Context dependent learning using aversive stimuli in Eastern and Western Hognose
snakes (Heterodon platyrhinos and Heterodon nasicus)
Seana McGuffey
Summer Honors Research 2003
Introduction
Little is known about the learning capacity of reptiles. Though much research has
been done involving learning in mammals and birds, learning in snakes is a topic that has
not been extensively explored. This project was proposed in the interest of advancing
knowledge regarding learning in reptiles, specifically snakes, and in understanding the
ways in which they learn using aversive stimuli to come to understand the anti-predator
behaviors of these organisms.
Research done by Burghardt and Greene (1977) using neonate hognose snakes
has indicated that these snakes habituate to a specific stimulus (predator). Burghardt and
Greene have also found variation between Heterodon platyrhinos and Heterodon nasicus
in response waning. Yet no research has been cited testing contextual cues with these
snakes to test learning. This project seeks to test the variability of response waning
within the new context, as well as measure learning in the snakes.
The anti-predator responses of Heterodon platyrhinos and Heterodon nasicus have
been explored minimally. The particular behaviors that comprise the responses of
feigned attack and feigned death are specific to these snakes. The number of snakes used
allows a limited comparison of individual variability, interspecies comparison and a
control group for variability in response to predators in the grass context compared with
the asphalt context.
There has been much speculation about the intelligence of reptiles, more about
specifically snakes because they are difficult organisms to study using Pavlovian
Conditioning, and the task of finding an effective stimulus is incredibly difficult. In
beginning to conduct studies that test capacity for learning in snakes, the ways snakes
learn can begin to be investigated. Understanding the circumstances from which snakes
take "learned behaviors" and how this information is remembered and processed allows
humans to begin to understand the snakes themselves. The aspect of this project dealing
with the defense mechanisms of the hognose snake and the circumstances that elicit these
responses allows an understanding of what the organism perceives to be a threat. Work
with these snakes will hopefully trigger interest in other snakes and how their learning
processes and defense mechanisms work and add information about a new organism to a
vast body of research concerning animal learning and behavior.
One goal in this particular study was to gather information about the little
investigated Western hognose snake in order to compare its behaviors to those of the
Eastern Hognose snake. An ethogram for the Eastern Hognose snake was developed by
Burghardt (Cognitive Ethology, 1991), but not for the Western Hognose. The ethogram
developed in this study for the Western Hognose snake consists of four stages; flight,
bluff, immobility and recovery. The flight stage includes the time during which the snake
gathers information about its surroundings, and attempts flight from a perceived threat.
The bluff occurs when the snake begins to puff the head and neck area, starts to hiss and
strike, which can be open or close mouthed, and the snake's body will begin coiling. The
immobility stage includes a complete coiling of the body, tucking the head under the
body, and a maintained puffed posture, which does not indicate any sign of breathing.
The final stage, recovery begins when the snake resumes normal breathing, lifts its head,
begins flicking its tongue, starts to uncoil and slithers away.
This project undertakes the problem of learning in snakes, specifically the Eastern
Hognose (Heterodon platyrhinos) and Western Hognose (Heterodon nasicus). Pavlovian
conditioning was the method by which learning in the snakes was tested for. Pavlovian
conditioning is a process in which a biologically irrelevant, arbitrary event, or
conditioned stimulus is paired with a biologically relevant event, or unconditioned
stimulus. The unconditioned stimulus elicits a biological response or unconditioned
response. When conditioning has occurred, the conditioned stimulus elicits a conditioned
response, which often resembles the unconditioned response. For Pavlov's dog, the
conditioned stimulus, or arbitrary event was a metronome, and the biologically relevant
event, or unconditioned stimulus was food powder. The unconditioned, natural response
of the dog was to salivate in the presence of the food powder. When conditioning had
occurred and an association had been formed between the sound of the metronome and
the presentation of the food powder, the salivation response was elicited by the
metronome even in the absence of food presentation. This was the conditioned response.
Contextual conditioning uses a new context as the conditioned stimulus in which
a biologically relevant event is presented in order that an organism forms an association
between the context and the event. When conditioning has occurred, an organism will
begin to show responsiveness as soon as it is placed in the new test context, even in the
absence of the stimulus. Research has been done investigating the fertility of female
Japanese quail (Coturnix japonica) based upon the context in which access to a
copulation partner was provided (Gibbons and Cusato, 2003). Females were divided into
a paired group in which the access to the male was provided in the context of the test
chamber, and an unpaired group in which access to the male was provided in the home
cage. Females in the paired group showed increased sexual receptivity in the form of
squats and produced more fertile eggs than did the unpaired group by the conclusion of
the study.
