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toxins
Article
Fangs for the Memories? A Survey of Pain in
Snakebite Patients Does Not Support a Strong Role
for Defense in the Evolution of Snake
Venom Composition
Harry Ward-Smith 1, Kevin Arbuckle 2, Arno Naude 3and Wolfgang Wüster 1 ,*
1Molecular Ecology and Fisheries Genetics Laboratory, School of Natural Sciences, Bangor University,
Bangor LL57 2UW, UK; harry_ws@hotmail.co.uk
2Department of Biosciences, College of Science, Swansea University, Swansea SA2 8PP, UK;
kevin.arbuckle@swansea.ac.uk
3Snakebite Assist, Pretoria ZA-0001, South Africa; afnaude@gmail.com
*Correspondence: w.wuster@bangor.ac.uk
Received: 13 February 2020; Accepted: 19 March 2020; Published: 22 March 2020
Abstract:
Animals use venoms for multiple purposes, most prominently for prey acquisition and
self-defense. In snakes, venom composition often evolves as a result of selection for optimization
for local diet. However, whether selection for a defensive function has also played a role in driving
the evolution of venom composition has remained largely unstudied. Here, we use an online survey
of snakebite victims to test a key prediction of a defensive function, that envenoming should result
in the rapid onset of severe pain. From the analysis of 584 snakebite reports, involving 192 species
of venomous snake, we find that the vast majority of bites do not result in severe early pain.
Phylogenetic comparative analysis shows that where early pain after a bite evolves, it is often lost
rapidly. Our results, therefore, do not support the hypothesis that natural selection for antipredator
defense played an important role in the origin of venom or front-fanged delivery systems in general,
although there may be intriguing exceptions to this rule.
Keywords: Defense; evolution; pain; selective pressure; snake; snakebite; survey; venom
Key Contribution:
Through an online survey of snakebite victims, we show that the venoms of
a wide range of venomous snakes do not cause the early, severe pain typically associated with
defensive venoms. This provides strong evidence against the hypothesis of widespread selection for
a defensive function as a major driver of snake venom evolution.
1. Introduction
“Bee stings hurt. So do wasp stings, scorpion stings, the bites of centipedes, and the venom injections
of many other animals, including snakes. To inflict pain is not necessarily to the advantage of
an animal that uses its venom strictly for incapacitation of prey. In fact, it may be to its disadvantage
because pain may induce increased struggling on the part of the prey. But venoms are also used
defensively, and it is in that context that they may derive their effectiveness largely, if not exclusively,
from their pain-inducing qualities. It is principally because venoms are painful that they can function
in defense”.—Eisner and Camazine [1]
Venoms are widespread across the animal kingdom, and have evolved numerous times
in a broad range of phyla [
2
], with further examples still being discovered regularly, such as
Toxins 2020,12, 201; doi:10.3390/toxins12030201 www.mdpi.com/journal/toxins
Toxins 2020,12, 201 2 of 19
venomous crustaceans [
3
] and frogs [
4
]. The biological functions of venomous secretions include
primarily predation and anti-predator defense, as well as intraspecific competition, reproduction,
and digestion
[2,5]
. While a primary function can be identified for most venom systems, many
venomous animals use their venoms for multiple purposes. In particular, animals with primarily
foraging venoms frequently employ these for anti-predator defense [2].
Among venomous animals, snakes have received the greatest amount of research attention, due to
their medical significance [
6
], and because the large volumes of venom secreted by many species greatly
facilitate toxicological research. Snake venoms are highly variable in composition at all taxonomic
levels, from ontogenetic variation within individuals [
7
] to geographic variation within species [
8
] and
differences between higher taxa. The mechanisms and selective drivers of this variation have attracted
extensive research attention.
Snakes use their venoms for both foraging and self-defense, but the relative importance of these as
drivers of venom evolution has remained poorly understood. The “life-dinner principle” [
9
] suggests
that defense, where the snake is fighting for its life, should take precedence over foraging efficiency,
where a suboptimal strategy would merely result in reduced energy intake. However, most of the
literature on the selective drivers shaping venom composition has focussed on the role of diet.
Studies in multiple taxa and using diverse approaches have accumulated a considerable body
of evidence that many snake venoms have evolved under selection to optimize their prey-specific
toxicity. Diet-related evolutionary effects were first discovered through correlations between venom
composition and diet in Calloselasma rhodostoma [
10
]. Direct functional evidence in the shape of
prey-specific lethality has been demonstrated on multiple occasions. For instance, the venoms of
naturally arthropod-eating species of Echis and Vipera are more toxic to invertebrate prey than those of
predominantly vertebrate-feeding congeneric species [
11
–
13
]. Prey-specific venom toxicity has also
been detected in the venoms of different species of Sistrurus [
14
], and across multiple species of New
World coral snakes (Micrurus) [
15
]. Among colubrid venoms, individual toxins with specific toxicity to
avian and lizard prey have been documented in Boiga spp. [
16
–
18
], Oxybelis fulgidus [
19
] and Spilotes
sulphureus [
20
]. Patterns of ontogenetic variation in venom composition in vipers have also been found
to reflect ontogenetic diet changes [
21
]. Moreover, many prey species have evolved various levels of
resistance to snake venoms [
22
–
24
], resulting in a toxic arms race that has led to prey-specific venom
evolution in the snakes [
25
]. While the link between diet and venom composition may not be universal
(e.g., Zancolli et al. [
26
]), the idea that venom composition is driven primarily by selection for prey
subjugation has become the dominant paradigm in snake venom evolution.
