The Evolution ofVenom Delivery

Systems in Non-Front-Fanged Colubroids

Dean Ostrofsky

1

Abstract

Recent research suggests that many colubroids utilize toxic

oral secretions for prey apprehension and defense against

antagonists (specifically humans in this study). It was

hypothesized that if a species is not prey and/or predator

specific it will exhibit a varied relationship between two

important morphological features, the Rear Fang length (RFL), and

the Head Length (HL). Alternatively, if a relatively consistent

relationship between the two morphological variables is observed,

the Venom Delivery System (VDS) can be considered prey and/or

predator specific. In this study, 4 cases studies were performed

(1: Diadophis punctatus of the Southern California Lineage 2: Heterodon

platirhinos 3: Heterodon nasicus 4: Borikenophis portoricensis), in which the

specificity of the VDS was analyzed by performing a linear

regression between the HL and the RFL. The R2 value was compared

to the geographic distribution of each samples origin to negate

geographic distribution as the function of the HL-RFL

relationship. Results indicated that specimens exhibiting

distinct predator specific behavior (B. portoricensis) express a more

consistent relationship between the HL and RFL than specimens

2

that have exhibited distinct physiological and/or behavior

evidence of prey specificity. Geographic distribution was

successfully negated as a factor affecting the HL-RFL

relationship excluding B. portoricensis. It has been concluded that

specificity of the VDS may be confined more so by predatory

threats than prey-specific apprehension.

Introduction

Among snakes, venom is utilized in numerous ways including

defense and disabling prey to prevent injury to the snake during

transport and digestion (Weinstein, et al. 2012). A wide

variation exists in venom delivery systems (VDS) of snakes. This

suggests that those systems have evolved in response to multiple

evolutionary pressures. Most studies of VDS have focused on

highly venomous, front-fanged, highly pressured delivery systems

in elapids (e.g. cobras), viperids (e.g. rattlesnakes), and

Atractaspis (Deufel et al. 2003; Young et al. 2004; Deufel et al.

2006; Cundall et al. 2006; Deufel et al. 2010).

Recently, many colubroids have been identified as venomous

(Mckinstry 1978; Weinstein et al. 2011). The VDS of such species

3

are characterized by low-pressure, posteriorly positioned, and

non-caniculated teeth (Weinstein et al. 2009). The Ring Neck

Snake (Diadophis punctatus), Eastern Hognose Snake (Heterodon

platirhinos), Western Hognose Snake (Heterodon nasicus) and Puerto

Rican Racer (Borikenophis portoricensis) meet these parameters.

This study strives to elucidate the evolution of the VDS of

three distantly related colubroid lineages, Diadophis punctatus

complex, Heterodon, Borikenophis portoricensis. More specifically, this

study aims to identify underlying evolutionary patterns in the

VDS morphology and how they relate to a species’ evolutionary

expression of prey apprehension specialization, or prey

specificity and defense within the system (Weinstein et al.

2010). Relevant life history for the focal taxa is discussed

below.

Western Lineages of Diadophis punctatus complex. Currently,

there are 11 identified subspecies of D. punctatus. Classified as a

whole, these snakes show little preference in habitat selection,

which may explain their transcontinental distribution (Fontanella

et al. 2008). In addition, their diets are varied vertebrates and

invertabrates. If the geographically isolated mtDNA lineages are

4

viewed as independently evolving entities (i.e., species),

however, hen the D. punctatus complex is a clade of prey specialists

rather than a single widely distributed prey and habitat

generalist. For example, whereas eastern lineages have a smaller

body size and consume smaller, mostly invertebrate prey, western

lineages exhibit large body size and consume large prey. There

has been evidence suggesting that, D. p. occidentalis, produces toxic

secretions similar in composition to viperid and elapid and which

is lethal to Thamnophis ordinoides (O’Donnell et al. 2007).

Heterodon. Heterodon platirhinos is restricted to the eastern half

of the continental U.S. with no subspecies and have a preference

to consume toads (Anaxyrus spp.). Heterodon platirhinos occurs almost

exclusively in loose-soiled habitats suitable for burrowing where

toads, its primary prey, also occur (Michener et al. 1989).