Aversive conditioning uses an unpleasant stimulus, such as a burst of air to the
face whereas appetitive conditioning uses a positive one such as access to food. Operant
conditioning trials often use electric shock with rats to condition the behavior of the rats
in attempting to access food. This aversive stimulus is paired with an attempt to access
food when a colored light is not on. When the light is on and the rat attempts to access
food, the food is given to the rat- this is an appetitive stimulus. The electric shock
encourages the rat not to attempt to access the food until the light is on, the light serves as
a cue to the rat to attempt to access the food. The shock is an aversive stimulus, and the
food is an appetitive one.
Background
Little research has addressed the issue of learning in reptiles, more specifically
snakes. Burghardt asserts that studies testing for learning in reptiles are difficult to
conduct using appetitive conditioning, as appetitive conditioning usually includes food as
reinforcement for certain behaviors. Most reptiles need to eat only weekly and some
even monthly, hindering the use of food as reinforcement. Some studies have been
conducted with the use of heat lamps connected to a lever as positive reinforcement for
reptiles, but this poses a problem in studies using snakes as they have no limbs to press a
lever and activate the heat lamp (Burghardt, 1977).
A 1988 study conducted by Burghardt and Greene using neonate Eastern Hognose
snakes, age thirteen days, was conducted using aversive stimuli. Snakes were placed in a
four-foot by four-foot sand filled arena. An experimenter gazed at the snake from a
distance of 25 centimeters, after ten seconds the snake was lightly touched on the head or
neck, after another ten seconds the snakeís body was lightly stroked, and after another ten
seconds elapsed, the subject was lifted and lightly shaken. The recovery was monitored
in the presence of a stuffed owl 20 centimeters in height placed one meter away from the
snake at a height of 80 centimeters. This experiment was then repeated the next day and
the latency of the immobility was measured either in the presence of a human gazing
directly at the snakes, in the presence of a human gazing away from the snakes, and in the
presence of a human crouched out of view of the snakes. The snakes whose recovery was
monitored under the direct gaze of the human and in the presence of the owl had
significantly longer recovery times than those that were monitored in the presence of a
human gazing away or crouched out of view. This study established a direct correlation
between the gaze of a predator and the recovery time of the snakes from their immobility
response.
Methods
Subjects
Subjects included four wild-born Eastern Hognose snakes (Heterodon
platyrhinos) that had been in captivity obtained from a commercial dealer, and two
captive-born Western Hognose snakes (Heterodon nasicus) also obtained from a
commercial dealer. Subjects were housed individually in ten gallon aquaria with a log
ìhiding areaî and a water dish, and each subject was identified by a letter corresponding
to the species and a randomly assigned number. Subject numbers were; E1, E2, E3, E4
(for Eastern Hognoses), W1 and W2 (for Western Hognoses). Within species groups,
each subject was randomly assigned to a paired or unpaired condition resulting in the
following pairing; E2, E4, W1 as paired snakes and E1, E3, W2 as unpaired snakes.
Length measurements for each subject are: E1 62.7 centimeters; E2 77.5 centimeters; E3
73 centimeters; E4 52.5 centimeters; W1 36 centimeters; W2 31.2 centimeters. Snakes
E2 and E4 were in poor condition upon arrival, appearing to suffer from malnourishment
and died soon after the start of the experiment.
Apparatus
All trials were run outdoors. An outside context was used because in a series of
pre-trials snakes were unresponsive to the tactile stimulus in an artificial, laboratory
setting. A pretrial conducted outdoors indicated that context was indeed a factor in the
behavior of the snakes. The snakes' habituation to humans was overcome by a change of
context from indoors to outdoors. An area of grass marked by stakes and yellow twine
corresponding to the field of view of the video camera used was the arena for the test
context. An area of asphalt identical in size, shape and orientation was indicated by duct
tape. A stopwatch was used to monitor the amount of time each subject spent in each
context, as well as to monitor the duration of the stimulus presentation and duration of the
response elicited. A human tactile stimulus was used which included light touching, light
tapping, lifting and light shaking of the subjects.
Procedure
Dependent variables manipulated were the species of the snake (Heterodon platyrhinos
and Heterodon nasicus) and the context in which the stimulus was presented- the grass
context or the asphalt context as well as pairing for the test context- paired snakes were
presented with the stimulus in the test context and unpaired snakes were presented with
the stimulus in a different context than the paired snakes. Independent variables to be
measured are: tongue flicks, immobility, latency to immobility, immobility duration,
hissing, puffing, striking, avert or empty of cloaca and head and neck flattening. During
trials, paired subjects were removed from the home cages and placed in a new context;
either a grass or asphalt context. Four stakes in the grass and yellow string marked the
test area in the grass and duct tape on the asphalt. In these new contexts, a human tactile
stimulus was presented during which direct gaze was maintained. Learning was also to
be measured: do the snakes begin to respond to the context in the absence of the
predator?