While the venom system of most non-front-fanged snakes is of limited effectiveness against
predators [
27
], we know from the global impact of snakebite that front-fanged venomous snakes
frequently use their venoms in self-defense, often to devastating effect. This is supported by the
evolution of highly specific defensive adaptations, such as hooding, tail vibration, scale rubbing and
the rattle [
28
]. The frequent evolution of venom resistance among snake predators [
25
,
29
], predator
avoidance of front-fanged snakes [
30
,
31
], the evolution of innate avoidance of characteristic venomous
snake colour patterns [
32
,
33
], and the evolution of Batesian mimicry of front-fanged snakes [
34
,
35
] all
indicate that venom can be an effective defense against at least some predators. However, whereas
adaptation of venom composition to natural prey has become a well-documented phenomenon,
we remain largely ignorant whether natural selection for defensive purposes may also have played
a role in driving venom composition [
36
]. Harry W. Greene recognized this deficit in 2013 [
28
]
predicting that “we’ll soon be asking if toxins had more to do with defense than heretofore realized”.
To test for selection for a defensive function, it is essential to first consider the requirements for
a defensive venom: for a venom to be effective in that role, it must repel a predatory attack sufficiently
rapidly for its producer to escape serious injury or death. This is most readily achieved through
the rapid infliction of pain beyond that expected from the physical trauma of the bite alone [
1
,
37
].
In human patients, these characteristics are evident from clinical cases involving many primarily
defensive animal venoms. For instance, virtually all venomous fish use their potent venoms solely
Toxins 2020,12, 201 3 of 19
for defense, invariably causing intense pain immediately upon envenomation [
38
–
42
]. Similarly,
the entirely defensive venoms of non-predatory hymenopterans such as honeybees are equally notable
for the immediate pain following the sting. Other invertebrates that use their venom for both predation
and defense nevertheless include specifically pain-inducing toxins in their venom. This includes
many scorpions [
43
] and centipedes of the genus Scolopendra that produce symptoms which, although
rarely fatal to humans, are characterized by intense pain immediately upon envenomation, caused by
a specific pain-causing toxin [
44
]. These offer examples of venom which are highly effective both in
predatory and defensive contexts.
Whereas rapid-onset pain is ubiquitous and well documented in the examples of clearly defensive
venoms, we lack systematic information on pain after snakebite. It is widely acknowledged that
snakebites often entail significant or extreme pain [
45
–
47
]. However, the timeframe of its development
is rarely stated. From anecdotal reports, we know that bites by many species result in great variation
in the level and time course of pain experienced, with some bites resulting in immediate intense pain
while others cause none [
48
]. Moreover, pain often appears to be a delayed symptom secondary to
other venom effects, such as severe swelling or local tissue destruction [
48
]. Indeed, some snakes are
notorious for the lack of early pain caused by their bites: for instance, in Bungarus envenomations,
which often occur while the victim is asleep, initial pain is often never felt [49].
Limited evidence exists of specific pain-inducing toxins in certain species. Bohlen et al. [
50
]
discovered the first snake venom toxin to specifically cause pain in the venom of Micrurus tener. MitTx
was found to have no other function than to activate acid-sensing ion channels (ASICs), producing
pain. MitTx has subsequently also been found in the venom of M. mosquitensis and M. nigrocinctus [
51
],
but interestingly, the closely related M. fulvius lacks MitTx [
52
,
53
], indicating that this pain-inducing
toxin is phylogenetically labile within Micrurus. More recently the Lys49 myotoxin BomoTx, found
in the venom of Bothrops moojeni, was discovered to induce intense pain [
54
] through the promotion
of ATP release, which consequently activates the P2X2 and/or P2X3 purinergic receptors. However,
the relationship between the presence or absence of these toxins and the actual pain experienced by
bitten adversaries has not been explored.
The very limited data currently available on the ability of different snake venoms to cause early
pain post-bite restricts our ability to infer the role of antipredator defense in driving the evolution of
snake venom composition. The assessment of pain from envenomation is potentially complicated
by taxonomic differences in nociceptor function and pain perception. However, the structure of
nociceptors appears to be highly conserved across both vertebrates and invertebrates, as does the
central processing of nociception, which gives rise to the perception of pain [
55
]. There are exceptions
to these rules, such as the lack of sensitivity to capsaicin in birds or to acidity in naked mole rats [
55
],
and specific resistance in some specialized predators of venomous organisms, such as the specific
blocking of scorpion venom-induced algesia documented in scorpion-feeding grasshopper mice
(Onychomys sp.) [
43
]. However, it seems highly likely that most predators are likely to show similar
patterns of nociceptor activation in response to venomous challenges, especially in terms of their
time-course. This also suggests that the pain experience of a human snakebite patient is likely to be
representative of that of other generalized predators.