Steroid secretions from the toad’s skin impedes upon the transfer

of ions across the synapse and it has been hypothesized that the

adrenal glands of H. platirhinos are the sites of production of

enzymes, which maintain the transportation of ions across the

synapse despite the toad's cardiotoxic secretions (Mohammadi et

al. 2013). This modification of the snake’s endocrine system

5

suggests constriction of the variability in the system because of

prey pressures. Though its sister taxon, H. simus, of the

southeastern U.S., does consume predominantly toads, (Goin 1947,

Ashton and Ashton 1981) its diet also includes frogs,

salamanders, and lizards (Martof et al. 1980, Ernst and Barbour

1989). Heterodon nasicus has a more specific diet than that of H.

simus, consisting predominantly of frogs and toads but does,

occasionally, consume small mammals, birds, and reptiles

(Averill-Murray 2006). There have been copious accounts of human

envenomation from both H. platirhinos and H. nasicus with symptoms such

as ecchymosis and edema around the affected area (Bragg 1960;

Mckinstry et al. 1978; Averill-Murray 2006; Weinstein et al.

2011). There have not been; however, cases of envenomation in

humans by H. simus (Morris 1985).

Borikenophis portoricensis . The Puerto Rican Racer’s (B.

portoricensis) geographic distribution is limited to Puerto Rico and

small neighboring islands (Danmorth 1925; Tolson et al. 2006;

Weldon et al. 2010). Unlike the prior two groups of interest,

there have been multiple medical reports related to the venom of

B. portoricensis, including symptoms such as edema, vomiting, and

6

ecchymosis (Heatwole et al. 1966; Rodriguez-Robles et al. 1992;

García-Gubern et al. 2010; Weldon et al. 2010). Weldon et al.

(2010) reported the toxicity of the venom of B. portoricensis to be

similar to that of elapids and viperids. The Puerto Rican Racer

utilizes its venom in an offensive manner by striking at

antagonists. While it consumes both squamates and mammals,

however, its venom composition is more reptile-specific

(Rodríguez-Robles et al. 1993). Closely related taxa tend to be

generalists, including Cubophis cantherigerus, which consumes

squamates, birds and mammals (Henderson 1984) and C. vudii whose

diet ranges from invertebrates (Henderson et al. 1993) to

amphibians (Rana pipens Rice et al. 1995).

In some systems shaped by evolutionary processes there lies

a threshold in which the adaptations in that system yield enough

efficiency for the pressure. Thus, any variance beyond the limits

of the system’s threshold would deem it unfit to complete the set

task. This would generate an unfavorable trait, which would not

be inherited by subsequent generations (Cundall 2000). The

optimal point of efficiency is rarely achieved because any one

system fits in respect to numerous tasks. The threshold can be

7

altered based on the ecosystem in which it is placed (Cundall

2000; Deufel et al. 2003). This concept is applicable to the VDS

of snakes. Venom, like the VDS can undergo evolution due to the

pressures of the given environment (Barlow et al. 2009). When it

was concluded that VDS are elastic in respect to morphological

variances (Deufel et al. 2006), most rear-fanged colubroids were

considered non-venomous and therefore were not included in

studies of VDS.

The aim of this study is to test a novel methodology for

quantifying prey/predator specificity of VDS. It was hypothesized

that a species that is not prey and/or predator specific will

exhibit a varied relationship between two important morphological

features, Rear Fang length (RFL), and Head Length (HL). I also

hypothesized that if a relatively consistent relationship between

these two morphological variables is observed, the VDS can be

considered prey and/or predator specific.

Methodology

Dipasadid Samples

8

All samples used in this study were whole fluid samples

preserved in a 70% ethanol solution and were part of the

collection of the American Museum of Natural History. This study

included four case studies, each case included an ingroup tested

for prey/predator specificity and 3-4 outgroups. There were four

species/complexes of interest.