The experiment ran for thirteen consecutive days, with a total of three test-trial
days and ten conditioning trial days. All trials were videotaped. Days one, seven, and
thirteen were designated test trial days in which each subject was placed in the grass
context for five minutes without the presentation of the stimulus because it was the grass
context that was being tested for a response in the paired snakes. Test trials were used to
measure the snakes' responsiveness to the context in the absence of the stimulus over the
course of the project. Subjects were transported from home cages to the outside contexts
on their ìhiding logsî and were lightly shaken off the log into the test area. Contact with
the experimenter was limited to the time for which the tactile stimulus was presented.
Conditioning trials ran for five consecutive days between test trial days one and seven,
and between test trial days seven and thirteen. Subjects were on a three day rotation
schedule so that no subject was run at the same time for any two consecutive days to
prevent temporal anticipation of the conditioning trials. Conditioning trials for the paired
subjects consisted of a presentation of the conditioned stimulus- the grass context- for
one minute before the presentation of the human tactile stimulus. The stimulus was
presented until the death-feign or immobility response was elicited, and trials were
deemed over when subjects recovered normal or full mobility. After the CS-US pairing
was presented, the paired snakes were placed in the asphalt context for eleven minutes.
In the unpaired condition, trials consisted of a presentation of the grass context for eleven
minutes, an elapse of at least forty-five minutes to prevent the grass context from serving
as a cue to the unpaired snakes, and the presentation of the human tactile stimulus
immediately as subjects were placed in the asphalt context. Paired snakes were given
access to the asphalt context and unpaired to the grass context in order to ensure that
individuals were given equal exposure to each context and to the stimulus. The unpaired
group served as a control in order to compare the response to the new grass context to
that of the paired group, it was necessary to ensure that both groups were treated the
same. The different contexts were used in order to ensure that it was an association
between the context and the stimulus that elicited the defense responses of the subjects
and that the defense behaviors of the subjects were not in response to the new context
itself. The stimulus was presented until the death-feign or immobility response was
elicited or for twenty minutes to ensure equal exposure to the stimulus across pairings.
Trials in which the response was elicited were taped until the subject resumed full
mobility.
Results
The data presented is from the second day of conditioning trials compared with
the data from the last day of conditioning trials for each subject that completed the study.
Eastern Hognose 1, Eastern Hognose 3 and Western Hognose 2 were in the unpaired
group. Western Hognose 1 was the only remaining subject in the paired group at the end
of the study. The figures presented show the latency to immobility Fig 1, and immobility
duration Fig 2, for each subject on the specified days. From day 3 to day 12 the duration
of the presentation of the human tactile stimulus decreased from ninety seconds to twenty
six seconds in Western Hognose 1, the only remaining paired snake. In the unpaired
group, where there was an immobility response, the duration of the presentation of the
human tactile stimulus was increased suggesting that learning had occurred in the paired
snake and had not in the unpaired snake. The response was elicited in the context much
faster in the paired condition than it was for the unpaired. The duration of the immobility
response, where the response was elicited, was longer for the paired snake than for the
unpaired group.
Discussion
The use of an aversive stimulus in conditioning trials allowed not only the
measure of learning in the organisms, but also an observation of the defense mechanisms
of the snakes. Though preliminary data collection indicates that learning has occurred in
the paired group of snakes, a more thorough study should be conducted using a larger
sample size in order to account for individual variability. Given the opportunity to
expand this study, it would be best to focus only on one species in data collection- to
conduct the study using only Heterodn platyrhinos or Heterodon nasics. Ideally, neonates
would be used as Burghardt and Greene have used in their studies in order to prevent
adult habituation to human experimenters- this problem was overcome by a change in
context in this study. Though there are many similarities across the two species, there are
differences from which an experimenter would benefit by investigating each species
individually. A more thorough study of the Western Hognost or Heterodon nasicus
individually would provide for a better comparison of the species with the Eastern
Hognose based upon the information already gathered. The best approach to further
study would be to use individuals from two different clutches of neonate Eastern
Hognose snakes in order to account for genetic similarity to prevent that a response not
be due to the genetic origination of the subjects but in large enough numbers that
individual variability to find similarities across all individuals in behavior is accounted
for as well. Though the snakes used in this project were experimentally naive, they had
habituated to humans and artificial environment. In using neonates, the study could be
conducted in a laboratory setting without losing the responsiveness of the subjects. Also
beneficial to a study of this nature would be a lengthier conditioning period and a posttest trial to determine the degree to which each organism retained the behaviors they had
learned throughout the course of the study.
The goal of beginning to develop an ethogram that maps the behaviors of the
Western Hognose snake was accomplished in this study, however no conclusive results
were reached on the question of learning in snakes. From the data available with this
sample, it appears that learning in snakes does indeed occur and has occurred in this
study, but more thorough investigation is necessary using more individuals in order that
an effective comparison can be made.
References
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Burghardt, G.M. & Greene, H.W. 1988. Predator simulation and duration of death
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Gibbons, K., & Cusato, B. (2003, April). Contextual conditioning of sexual behavior in
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