Since the testing of nociceptor activation in the laboratory is time-consuming and may be difficult
to relate to the perceived level of pain
in vivo
[
44
], we sought instead to assess the defensive potential
of different snake venoms by using human snakebite victims as a model system that allows data on
pain perception to be recalled and directly communicated. An increasing number of humans interact
regularly and intentionally with venomous snakes in a professional capacity or as part of leisure
activities, and as a result, numerous bites by a wide variety of snake species occur every year [
56
–
58
].
These well-informed bite victims represent a potentially valuable source of information on snakebite
symptoms, as they are capable of providing positive identification of the snake species, and, due to their
awareness of the risks of their activities, they are likely to be on average less susceptible to fear-induced
memory distortions than unprepared victims of entirely unexpected ‘accidental’ bites. The large body
Toxins 2020,12, 201 4 of 19
of collective experience of snakebites among reptile workers thus represents an unparalleled source of
information on the development of pain after snakebite.
Here, we exploit this collective reservoir of knowledge through the use of a questionnaire
that seeks to establish the severity and, more importantly, the time course of pain development in
patients envenomed by a diversity of snake species spanning the phylogenetic breadth of venomous
caenophidians. We postulate that any venom at least partly shaped by selection for antipredator
defense should cause pain of rapid onset to deter a predator in the critical early stages of any encounter,
potentially giving the snake a chance to escape before being seriously injured or killed. While the
presence of early pain after a bite does not necessarily indicate adaptation to a primarily defensive
function, absence of early pain would preclude such a role. We also predict that any at least partly
defensive venom should generate a trajectory of pain that would be consistent between patients:
although the perceived intensity of pain from a bee sting may vary between individuals, they invariably
cause early pain, and the same would be expected of other defensive venoms. The aim of this survey is
thus not to compare absolute pain levels across snake species, but instead to begin to understand the
pain trajectory as an ecologically informative attribute of snakebite in the context of defense.
2. Results
The distribution of sex to age of the 584 individual bite reports received in this study are shown
in Table 1. In all snake families, mean pain levels within one and five minutes after the bite were
considerably lower than the maximum pain level reported in the later phases of envenoming (Figure 1).
The pain became too distracting for normal activities within the ecologically crucial first 5 min in only
14.55% of bite victims, and later than 5 min in another 30.82% (Figure 2). Remarkably, 54.62% reported
never experiencing pain great enough to make normal activities impossible. Moreover, the pain
experienced by different individuals bitten by the same species varied immensely, not only in its
absolute level but also in its trajectory. Figure 3shows the mean and individual pain trajectories for 12
representative and well-sampled species from all snake families. While absolute pain levels are likely
to vary subjectively, the trajectory of pain development also varied extensively within many species
(e.g., Agkistrodon contortrix,Vipera berus,Atractaspis bibronii), but was much more consistent in others
(e.g., Crotalus atrox,Bitis arietans) (Figure 3).
Table 1. Distribution of bites received by sex to age of victim at the time of bite.
Age (years) Male Female Unreported Total
Total, n(%) 523 (89.6%) 51 (8.7%) 10 (1.7%) 584 (100%)
11–20, n(%) 129 (22.1%) 11 (1.9%) 1 (0.2%) 141 (24.3%)
21–30, n(%) 164 (28.1%) 27 (4.6%) 6 (1.0%) 197 (33.7%)
31–40, n(%) 102 (17.5%) 7 (1.2%) 1 (0.2%) 110 (18.8%)
41–50, n(%) 69 (11.8%) 3 (0.5%) 2 (0.3%) 74 (12.7%)
51–60, n(%) 41 (7.0%) 0 0 41 (7.0%)
≥61, n(%) 14 (2.4%) 3 (0.5%) 0 17 (2.9%)
Unreported, n(%) 4 (0.7%) 0 0 4 (0.7%)
Toxins 2020,12, 201 5 of 19
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Figure 1. Mean pain Numeric Rating Scale (NRS) (on a 0–10 scale) across four major clades of
venomous snakes within the first minute after the bite, within 1–5 minutes after the bite, and the
maximum pain experienced at any time.
Figure 2. Percentage of bites where pain became too intense/distracting to continue with
intended/normal activities across three time periods; early (within 5 mins), late (after over 5
minutes), and never.
Figure 1.
Mean pain Numeric Rating Scale (NRS) (on a 0–10 scale) across four major clades of venomous
snakes within the first minute after the bite, within 1–5 min after the bite, and the maximum pain
experienced at any time.
Toxins 2020, 12, x FOR PEER REVIEW 5 of 20
Figure 1. Mean pain Numeric Rating Scale (NRS) (on a 0–10 scale) across four major clades of
venomous snakes within the first minute after the bite, within 1–5 minutes after the bite, and the
maximum pain experienced at any time.
Figure 2. Percentage of bites where pain became too intense/distracting to continue with
intended/normal activities across three time periods; early (within 5 mins), late (after over 5
minutes), and never.
Figure 2.
Percentage of bites where pain became too intense/distracting to continue with
intended/normal activities across three time periods; early (within 5 min), late (after over 5 min),
and never.