The Southern California lineage (SCA) of the Diadophis punctatus

complex (n=49) Heterodon platirhinos (n=52), H. nasicus kennerlyi (n=24), and

Borikenophis portoricensis (n=19) were the 4 ingroups. The sister taxon

and one or more additional outgroup (assuming previously

published hypotheses of relationship) were used to establish

ingroup polarity (prey/predator specific expression vs. no

prey/predator specific expression) of character states. For the

ingroup D. punctatus SCA, the Coastal California Lineage (CCA) of D.

punctatus (n=10) served as the sister outgroup, the Great Plains

lineage (GP) and the Northeastern lineage of D. punctatus (NE)

(n=99) served as the tertiary outgroup, and H. simus (n = 21) was

the quaternary outgroup. For the purposes of this study, each

distinct mtDNA lineage recovered by Fontanella et al. (2007) was

treated as a species (following Frost and Hillis 1990; Wiens and

9

Penkrot 2002), which permitted the separation of the lineages

into in/outgroups.

For H. platirhinos, H. simus was used as the primary outgroup, D.

punctatus NE as the secondary outgroup, and Farancia abacura (n = 28)

as the tertiary outgroup (Vidal et al. 2010).

For H. nasicus, H. simus was used as the primary outgroup, D.

punctatus NE (n=99) as the secondary outgroup, and Farancia abacura

(n = 28) as the tertiary outgroup (Vidal et al., 2010).

For B. portoricensis, Cubophis cantherigerus (n=31), Cubophis vudii (n =

27), and Alsophis rufiventris (n = 4) comprise the first outgroup and

Antillophis andreai (n = 65) the second outgroup (Hedges et al.,

2009). The current knowledge of prey and/or predator specificity

of each in and outgroup is outlined in Figure 1.

Cases

Groups Species Prey Specificity

Predator Specificity

Specificity ofGroups

10

1) Ingroup D. punctatus SCA Specialista Generalista

Outgroup D. punctatus CCA Specialista Generalista

Outgroup D. punctatus NE Generalist* Generalist*Outgroup D. punctatus GP Generalist* Generalist*Outgroup H. simus Specialistb Generalistc

2) Ingroup H. platirhinos Specialistd,e,f Specialistd,g,c,h

Outgroup H. nasicus Specialistg Specialistg,c,i

Outgroup H. simus Specialistb Generalistc

Outgroup F. abacura Specialisti Generalist*3) Ingroup H. nasicus Specialistg Generalistg,d,h

Outgroup H. simus Specialistb Generalistc

Outgroup D. punctatus NE Generalist* Generalist*Outgroup F. abacura Outgroupi Generalist*

4) Ingroup B. portoricensis Generalistj.k.l Specialistd,m,n

Outgroup C. cantherigerus Generalistk,l,o Generalist*Outgroup C. vudii Generalist,l,o Generalist*Outgroup A. rufiventris Generalistk,l,o Generalist*Outgroup A. andreai Generalistk,l,o Generalist*

Quantification of Prey/Predator Specificity

The HL of a given venomous snake has been utilized to

determine the maximum height of prey consumed by that specimen

(Cundall et al. 2006; Pough et al. 1983). The rear fang of a

Figure 1: Specificity of Groups All reports on specificity expression are based on previous literature. a) O’Donnell et al. 2007 b) Goin 1947 c) Morris 1985 d) Weinstein et al. 2011 e) Michener et al. 1989 f) Mohammadi et al. 2013 g) Averill-Murray et al. 2006 h) Bragg 1960 i) Meade 1940 j) Rodríguez-Robles 1993 k)

11

Dipasadidae is the specimen’s means of delivering its venomous

secretions. By comparing the morphological aspect that determines

the size of prey a venomous snake can subdue (HL) to the

dentition used to deliver venomous secretions (Rear Fang Length

(RFL)), it may be plausible to quantify the extent of a rear

fanged VDS prey/predator specificity.

HL and RFL measurements (millimeters) were taken with a

ruler modified to facilitate the measurement of features without

causing harm to the specimen. The proxy for HL was the distance

from the central dorsal point, lateral from the quadrate bulge,

to the snout. The end of the ruler was aligned with the frontal-

most point of the snout and the measurement was taken between the

two bulges on the dorsal side of the head. The proxy for RFL

measurement was from the tooth-maxilla junction to the tip of the

tooth. The average RFL was taken because many studied Dipsadidae

tend to have dense groupings of rear fangs, the adjacency of

which facilitates capillary movement of the oral secretions along

the teeth and into the prey item (they also can serve as

replacement teeth).

