Consequences of Abiotic and Biotic Factors

50 Pages·2012·0.4 MB·English

by  Enrollment Services

CONSEQUENCES OF ABIOTIC AND BIOTIC FACTORS ON
RATTLESNAKE FORAGING BEHAVIOR
_______________
A Thesis
Presented to the
Faculty of
San Diego State University
_______________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Biology
with a Concentration in
Ecology
_______________
by
Matthew Allen Barbour
Spring 2012

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Copyright © 2012
by
Matthew Allen Barbour
All Rights Reserved
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ABSTRACT OF THE THESIS
Consequences of Abiotic and Biotic Factors on Rattlesnake
Foraging Behavior
by
Matthew Allen Barbour
Master of Science in Biology with a Concentration in Ecology
San Diego State University, 2012

Much research in behavioral ecology has focused on trying to understand how the
environment influences animal foraging behavior. These environmental influences may
manifest themselves through both abiotic and biotic factors. For my thesis research, I
examined how certain abiotic and biotic factors influence free-ranging rattlesnake foraging
behavior. While foraging, rattlesnakes will search for ambush sites and wait for hours,
sometimes days, for unsuspecting prey to come within striking distance. However, we know
virtually nothing about the behavior rattlesnakes’ exhibit during this waiting period. In
chapter 1, I examined how an abiotic factor, natural light availability, influences rattlesnake
behavior during this waiting period. I used a combination of radio telemetry and portable
video surveillance cameras to quantify the behaviors exhibited by rattlesnakes while waiting
for prey during both day and night periods. The two most common behaviors I observed
were chemosensory probes, a behavior I describe in detail for the first time, and mouth gapes.
The frequency of these behaviors changed from day to night periods. The rate of
chemosensory probing increased by 82% from day to night. Likewise, the percentage of
hours with one or more mouth gapes increased by 35% from day to night periods. Nearly
half of all mouth gapes were followed immediately with a chemosensory probe, suggesting
that mouth gaping also serves a chemosensory function in this context. Our results suggest
that chemical cues play an increasingly important role in mediating rattlesnake foraging
behavior at night, a pattern that may be true for many sit-and-wait predators that ambush prey
opportunistically throughout the day and night. In chapter 2, I examined how a biotic factor,
ground squirrels approaching and tail-flagging toward rattlesnakes, influences rattlesnakes
foraging behavior. I conducted this study using manned and unmanned video cameras to
record behaviors of radio-tagged, free-ranging rattlesnakes foraging in ground squirrel
colonies. I found that squirrel tail-flags deter snake predation on two different time scales. At
the time of the interaction, snakes did not strike at tail-flagging squirrels unless they
approached too close—about 1/3 of the distance of their typical strike range. This is likely
because tail-flagging is reliably associated with squirrel vigilance and their readiness to
dodge a snake strike. Tail-flagging also influenced subsequent behaviors of rattlesnakes.
Tail-flagging by adult squirrels increased the probability that snakes would leave the area;
however, tail-flagging from pups did not cause snakes to leave the area. Snakes did not
respond to tail-flagging interactions with pups because they actually experienced an
increased probability of striking a squirrel in the area. These differences in snake responses
are likely shaped by the differential effects of pup and adult tail-flagging on nearby squirrels,
as well as pups being associated with high prey density areas. Taken together, my thesis
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research has shed light on how certain abiotic and biotic factors influence rattlesnake
foraging behavior, a group of predators we know little about.
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TABLE OF CONTENTS
PAGE
ABSTRACT ............................................................................................................................. iv
LIST OF TABLES ................................................................................................................. viii
LIST OF FIGURES ................................................................................................................. ix
CHAPTER
1 DIEL CYCLES IN CHEMOSENSORY BEHAVIORS OF FREE-RANGING
RATTLESNAKES LYING IN WAIT FOR PREY.......................................................1
Introduction ..............................................................................................................1
Methods....................................................................................................................3
Study Sites and Animals ....................................................................................3
Field Videography .............................................................................................4
Ethology .............................................................................................................5
Data Analysis .....................................................................................................7
Results ......................................................................................................................8
Chemosensory Probing ......................................................................................8
Mouth Gaping ..................................................................................................10
Discussion ..............................................................................................................11