Toxins 2020,12, 201 6 of 19
Toxins 2020, 12, x FOR PEER REVIEW 6 of 20
Figure 3. Variation shown of NRS at early, mid and max levels of pain of some of the best-sampled
species, and species with unusual patterns. Data plotted of 11 point NRS, where 0 = no pain felt at all
and 10 = the maximum level of pain imaginable. Crotalinae: a) Agkistrodon contortrix [n = 28], b)
Crotalus atrox [n = 24], c) Crotalus horridus [n = 13]; Viperinae: d) Bitis arietans [n = 14], e) Causus
rhombeatus [n = 5], f) Vipera berus [n = 40]; Elapidae: g) Demansia psammophis [n = 9], h) Notechis scutatus
[n = 9], i) Pseudechis porphyriacus [n=9]; Atractaspidinae: j) Atractaspis bibronii [n = 14]; Colubridae: k)
Figure 3.
Variation shown of NRS at early, mid and max levels of pain of some of the best-sampled
species, and species with unusual patterns. Data plotted of 11 point NRS, where 0 =no pain felt at
all and 10 =the maximum level of pain imaginable. Crotalinae: (
a
)Agkistrodon contortrix [n=28],
(
b
)Crotalus atrox [n=24], (
c
)Crotalus horridus [n=13]; Viperinae: (
d
)Bitis arietans [n=14], (
e
)Causus
rhombeatus [n=5], (
f
)Vipera berus [n=40]; Elapidae: (
g
)Demansia psammophis [n=9], (
h
)Notechis
scutatus [n=9], (
i
)Pseudechis porphyriacus [n=9]; Atractaspidinae: (
j
)Atractaspis bibronii [n=14];
Colubridae: (
k
)Heterodon nasicus [n=14], (
l
)Hydrodynastes gigas [n=13]. Black dashed lines plot
the mean trajectory and value at each time point. Note the relatively flat lines in Causus rhombeatus,
Demansia psammophis and Hydrodynastes gigas, indicating a relatively early onset of pain.
Toxins 2020,12, 201 7 of 19
Consistent with these results, and despite the phylogenetically widespread nature of
envenomations causing early pain, our ancestral state estimates suggest that the majority of the
history of venomous reptiles has been characterized by venoms causing little pain, particularly no
early pain (Figure 4). Nevertheless, there are two prominent exceptions to this pattern: Elapidae
and the New World radiation of pit vipers. Interestingly we estimate that these two deeper origins
of early-pain-inducing venoms arose in different ways. In the ancestor of elapid snakes, the venom
most likely caused early pain with little intraspecific variation, whereas in New World pit vipers
intraspecific variation consisting of all three possible states (no, early, and late pain) is the estimated
ancestral state (Figure 4). The estimated transition rates between states also suggest little evidence for
a pervasive influence of a defensive function over the evolutionary history of venomous reptiles in
general (Table 2). Specifically, states which include early pain (with or without intraspecific variation)
tend to have higher transition rates which involve loss of early pain, suggesting it is not maintained by
strong selection. Note that transition rates are not clade-specific but apply across the whole tree, so they
do not preclude an effect of antipredator defense in particular clades (such as elapids as highlighted
above), but suggest limited influence of defense in general.
The results from our variance partitioning analysis (using phylogenetic mixed models) suggest
that most of the variation in levels of pain depends on the bitten individual (for immediate and early
pain) and the phylogenetic history of the snake species which inflicted the bite (for the maximum pain
resulting from the bite) (Figure 5). Phylogeny had a much stronger influence on the severity of pain than
species-specific effects, which suggests that particular clades have characteristic venom compositions
that influence the level of pain experienced from a bite. Nevertheless, despite explaining ~95% of
the variance in maximum pain throughout the bite, any influence related to the snake responsible for
the bite is relatively minor (~25%) for early pain-induction compared to victim characteristics (~75%).
Because early pain is likely to be a key component of a defense-driven venom, our results suggest that,
although there may be important differences between different clades of snakes, the overall evidence
of selection for defense is limited. Note that we did find the predicted consistency across individual
bites, which explains almost none of the variation (~0.3% for early pain and ~1.5% for maximum
pain; Figure 5), but if we assume that humans are sufficiently analogous to other predators then the
effect of the individual bitten suggests that early pain is likely to be particularly severe only in some
bitten individuals.
Table 2.
Estimated transition rates between state combinations of pain trajectories according to the
fitted model behind our ancestral state estimates. Rates are given as probabilities of transitioning from
the state in the row to the state in the column of the table per million years of evolution. States which
include early pain-induction are highlighted in bold. The diagonal is marked with - to signify that
there is no transition rate from one state to itself since no change happens in that case.
From\To None None+Early Early Early+Late None+Early+Late None+Late Late
none - 0.004 0 0.011 0.003 0 0.041
none+early 0 - 0.032 0 0 0.205 0
early 0 0.125 - 0 0 0 0
early+late 0 0 0 - 0 0.070 0.244
none+early+late
0.057 0 0.016 0 - 0.043 0.008
none+late 0.018 0 0.042 0 0 - 0.111
late 0.141 0 0.032 0.022 0.069 0 -
Toxins 2020,12, 201 8 of 19
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Figure 4. Cont.
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Figure 4. Ancestral state estimates for pain trajectories showing either full coding of each individual
combination of states (a) or simplified into either inducing early pain (with or without variation) or
not (b). Venoms that cause early pain occur frequently across the current diversity of venomous
reptiles but with two major exceptions (Elapidae and New World pitvipers) are mostly independent
origins.
