ecologists whose goal was to identify chemical cues (particularly those derived from prey) that are detected by snakes. Consequently, high sensitivity and ecological face validity are characteristic of these paradigms. At the same time they have another less desirable characteristic; they tend to focus on only the terminal aspects of feeding episodes, measuring the acts of attacking and devouring nearby prey rather than earlier components of feeding episodes such as foraging, stalking, or selecting locations where prey are likely to appear. This brings up our third point that attractants and repellents identified in standard laboratory tests have had smaller effects in the field than we had hoped, not because the laboratory tests were fallacious, but because chemical cues that determine terminal aspects of predation do not necessarily influence earlier components of predatory episodes. Implications of this point will be addressed in the discussion. METHODS Wilde (1938) introduced a technique for assessing snake response to chemical cues; and in recent years Arnold (1980, 1981), Burghardt (1970, 1980), Cooper (1989, 1990), and others have exploited this technique to study the effects of ontogenetic, genetic, and phylogenetic effects on chemoreception. In its simplest form, the method involves soaking cotton-tipped applicators with chemical extracts and then presenting the applicators to the lips of snakes for 60 sec, while tongue flicks and latency to bite the applicator are recorded. Cooper and Burghardt (1990) studied the efficiency of various dependent variables to compare snakes' responses to chemical cues derived from prey with other responses to various control solutions. The most recent review of the literature on reptile nasal and vomeronasal chemoreception was provided by Halpern (1992). We have used a slight variation of the applicator technique to study response of brown tree snakes to chemical cues. Because the brown tree snake is a large and aggressive snake, it is not possible to suspend an applicator in front of the snake for 60 sec without risking either an escape or an attack aimed at the investigator. Consequently, we simply touch the snake gently on the lips with the applicator and then close the cage, recording tongue flicks during the next 10 min. Chemicals that stimulate more tongue flicking than does water are said to produce positive responses, whereas chemicals followed by fewer tongue flicks than occur after application of water are said to produce negative responses. Finally, chemicals associated with rates of tongue flicking not different from rates seen with water are said to evoke no response. In our screening program, a positive response signals a potential attractant, provided that the same chemical applied to a euthanatized rodent either accelerates predatory attack or does not disrupt such an attack. A negative response signals a potential repellent, provided that the same chemical applied to a euthanatized rodent either disrupts predatory attack (usually by increasing its latency) or forestalls the attack entirely. Chemicals with these characteristics in laboratory tests are then subjected to field trials on Guam. REPTILES, CHAPTER 11 1 23 OVERVIEW OF RESULTS Most chemicals tested in our laboratory have produced no response in brown tree snakes, but a few have been followed by significant positive responses. Examples of the latter include Russ Carmen's Triple Threat, Synthetic Fermented Egg, and Beaver Lure (Chiszar et al. 19%). These and numerous other lures for fur-bearing mammals are available from M & M Fur Company, Box 15, Bridgewater, SD 57319. During our studies we also tested whole blood using cotton-tipped applicators, with the result that brown tree snakes exhibited strong positive responses to blood from several mammals, including humans (Chiszar et al. 1991, 19930). The substances that evoked positive responses in the applicator test also passed our second test in that their presence on mouse fur either accelerated predatory attacks or did not disrupt them. Consequently, these materials were tested as attractants in field trials on Guam. The result of the field tests was that significantly more brown tree snakes were trapped using these "attractive" materials as baits than when traps were unbaited or baited with odiferous but biologically irrelevant chemicals such as floral-based perfumes. Success was mitigated, however, by the fact that the trap grids also contained units baited with live mice, and these traps caught far more brown tree snakes than did any of the traps baited with chemical cues (Rodda and Chiszar 1993). The only substance that we examined in the applicator tests that might have repellent effects is synthetic monkey pheromone (SMP), supplied to us by U.S. Department of Agriculture's Denver Wildlife Research Center. This material elicited vigorous jaw rubbing, as snakes attempted to remove the chemical from their lips; and, when SMP was applied to rodent fur, predatory episodes were disrupted, with most snakes refusing to attack (Byall et al. 1993, Chiszar et al. 1996). No field tests have yet been conducted to determine if brown tree snakes or any other species will avoid areas containing SMP. It is noteworthy that we initially tested SMP as a potential attractant because to humans its odor resembled those of some of the mammal lures that produced positive responses in applicator tests. Hence, our identification of SMP as a possible repellent was a surprise, similar to surprises