Statistical Analysis

12

The consistency of the HL-RFL relationship was quantified

for each specimen studied. For each group of interest, a

scatterplot was created of HL versus RFL. A linear regression

analysis was performed by calculating a trend line, which

represented the model relationship between HL and RFL with the

most consistent relationship for each species. The deviation from

the model was determined by calculating the coefficient of

determination (R2), as this value nears 1 the consistency more

closely reflects the model’s direct relationship.

Analysis of Life History

The location where each sample was collected was recorded

and tallied. The R2 value calculated for the HL-RFL relationship

for each group was compared to the geographic distribution to

determine if variations in the HL-RFL relationships were a

function of specificity or geography.

Results

Diadophis punctatus complex:

13

The Coefficient of Determination (R2) calculated through

Linear Regression Analysis of the HL-RFL relationship for the SCA

lineage (ingroup) equals 0.384. R2 values for CCA, NE, and GP

lineages (outgroup 1) were calculated to be 0.242, 0.232, and

0.251, respectively. For H. simus (outgroup 2) samples expressed a

similar deviation to the SCA lineage with a R2 value of 0.377.

(Fig. 2).

To mitigate the effects of geography R2 values is a function

of geographic distribution the localities of all samples in each

case study were analyzed for geographic proximity of the

collection locations (Fig. 2).

Heterodon platirhinos

The linear regression of the HL-RFL relationship reveals

that H. platirhinos (ingroup) has an R2 value of 0.767. H. nasicus, H.

simus, and F. abacura (outgroup 1) have R2 values of 0.639, 0.377,

and 0.664, respectively (Fig. 2).

Heterodon nasicus:

14

The linear regression analysis of the HL-RFL relationship

reveals that H. nasicus (ingroup) has an R2 value of 0.639. H. simus,

the NE lineage of D. punctatus, and F. abacura have R2 values of

0.377, 0.232, and 0.664, respectively (Fig. 2).

Borikenophis portoricensis:

The linear regression of the relationship between HL and RFL

show that B. portoricensis (ingroup) expressed a deviation with an R2

value of 0.817. C. cantherigerus, Cubophis vudii, A. rufiventris, and

Antillophis andreai (outgroup 1) expressed R2 values of 0.427, 0.529,

0.515, and 0.253, respectively (Fig. 2).

Species R2 Geographic Distribution (Counties, States)

D. punctatus SCA

0.38361*

7,3a,b,c

D. punctatus CCA

0.24153**

2,1c

D. punctatus NE 0.23169*

9,2d,e

D, punctatus GP 0.25102*

8,3f,g,h

H. simus 0.37688*

10,5a,i,j,k,,l

H. platirhinos 0.76693*

19,4l,m,e,d

HL-RFL Analyses and GeographicDistributions

15

H. nasicus 0.63867*

4,2a,b***

H. simus 0.37688*

10,5a,i,j,k,l

F. abacura 0.66373*

15,6i,j,,l,n,o,p

H. nasicus 0.63867

4,2a,b***

H. simus 0.37688*

10,5a,i,j,k,l

D. punctatus NE

0.23169*

9,2d,e

F. abacura 0.66373*

15,6i,j,l,n,o,p

Caribbean LocalitiesB. portoricensis

0.81662*

2q,r

C. cantherigerus

0.4267*

1s

C. vudii 0.51491*

5t,u,v,,w,x

A. rufiventris 0.51866**

2y,z

A. andreai 0.25342*

1s

Discussion

In this study,

HL-RFL

Geographic Distribution as a Function of Prey/Predator Specificity

Figure 2: HL-RFL Analyses and Geographic Distribution The R2 calculated through Linear Regression Analysis of the HL-RFL relationship for the 4 case studies indicated relatively higher R2 values for the ingroups. Geography was originally documented for purposes other than this study. (* denotes p<.05; ** denotes p>.05; *** denotes documentation of inhabiting Mexico) a – Arizona, b – New Mexico, c –

16

High R2 Low R2

Wide Geographic Distribution

Geography is insignificant for specificity despite wide distribution.

Geography may be a contributing factor to specificity; role of ratio indeterminate.