The results from our variance partitioning analysis (using phylogenetic mixed models) suggest
that most of the variation in levels of pain depends on the bitten individual (for immediate and early
pain) and the phylogenetic history of the snake species which inflicted the bite (for the maximum
pain resulting from the bite) (Figure 5). Phylogeny had a much stronger influence on the severity of
pain than species-specific effects, which suggests that particular clades have characteristic venom
compositions that influence the level of pain experienced from a bite. Nevertheless, despite
Figure 4.
Ancestral state estimates for pain trajectories showing either full coding of each individual
combination of states (
a
) or simplified into either inducing early pain (with or without variation) or not
(
b
). Venoms that cause early pain occur frequently across the current diversity of venomous reptiles
but with two major exceptions (Elapidae and New World pitvipers) are mostly independent origins.
Toxins 2020,12, 201 10 of 19
Toxins 2020, 12, x FOR PEER REVIEW 10 of 20
explaining ~95% of the variance in maximum pain throughout the bite, any influence related to the
snake responsible for the bite is relatively minor (~25%) for early pain-induction compared to victim
characteristics (~75%). Because early pain is likely to be a key component of a defense-driven venom,
our results suggest that, although there may be important differences between different clades of
snakes, the overall evidence of selection for defense is limited. Note that we did find the predicted
consistency across individual bites, which explains almost none of the variation (~0.3% for early pain
and ~1.5% for maximum pain; Figure 5), but if we assume that humans are sufficiently analogous to
other predators then the effect of the individual bitten suggests that early pain is likely to be
particularly severe only in some bitten individuals.
Figure 5. Variance in the magnitude of immediate, early, and maximum pain experienced by
snakebite victims as explained by variation between individual bites, individual victims, snake
species, and snake phylogeny. Note that individual bites are only a minor source of variation after
accounting for the snake and the bitten individual, and there is a stark difference between variance
components of the level of pain within the first 5 minutes (which mostly varies based on the bitten
individual) and the maximum level of pain experienced over the course of the envenomation (which
is mostly related to the phylogenetic history of the snake species involved).
3. Discussion
In summary, our results provide little evidence of pervasive selection for a defensive function in
the evolution of snake venoms. The overall pattern from envenomed bites suggests that the majority
of bites cause relatively little early pain, compared to the much higher levels of pain experienced
later during the course of the envenoming. Strikingly, in the vast majority of bites sustained by our
Figure 5.
Variance in the magnitude of immediate, early, and maximum pain experienced by snakebite
victims as explained by variation between individual bites, individual victims, snake species, and snake
phylogeny. Note that individual bites are only a minor source of variation after accounting for the snake
and the bitten individual, and there is a stark difference between variance components of the level of
pain within the first 5 min (which mostly varies based on the bitten individual) and the maximum level
of pain experienced over the course of the envenomation (which is mostly related to the phylogenetic
history of the snake species involved).
3. Discussion
In summary, our results provide little evidence of pervasive selection for a defensive function in
the evolution of snake venoms. The overall pattern from envenomed bites suggests that the majority
of bites cause relatively little early pain, compared to the much higher levels of pain experienced
later during the course of the envenoming. Strikingly, in the vast majority of bites sustained by our
respondents, pain only became too distracting for other activities much later than during the first
few minutes, and, even more surprisingly, in 54.62%, this never happened. This suggests that the
venoms of these snakes would be ineffective in deterring a continued attack by a predator within
an ecologically relevant timeframe.
Moreover, respondents bitten by some species had pain experiences that cannot be attributed solely
to inter-individual differences in pain sensitivity, but that instead suggest intraspecific differences in
venom activity. Even though we only considered bites with evidence of envenoming, some respondents
bitten by species such as Crotalus atrox,Vipera berus and Notechis scutatus reported no pain whatsoever
in the early or even later stages of envenoming, while others reported a strong later increase in pain,
Toxins 2020,12, 201 11 of 19
or even high early pain levels (Figure 3). Even accounting for individual differences in pain sensitivity,
these extreme differences are difficult to reconcile with being due to identical venoms. Instead, they
suggest intraspecific variation in venom composition with regard to algesic activity. This would be
unexpected in a scenario of pervasive selection for a defensive function.
Similarly, the phylogenetic comparative analyses found little support for strong selection for
a defensive function across the clade as a whole, and certainly not early in the caenophidian
(or toxicoferan) radiation. Early pain as a consequence of venom appears to have evolved repeatedly,
in particular, we find evidence for deeper origins at the base of the Elapidae and the New World pitvipers
(with these deeper origins being far less likely to be explained by noise in the data). The evolution of
consistent early pain in the Elapidae may be related to elapid venoms being typically more neurotoxic
and so potentially targeting pain receptors directly (either as a directly selected or exapted effect),
whereas in vipers the pain may be the result of SVMPs or similar toxins breaking down tissue and
so is under weaker (if any) direct selection and is consequently more variable. This interpretation
is consistent with our results from Figure 4, as the origin of early pain in elapids is estimated to be
fairly consistently early pain, whereas in New World pitvipers it is estimated that bites could variably
cause early, late, or no pain. If true, it suggests that elapid snakes are the best clade upon which to
focus future efforts on understanding defense-driven evolution of pain. It also opens the intriguing
possibility that spitting cobras (as the only snakes with unambiguously defensive adaptations of venom
use) may have been exapted for defensive use of venom via early-pain inducing elapid ancestors.