encountered by Clark (this volume). A product called Snake-a-Way@ is currently marketed as a snake repellent. It had no effects in tests reported by McCoid et al. (1993) DISCUSSION While our laboratory experiments on attractants have produced insights into the chemical control of snake predatory behavior (e.g., Chiszar et al. 1993b), it is disturbing that substances eliciting strong positive responses in the laboratory had only modest effects in the field. Although we have not tested putative repellents in the field, Nishimura and Kamura (1993) did so with habu (Trimeresums Jlavoviridis). Substances that had repellent effects in laboratory tests had insignificant effects in the field in that snakes were not stimulated to leave their refugia (crevices in rock walls) when the areas were heavily sprayed with the presumed repellent. Accordingly, we urge that attention be given to the reasons why laboratory and field tests have produced such discrepant outcomes. 124 ATTRACTANTS AND REPELLENTS FOR SNAKES Two classes of phenomena are probably responsible for these differences: (1) Because the generally impoverished and unstimulating nature of most captive environments cause normally important but episodic events, such as presentation of prey, to take on even greater significance than would be the case in nature, snakes and other animals long maintained in captivity become supersensitive or superresponsive to certain cues, especially those arising from prey (Hediger 1964, Sharpless 1975). Laboratory paradigms for study of responses to chemical cues have focused, perhaps inadvertently, on aspects of behavior that are strongly influenced by captivity-induced supersensitivity, especially the terminal aspects of predatory episodes. This focus on terminal aspects of predation has probably increased our estimates of the magnitude of response to chemical cues relative to estimates that would be obtained with wild snakes. (2) Response of brown tree snakes to attractants and repellents in the field depend upon the ability of these stimuli to influence searching behaviors, precisely those aspects of predation that have been ignored in laboratory studies, which have measured only attacking and ingestive behaviors. If initial aspects of searching are not strongly influenced by the same stimuli that modulate terminal attack and swallowing components, then use of these stimuli to attract foragers to traps is not likely to be successful. For example, it appears that the brown tree snake relies heavily upon vision during initial searching behavior and that chemical senses become involved only later as predatory attack becomes imminent. Hence, a truly effective artificial attractant probably must have both visual and chemical elements. Laboratory research on repellents is especially likely to be affected by the habituation effects that are inevitable aspects of captivity. While long-term captives become strongly responsive to preyderived cues, the snakes become unresponsive to previously threatening stimuli associated with humans and with the laboratory environment (Chiszar et al. 1995). Hence, captive snakes might easily be induced to leave a refuge by a mildly noxious odor, whereas a wild snake would not do so because the countervailing forces keeping the animal in its refuge are stronger than are the mildly aversive ones added to the refuge by the investigator. Interactions of this sort are probably common, greatly complicating the task of discovering repellents that would be effective in the field. Our view is that laboratory studies of attractants must be redesigned to measure initial, secondary, and other aspects of predation as well as its terminal components, attack and ingestion. We must create empirically verifiable models of the sequence in which sensory modalities participate in the guidance of foraging activities, and development of attractants must be informed by these models. Laboratory studies on repellents also must focus on searching snakes as well as hiding snakes. After all, even if we drive all resident snakes out of refugia in sensitive areas, this victory will remain incomplete if new foragers enter because cues which influence inactive, hiding snakes have little effect on actively foraging immigrants. At the same time, it is necessary to REPTILES, CHAPTER 11 125 consider the habituated nature of long-term captive snakes when we evaluate repellent effects. Perhaps the best way to do this is to replicate studies of promising repellents with freshly caught snakes to verify that they behave as do long-term captives. Thus, while discrepancies between laboratory and field data have led to some disappointment in the study of snake attractants and repellents, we believe that the problems are not insurmountable and that they can be addressed by developing new, more complete experimental designs. In many respects, our ideas are convergent with those of Clark (this volume), who has developed new test procedures to measure movement patterns such as those that occur during foraging and refuge seeking. LITERATURE CITED Arnold, S. J. 1980. The microevolution of feeding behavior. Pages 409-453 in A. C. Kamil and T. D. Sargent, eds. Foraging behavior: ecological, ethological and psychological approaches. Garland STPM Press, NY. . 1981. Behavioral variation in natural populations. I. 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Experiments on Chemical Control of Behavior in Brown Tree Snakes

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