Limited Geographic Distribution

Geography may be a contributing factor to specificity; role of ratio indeterminate.

No relation between geographic distribution and specificity.

relationships were analyzed by linear regression. It should be

acknowledged that there may be multiple evolutionary pressures

other then those from prey and predators that can alter the

coefficient of determination as well as many other possible

phenotypic expressions derived from the same selective pressures.

Though it has been recognized that geographically isolated

samples may be the result of an irregular and/or unnatural force,

all available localities contributed to the recorded geographic

distribution. By taking into account geographic distribution, one

major source of uncertainty is being addressed. Geographic

distribution could only be eliminated as a function of

specificity if the coefficient of determination for an ingroup

was high (relative to its outgroups) despite a wide geographic

distribution. While a high R2 and a limited geographic

distribution would signify an indeterminable influence of

distribution upon specificity, a high R2 value and a wide

geographic distribution would mean specificity would mean

specificity would be so defined despite other environmental

factors If there is an inconsistent relationship between the HL

Figure 3: Geographic Distribution as a Function of Prey/Predator Specificity Support for the test of predator/prey specificity model tested herein is supported when ingroups have a high coefficient of determination and wide distribution, while outgroups would have either a high R2 and limited geographic distribution or vice versa. No group should have a low R2 and a limited distribution.

17

and RFL (low R2 value), then there is no relationship between

geographic distribution and the coefficient of determination

(Fig. 3).

Diadophis punctatus complex : Relative to all other outgroups the SCA

lineage of D. punctatus exhibited a greater linear relationship (R2:

0.384) between HL and RFL. This consistency is supported in the

documented characteristics of venom composition of this species.

This is seen in D. punctatus’ venom composition and preference to

consume squamates, specifically slender prey (O’Donnell et al.

2007, Weaver 2010). The consideration of the snake’s girth

relative to its transcontinental distribution suggests larger

individuals have inherited the capability to overcome much larger

prey, and, therefore, much more dangerous prey. This relationship

between girth and prey size highlights the evolutionary pressure

from which the observed relative consistency of the SCA lineage’s

VDS is derived.

The geographic distribution of the samples used in this

study of the SCA lineage of D. punctatus is larger than that

observed of its sister taxon, the CCA lineage of D. punctatus. This

18

might be expected to yield an inconsistent HL-RFL relationship,

as the environmental pressures, due to a wide geographic

distribution, would generate diversity in VDS. The preference for

squamates has constrained the system to a higher degree of

specificity than that of the VDS of the CCA lineage regardless of

the geographic distribution observed. This suggests that the

geographic distribution of these samples may not be a factor in

the calculated consistency of the HL-RFL relationship. The other

two outgroups, the NE and GP lineages of D. punctatus have a wide

geographic distribution spanning over areas larger than either of

the California lineages. In this case, the variability of

environmental pressures- a result of geographic distribution- may

explain the relatively inconsistent relationship between the two

morphological components of the VDS for these outgroups.

Heterodon platirhinos/ Heterodon nasicus: Both ingroups exhibit

consistent morphology (H. platirhinos R2: 0.767, H. nasicus R2: 0.693)

with regard to the VDS. It is imperative to note that both of

these species have documentation of human envenomation (Kroll

1976; Morris 1985; Averill-Murray 2006, Weinstein et al. 2011),

19

unlike the D. punctatus complex. This may suggest that the VDS of

these species are a well-developed defensive mechanism as well,

which may explain why these two species have been documented

occasionally subduing prey other than Anaxyrus spp. (Edgren 1955).

Compared to the degree of consistency measured in the first case

study, both H. platirhinos and H. nasicus have a much more specified VDS

(as seen by the greater R2 value). This is a result of both their

preference of toads (Kroll 1976; Michener et al. 1987; Averill-

Murray 2006; Cooper et al. 2007; Mohammadi et al. 2013) and the

known potency of their oral secretions (Bragg 1960; Morris 1985;

Averil-Murray 2006; Weinstein et al. 2011). The two evolutionary

pressures (diet and defense) have further constrained the

threshold in which the VDS may serve its proper functions. The

consistency observed in the two ingroup taxa is not reflected in

the VDS of H. simus (outgroup). There have been no documented cases

of human envenomation for H. simus (Morris 1985), thus suggesting

the absence of specification with respect to defensive behavior

or behavior pertaining to subduing, or defending against,

relatively larger antagonists. In this case large body size in

Farancia may be an adaptation for preying on Amphiuma (Meade

20

1940), which are also large. Both environmental constraints may

contribute to the consistency in a VDS, however, because Farancia

abacura does not have a VDS, the greater R2 value may reflect prey

specificity associated with consuming Amphiuma.