Our estimated transition rates for pain trajectories find that the rate of loss of early pain was
systematically higher than its rate of gain. This again suggests a lack of widespread selection pressure
for a defensive function, as the early pain necessary for defense both evolves (relatively) infrequently
and seems to have little selection pressure maintaining it when it does.
While the general pattern argues against a pervasive selection for defense, some taxa with
divergent patterns such as relatively flat pain trajectories are worth noting, in particular Causus
rhombeatus,Hydrodynastes gigas and especially Demansia psammophis (Figure 3). To identify outliers with
potentially more defensive venoms, we explored the onset of incapacitating pain in the better-sampled
species (N
≥
5). Out of these 34 species, only four caused early incapacitating pain in more than 40%
of all cases, andin more than 50% of those cases in which incapacitating pain occurred at all: Causus
rhombeatus,Agkistrodon piscivorus,Pseudechis australis and the combined bites of all Demansia species.
The individual variation in pain perceptions in the early stages of a bite may be relevant here as pain
may only be experienced as incapacitating by a minor-moderate proportion of victims. In principle this
could still lead to effective defense against some proportion of the predator community, but studies on
variability in pain responses in natural predators are needed to further examine this possibility. In any
case, even if effective against some predators, the inconsistency of the results adds to the weight of
evidence against a strong role for defense in snake venom evolution in general. Moreover, we also
stress again that early pain is necessary, but not sufficient, to infer selection for a defensive function,
since it may also represent a mere side-effect of another venom activity.
In one of the few studies explicitly addressing the relationship between venom toxicological
function and defensive adaptations, Panagides et al. [
59
] noted an association between the defensive
adaptations of cobras (Naja) and relatives and venom cytotoxicity. They interpreted cytotoxicity as
a defensive adaptation, on the assumption that it would be associated with greater pain. Our data do
not support this inference, as the average pain trajectories of all Naja species in our dataset (but not
Hemachatus) display the typical pattern of much lower pain in the first five minutes after the bite
than later (Figure 6). Out of 26 Naja bites (all species), only four (15%), each by a different species,
resulted in early incapacitating pain, and 19 (73%) never reached that pain level. While the sample
sizes for the individual species are small, the emerging pattern does not support strong selection for
a defensive function, contrary to the interpretation of Panagides et al. [
59
]. This may be because the
assumption that cytotoxicity is a good proxy of early pain in cobras is incorrect. The clearest example
of adaptations for defensive use of snake venoms is in venom spitting in cobras, which suggests
Toxins 2020,12, 201 12 of 19
that spitting cobras should cause more rapid early pain thannon-spitting species. Unfortunately,
our sampling is insufficient to determine whether this is the case.
Toxins 2020, 12, x FOR PEER REVIEW 12 of 20
adaptations of cobras (Naja) and relatives and venom cytotoxicity. They interpreted cytotoxicity as a
defensive adaptation, on the assumption that it would be associated with greater pain. Our data do
not support this inference, as the average pain trajectories of all Naja species in our dataset (but not
Hemachatus) display the typical pattern of much lower pain in the first five minutes after the bite
than later (Figure 6). Out of 26 Naja bites (all species), only four (15%), each by a different species,
resulted in early incapacitating pain, and 19 (73%) never reached that pain level. While the sample
sizes for the individual species are small, the emerging pattern does not support strong selection for
a defensive function, contrary to the interpretation of Panagides et al. [59]. This may be because the
assumption that cytotoxicity is a good proxy of early pain in cobras is incorrect. The clearest example
of adaptations for defensive use of snake venoms is in venom spitting in cobras, which suggests that
spitting cobras should cause more rapid early pain thannon-spitting species. Unfortunately, our
sampling is insufficient to determine whether this is the case.
Figure 6. Mean pain trajectories for species of Naja and relatives. Note typical pain trajectory with
relatively low early pain compared to the maximum later pain levels. Species with n = 1 were
excluded.
In contrast to the overall conclusions of this study, the evolution of specifically
nociceptor-targeted toxins, such as BomoTX in Bothrops moojeni [54] and MitTX in Micrurus tener [50],
strongly suggests a defensive function in those species. However, at least in coral snakes, the
phylogenetically inconsistent distribution of this toxin argues against consistently strong selection
for defense in this clade: MitTX is present in M. tener, M. nigrocinctus and M. mosquitensis, but absent
in M. fulvius, the sister species of M. tener. A similar dimeric toxin is also present in the more
distantly related M. dumerili and M. frontalis [60], and apparent homologues have been found in
additional venoms [61], but again without a clear phylogenetic pattern.
The effect of these specifically algesic toxins on pain levels and trajectories in vivo remains
largely unexplored. In a series of 39 bites by M. fulvius, which lacks MitTX, local pain appeared to be
largely absent [62]. However, in another study [63], at last one patient bitten by the same species
reported radiating pain. In comparison, 42.7% of 82 M. tener bites in Texas were followed by local
pain (on an unknown timescale), but this was severe enough to require analgesia in only 15.9% [64].