Chemosensory Probing ....................................................................................11
Mouth Gaping ..................................................................................................13
Conclusion .......................................................................................................14
2 CONSEQUENCES OF GROUND SQUIRREL SIGNALING AT
MULTIPLE STAGES OF RATTLESNAKE FORAGING ........................................15
Introduction ............................................................................................................15
Methods..................................................................................................................17
Study Animals ..................................................................................................18
Natural Observations .......................................................................................18
H1: Tail-flagging Deters Snake Strikes by Signaling Escape Ability .............19
H2: Displays Deter Continued Hunting by Reducing Probability of
Encountering Unwary Squirrels .......................................................................23
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Results ....................................................................................................................24
Discussion ..............................................................................................................27
Conclusion .............................................................................................................32
ACKNOWLEDGEMENTS .....................................................................................................33
REFERENCES ........................................................................................................................35
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LIST OF TABLES
PAGE
Table 1. Output of Cox Proportional Hazards Regression Models of (1) the Time until
Snakes Abandon Hunting Areas and (2) the Time Until Snakes Strike
Squirrels .......................................................................................................................27
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LIST OF FIGURES
PAGE
Figure 1. Sequence of events during a typical chemosensory probe of C. ruber. ......................6
Figure 2. Change in median number of probes per hour between day and night
observation hours of foraging snakes. ..........................................................................9
Figure 3. Average (±SE) number of chemosensory probes per hour for different
periods throughout the diel cycle of foraging rattlesnakes (all individuals
combined). .....................................................................................................................9
Figure 4. Change in percent hours with at least one mouth gape between day and
night observation hours of foraging snakes. ................................................................10
Figure 5. Variable importance plots from Random forest model of six predictor
variables influencing snake decisions to strike at squirrels within range. .................25
Figure 6. Rattlesnake responses (strike or no strike) to squirrels that either tail-flagged
(black ‘x’) or not (grey ‘o’) at various distances within their observed strike
range (31 cm). ............................................................................................................25
Figure 7. Bar chart of the percentage of tail-flagging and non-tail-flagging squirrels
that attempted to dodge snake strikes. . ......................................................................26
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CHAPTER 1
DIEL CYCLES IN CHEMOSENSORY BEHAVIORS
OF FREE-RANGING RATTLESNAKES LYING IN
WAIT FOR PREY
INTRODUCTION
Understanding constraints on animal foraging has been a central research theme in
behavioral ecology research for the past 40 years (Schoener 1971). An important first step
toward this goal is to examine how organisms perceive and respond to their foraging
environment (Barbosa and Castellanos 2005). This process may be relatively simple for
foragers that rely primarily on one sensory system to both search for and capture prey (e.g.,
vision for terrestrial birds such as sparrows) (Getty and Pulliam 1993). Predators that rely on
a particular sensory system are constrained by changing environmental conditions (e.g.,
ambient light) that hinder sensory capabilities (Clarke 1983; Weissburg and Zimmer-Faust
1993). However, many organisms rely on multiple sensory systems during the foraging
process and may be able to compensate for deprivation of particular sensory systems
(Barbosa and Castellanos 2005). For example, threespine sticklebacks (Gasterosteus
aculeatus) rely on visual cues in clear water to capture prey, but switch to chemical cues
under turbid conditions (Webster et al. 2007). This switch in sensory systems has important
consequences on the mechanism and dynamic of predator-prey interactions (Barbosa and
Castellanos 2005; Weissburg and Browman 2005).
In this study, we quantified the diel cycles in chemosensory foraging behavior of free-
ranging red-diamond (Crotalus ruber) and northern Pacific (Crotalus oreganus oreganus)
rattlesnakes. Analysis of rattlesnake diets suggest they are cathemeral hunters (i.e. active
during day and night) (Fitch 1949; Taylor 2001; Clark 2002; Glaudas et al. 2008) and
consequently experience different sensory constraints depending on the time of day. We
chose C. ruber and C. o. oreganus because, although they are cathemeral, their diets differ in
composition of nocturnally and diurnally active prey. Specifically, C. ruber’s diet is