Borikenophis portoricensis: This lineage is geographically restricted,

occurring only in Puerto Rico, and VDS exhibits the strongest

relationship between HL and RFL (R2: 0.817). Borikenophis portoricensis

and its close relatives prey on a wide variety of small

vertebrates (Henderson et al. 1988, Schwartz et al. 1991). There

have been multiple cases of human aggression and envenomation

from B. portoricensis, some of which have been potent enough to cause

significant distress to the victim (Weinstein et al. 2011,

Heatwole et al. 1966; Rodriguez-Robles et al. 1992; García-Gubern

et al. 2010; Weldon et al. 2010). The VDS of B. portoricensis, seems

specialized system for defense; however, its R2 value (0.817)

suggests defensive behavior rather than prey specificity. One of

the outgroups, A. rufiventris, also exhibited a consistency in the

relationship between HL and RFL (R2: 0.515), but small sample

21

size may be a confounding factor in this case considering that it

has characteristics which parallel those of all the other

outgroups that expressed a lower consistency in the VDS. Small

sample size is also the most probable explanation for the high p

value.

To further investigate suggestions of prey specificity/

predator orientation of the VDS it might be constructive to

consider the number of annual documented cases of human

envenomation as well as the number and types of species each

species consumes (as previously mentioned, the tree of D. punctatus

Expression of Specification of VDS

Figure 4: Expression of Specifications in Response to Given Evolutionary Pressures As the quadrant gets darker, the relationship between the HL and the RFL becomes more consistent.

22

is that of mtDNA, which may not accurately reflect the evolution

of the species, but more so of the mitochondria itself, so in the

future, it would be beneficial to look at an updated nuclear DNA

phylogeny of D. punctatus. Let it also be noted that the HL

considered in this study may not qualify as such in other studies

of jaw morphology. The parameters for HL in this study were a

proxy, as it is recognized that snake skulls grow in multiple

dimensions. In certain cases sampling was an issue; perhaps

adding more localities and increasing the sample size can better

evaluate the influence of geographic distribution on the

variability of the VDS morphologies of these species evaluated,

and the accuracy of the R2 value representing relationship

between the HL and the RFL can be improved. Lastly, some

measurements of snake teeth might be confounded because they were

taken from replacement teeth that were not fully developed.

Conclusion

23

This methodology is novel and may be useful in elucidating

prey/predator specificity of the VDS in certain cases. Prey

specificity and defense are both forms of environmental

pressures, which tend to constrain the threshold that the

relationship between the HL and the RFL can vary. When the

pressures are concurrent (e.g. H. platirhinos, H. nasicus), the system

must be evolutionarily fit to complete both tasks; and thus, when

neither is present, the system has room to vary because the

variability will not affect the fitness of the system.

Acknowledgements

I would like to thank my mentor, Dr. David Kizirian for his

guidance, dedication, and support throughout this study while

providing me with the proper materials to adequately complete my

research. I would like to thank Dr. David Cundall for reviewing

my manuscript. I would also like to thank my research teacher,

Mrs. Barbara Franklin for her guidance and support throughout

this entire research experience. Finally I would like to thank my

24

friends and family for their love and support throughout the

entirety of my research.

Bibliography

Averill-Murray, Roy C.. "Natural History of the Western Hog-nosedSnake (Heterodon nasicus) with Notes on Envenomation." Sonoran Herpetologist 19.9 (2006): 98-101. Print.

Bragg, Aurthur. "Is Heterodon Venomous?." Herpetologica 16.2 (1960): 121-123. Print.

Close, Matthew, and David Cundall. "Mammals as Prey: Estimating Ingestible Size." Journal of Morphology 273 (2012): 1042-1049. Print.