A number of otherwise symptomatic patients in the latter series did not report pain, suggesting a
lack of the kind of consistent pattern of early pain following fish or honeybee envenoming. Our
Figure 6.
Mean pain trajectories for species of Naja and relatives. Note typical pain trajectory with
relatively low early pain compared to the maximum later pain levels. Species with n=1 were excluded.
In contrast to the overall conclusions of this study, the evolution of specifically nociceptor-targeted
toxins, such as BomoTX in Bothrops moojeni [
54
] and MitTX in Micrurus tener [
50
], strongly suggests
a defensive function in those species. However, at least in coral snakes, the phylogenetically inconsistent
distribution of this toxin argues against consistently strong selection for defense in this clade: MitTX
is present in M. tener,M. nigrocinctus and M. mosquitensis, but absent in M. fulvius, the sister species
of M. tener. A similar dimeric toxin is also present in the more distantly related M. dumerili and M.
frontalis [
60
], and apparent homologues have been found in additional venoms [
61
], but again without
a clear phylogenetic pattern.
The effect of these specifically algesic toxins on pain levels and trajectories
in vivo
remains largely
unexplored. In a series of 39 bites by M. fulvius, which lacks MitTX, local pain appeared to be
largely absent [
62
]. However, in another study [
63
], at last one patient bitten by the same species
reported radiating pain. In comparison, 42.7% of 82 M. tener bites in Texas were followed by local
pain (on an unknown timescale), but this was severe enough to require analgesia in only 15.9% [
64
].
A number of otherwise symptomatic patients in the latter series did not report pain, suggesting a lack
of the kind of consistent pattern of early pain following fish or honeybee envenoming. Our sample of
Micrurus bites is insufficient to add to this discussion, except to note that two bites by M. nigrocinctus
resulted in little early pain.
Another factor arguing against pervasive selection for defense is the atrophy of the venom
apparatus in snakes feeding on undefended prey. Among non-front-fanged snakes, the bird egg
specialist Dasypeltis is phylogenetically nested in a clade of venomous opisthoglyphous genera such
as Boiga and Telescopus [
65
], but its venom apparatus is atrophied [
66
]. Among front-fanged snakes,
several elapid lineages that have specialized on the consumption of fish eggs (Emydocephalus spp. and
Aipysurus eydouxii) display a greatly reduced venom apparatus and a series of deleterious mutations in
their main toxin genes [
67
–
69
]. This suggests that, in the absence of a foraging function, there were no
further selective pressures for the retention of a venom apparatus.
Toxins 2020,12, 201 13 of 19
Inevitably, studies like the present one, that are based on the recollections of individuals that
lived through a potentially traumatic experience, are likely to result in noisy data with multiple
potential sources of error. These include faulty memory, subjective biases, individual differences in
pain perception and tolerance, misidentification of snakes, and noise from a wide variety of unknown
factors, such as site of bite, quantity and depth of venom injection, individual venom variation etc.
Nevertheless, retrospective reports of pain intensity are commonly used and, given sufficient sample
sizes, are often sufficiently reliable for epidemiological studies (e.g., Brauer et al. [70]).
Despite the inevitable statistical noise in survey data, they have allowed us to exploit the large
body of collective experience accumulated by the herpetological community to assess the algesic
properties of snakebites across a considerable breadth of snake diversity in a manner unachievable
by other means. They have yielded a strongly supported and consistent pattern of limited early pain
after snakebites, compared to higher maximum pain later, and a lack of early incapacitation from pain.
This study thus adds to the evidence that venom in snakes has evolved for primarily foraging purposes
and suggests that any effectiveness as a defensive adaptation is restricted to particular cases rather
than a general (or early) driver of venom evolution.
This leads to the question of why, against the predictions of the life-dinner principle, selection for
defense did not play a greater role in the evolution of snake venom. It may be that for the most part,
biting is the final strategy in a snake’s defensive arsenal, because contact with the predator increases
the risk of injury to the snake [
71
]. To reduce the necessity for this risk, snakes have evolved other
defensive strategies that they employ before biting to deter and evade predation [
72
,
73
]. The evolution
of behaviour to utilise alternative defensive strategies prior to biting may have reduced the selective
pressures of defense upon the composition and toxicological effects of snake venoms.
Another reason may lie in the extremely lethal power of many front-fanged venomous snakes [
74
].
Numerous venomous organisms, such as insects and most fish, rely on painful rather than lethal
venomous defenses, where individual predators are deterred by pain resulting from individual stings,
and each sting needs to cause pain to generate that deterrence. Front-fanged venomous snakes have
sufficient lethal potential to incapacitate or kill many predators. As a result, rather than relying on
deterrence of individual predators through pain, deterrence may also develop through social learning
in some predators witnessing the death, suffering or incapacitation of a conspecific or relative [
34
,
75
],
or through natural selection for innate avoidance [
32
,
33
]. Neither of these mechanisms requires early
pain or other specifically defensive adaptations of venom composition, but mathematical models have
suggested instead that a quantitative increase in lethality may be selected for under some scenarios [
76
].