Cundall, David. “Drinking in snakes: kinematic cycling and water transport.” The Journal of Experimental Biology 203 (2000): 2171-2185. Print.

Cundall, David, and Abigail Pattishall. "Foraging Time Investmentin an Urban Population of Watersnakes (Nerodia sipedon)." Journal of Herpetology 45.2 (2011): 174-177. Print.

Cundall, David, and Alexandra Deufel. "Influence of the venom delivery system onintraoral prey transport in snakes." Elsevier 245 (2006): 193-210. Print.

Danforth, Stuart T.. "Porto Rican Herpetological Notes." Copeia 147 (1925): 76-79. Print.

Deufel, Alexandra, and David Cundall. "Feeding in Atractaspis (Serpentes: Atractaspididae): a study in conflicting functional constraints." Zoology 106 (2003): 41-63. Print.

25

Deufel, Alexandra, and David Cundall. "Functional Morphology of the Palato-Maxillary Apparatus in "Palatine Dragging" Snakes (Serpentes: Elapidae: Acanthophis, Oxyranus)." Journal of Morphology 271 (2010): 73-85. Print.

Edgren, Richard A. “The Natural History of the Hog-Nosed Snakes, Genus Heterodon: A Review.” Herpetologica 11.2 (1955): 105-117. Print.

Fontanella, Frank M., Chris R. Feldman, Mark E. Siddall, and Frank T. Burbrink. "Phylogeny of Diadophis punctatus: Extensive lineage diversity and repeated patterns of historical demography in trans-continental snake." Elsevier 46 (2008): 1049- 1070. Print.

Fry, B.G., and W. Wüster. "Assembling an Arsenal: Origin and Evolution of the Snake Venom Proteome Inferred from Phylogenetic Analysis of Toxin Sequences." Molecular Biology and Evolution 21.5 (2004): 870-883. Print.

Fry, Bryan G., Nicholas Vidal, et al. "Early evolution of the venom delivery system in lizards and snakes." Nature 439.2 (2006): 584-588. Print.

Fry, Bryan G., Holger Scheib, et al. "Evolution of and Arsenal Structural and functional diversification in the advanced snakes (Caenophidia)." Molecular & Cellular Proteomics 7.2 (2008): 215-246. Print.

García-Gubern, Carlos et al. "Is the Puerto Rican Racer, Alsophis portoricensis, Really Harmless? A Case Report Series." Wilderness & environmental medicine 21.4 (2010): 353-356. Print.

Goin, Coleman J. “A note on the food of Heterodon simus.” Copeia 1947. 4 (1947): 275-275.

Heatwole, Harold, and Ovan B. Banuchi. "Envenomation by the Colubrid Snake, Alsophis portoricensis." Herpetologica 22.2 (1996): 132-134. Print.

26

Henderson, Robert W., and Robert Powell. Natural history of West Indian reptiles and amphibians. University Press of Florida, 2009.

Henderson, R. W., and R. A. Sajdak. "Diets of West Indian racers (Colubridae: Alsophis): composition and biogeographic implications." Contributions to West Indian herpetology: a tribute to

Albert Schwartz (1996): 327-338.

Ismail, Mohammad, Abdullah M. Al-Berkairi, Ayman M. El-Bedaiwy, and Mohammad A. Abd-El Salam. "The ocular effects of spitting cobras: I. The ringhals cobra (Hemachatus haemachatus) Venom-Induced corneal opacification syndrome." Clinical Toxicology 31.1 (1993): 31-41. Print.

Jackson, Kate. "The evolution of venom-delivery systems in snakes." Zoological Journal of the Linnean Society 137 (2003): 337-354. Print.

Kardong, Kenneth V.. "The evolution of the venom apparatus in snakes from colubrids to viperids and elapids." Mem. Inst. Butanan46 (1982):105-118. Print.

Kroll, James C.. "Feeding Adaptations of Hognose Snakes." The Southwestern Naturalist 20.4 (1976): 537- 557. Print.

Mackessy, Stephen P.. Handbook of venoms and toxins of reptiles. Boca Raton: CRC Press, 2010. Print.

Mckinstry, D. "Evidence Of Toxic Saliva In Some Colubrid Snakes Of The United States." Toxicon 16.6 (1978): 523-534. Print.

Meade, George P.. “Observations on Louisiana Captive Snakes.” Copeia 1940.3 (1940): 165-168. Print .

Michener, Martin C., and James D. Lazell Jr.. "Distribution and Relative Abundance of the Hognose Snake, Heterodon platyrhinos, in Eastern New England." Journal of Herpetology 23.1 (1989): 35-40. Print.

27

Mohammadi, S., K. A. McCoy, D. A. Hutchinson, and A. H. Savitzky."Independently evolved toad-eating snakes exhibit sexually dimorphic enlargement of adrenal glands." Journal of Zoology 290.4

(2013): 237-245. Print.

Morris, Michael A.. "Envenomation from the Bite of Heterodon nasicus(Serpentes: Colubridae)." Herpetologica 41.3 (1985): 361-363. Print.

Odonnell, R, K Staniland, and R Mason. "Experimental Evidence That Oral Secretions Of Northwestern Ring-necked Snakes (Diadophis punctatus occidentalis) Are Toxic To Their Prey." Toxicon 50.6

(2007): 810-815. Print.

Pyron, R. Alexander, Frank T. Burbrink, et al. "The phylogeny ofadvanced snakes Colubridea), with discovery of a new subfamily and comparison of support methods for likelihood trees." Elsevier 58 2011): 329-342. Print.

Rice, Timothy F., and Douglas H. Taylor. “A simple Test of Prey Discrimination That Demonstrates Learning in Postlarval Ranid Frogs.” Journal of Herpetology 29.2 (1995): 320-322. Print.

Rojer, Anna. Biological inventory of Saba. Curaçao: Carambi Foundation, 1997. Print.

Rodriguez-Robles, Javier A., and Richard Thomas. "Venom Function in the Puerto Rican Racer, Alsophis portoricensis (Serpentes: Colubridae)." Copeia 1992.1 (1992): 62-68. Print.

Rodríguez-Robles, Javier A., and Manuel Leal. "Effects of Prey Type on the Feeding Behavior of Alsophis portoricensis (Serpentes: Colubridae) ." Journal of Herpetology 27.2 (1993): 163-168. Print.

Schwartz, Albert, and Robert W. Henderson. Amphibians and reptiles of the West Indies descriptions, distributions, and natural history. Gainesville: University of Florida Press, 1991. Print.

28

Tolson, Peter J., and Robert W. Henderson. "An Overview Of Snake Conservation In The West Indies." Applied Herpetology 3.4 (2006): 345-356. Print.

Weaver, Robert Elbert. Chemosensory and behavioral ecology of the dipsadid snakes: Contia tenuis, Diadophis punctatus, and Hypsiglena chlorophaea. 71.10(2010) . Print

Weldon, Caroline L., and Stephen P. Mackessy. "Biological And Proteomic Analysis Of Venom From The Puerto Rican Racer (Alsophis portoricensis: Dipsadidae)." Toxicon 55.2-3 (2009): 558-569. Print.

Warrell, DA, and LD Ormerod. "Snake venom ophthalmia and blindness caused by the spitting cobra (Naja nigricollis) in Nigeria.." The American Journal of Tropical Medicine and Hygiene 25.3 (1976):525- 529. Print.

Weinstein, Scott A.. "Replies to Fry et al. (Toxicon 2012 XX, 1-15). Part A. Analyses of squamate reptile oral glands andtheir products: A call for caution in formal assignment of terminology designating biological function." Elsevier 30 (2012): 1-10. Print.

Weinstein, Scott A.. "Venomous" bites from non-venomous snakes a critical analysis of risk and management of "colubrid" snake bites. London: Elsevier, 2011. Print.

Young, Bruce A., Karen Dunlap, Kristen Koeing, and Meredith Singer. "The buccal buckle: the functional morphology of venom spitting cobras." The Journal of Experimental Biology 207 (2004): 3483- 3494. Print.

Young, B.A., J. Lalor, and J. Solomon. "Comparative biomechanicsof an ophidian defensive behavior: head triangulation in hognose snake (Heterodon) and an egg-eating snake (Dasypeltis)." Journal of Zoology 248.2 (1999): 169-177. Print.

29

Pictures:

30

31