Despite the lack of pervasive selection for defense revealed here, the role of snake venom in
antipredator defense, and the ecological and evolutionary factors that may influence such interactions,
remain potentially rewarding subjects for further investigation. Currently, we lack even the most basic
quantitative data on the use of venom in interactions between snakes and their predators, including
any indication of how often snakes ever employ venom defensively, and how frequently this use of
venom affects the outcome of these encounters. Although our current study suggests that defense has
not been the primary driver of snake venom evolution in general, particularly early in the history of
the clade, we also suggest that some exceptions may exist in certain groups. Moreover, groups that
diverge from the majority of snakes in their use of venom, especially spitting cobras, may represent
rewarding targets for more detailed investigations of when, why, and how antipredator defense might
act as an important factor in snake venom evolution.
4. Materials and Methods
4.1. Questionnaire and Data Collection
To obtain data on the collective experience of snakebite pain from the herpetological community,
we designed a questionnaire to chart the time course of pain development in envenomations.
Respondents were asked to identify the species of snake they were bitten by and rate their level of
Toxins 2020,12, 201 14 of 19
pain experienced at three time periods: immediate (within 1 min post-bite), early (1 to 5 min post-bite)
and later (maximum level of pain), utilizing an 11 point (0–10) Numeric Rating Scale (NRS) to give
the most reliable account of pain experienced [
77
–
79
]. Respondents were then asked when the pain
became ‘too distracting’ (distraction index), defined as how long after the bite the level of pain became
too intense to continue with intended/normal activities: early (subgroups of: immediately, <1 min and
1 to 5 min), late (>5 min), or never.
Other questions related to the sex and age of the respondent at the time of the bite, the site of
the envenomation on the body, and the sex, size and life stage of the snake responsible. No other
medical symptoms were asked for, but many participants chose to include them, within the subsequent
comment section. Where a respondent had received multiple envenomed bites, separate reports were
collected for each event. Bite reports from both wild (51.2%) and captive (48.8%) snakes were included
in the analyses. “Dry” bites without clinical symptoms of envenoming were excluded.
Professional herpetologists, herpetoculturists and herpetological fieldworkers were targeted to
reduce the variance in pain perception in reports. To reach the largest audience of herpetologists
possible, the questionnaire was created and distributed electronically using Google Forms (https:
//goo.gl/forms/A8FdnjVRqMnUnV162). The survey became publicly available in November 2016 and
was extensively advertised via e-mail and shared on over 130 herpetological Facebook groups and via
Twitter. Here, we analyze responses to the questionnaire received until December 2018. Bites were
excluded from our analyses where the victim, at the time of the bite, was <10 years of age, or where
a person submitted a report of snakebite sustained by a third party. After removal of reports based
on the above criteria, and obviously erroneous or facetious entries, our final dataset used in further
analyses contained reports of 584 individual bites, inflicted on 368 individual respondents by a total of
192 snake species.
Inevitably, self-reported, survey-based measures of pain will contain substantial statistical noise.
However, as there is no obvious a priori reason to expect bias (systematic errors in a direction that
are likely to mislead attempts to answer our specific questions), such noise is likely to be random.
The effect of this should simply be to reduce the signal-to-noise ratio in the data, but our large sample
size at all levels (bites, people, and snake species) should still provide sufficient statistical power to
provide meaningful results. The full survey is available in Figure S1.
All subjects gave their informed consent for inclusion before they participated in the study.
The protocol was approved by the Bangor University, College of Natural Sciences Ethics Committee on
14th November 2016 (CNS2016HWS01).
4.2. Data Analysis
Because we were using data across multiple species, we used a phylogenetic comparative
approach to investigate the patterns of pain-inducing venoms across species and time [
80
]. We obtained
a phylogeny from the TimeTree database [
81
] based on the list of species from our dataset. For this
purpose (and hence subsequent comparative analyses) five records were removed as they represented
unidentified species or hybrids. Thirty-nine species that were present in the dataset but not the TimeTree
database were either added manually to the phylogeny at an appropriate position sister to congeneric
species (10 species) where possible, or were replaced by ‘phylogenetically equivalent’ species [
82
]
to download the tree (29 species). In the case of phylogenetic equivalent species, we corrected the
replacement names to match our dataset after obtaining the phylogeny, so the names in our figures
match the dataset. Overall, the final phylogeny and subsequent comparative analyses contained 192
species. Both the phylogeny and list of species, with notes on how species which were present in the
dataset, but not the TimeTree database were included, is provided in Table S1. All comparative analyses
were conducted in R v3.6.0 [83]. R Code used for analysis is provided in Supplementary Materials.
We used two types of comparative analyses in this study. First, we investigated how our
‘distraction index’ (no pain [‘never’ =pain never became distracting], rapid onset of pain [‘early’
=
≤
5 min], or delayed onset of pain [‘late’ = > 5 min]) has evolved over the evolutionary history
Toxins 2020,12, 201 15 of 19
of venomous reptiles using ancestral state estimation. We used Bayesian stochastic mapping [
84
]
implemented in phytools v0.6.99 [
85
] to estimate ancestral states. Because we had intraspecific variation
in our trait of interest (distraction index), we coded our species to include this ‘polymorphism’ with 7
possible states: none, early, late, none+early, none+late, early+late, or none+early+late. This coding
also allows us to group these states into those which include early pain onset and those that don’t after
the analysis, improving the interpretability of the results in the context of our questions. Ancestral state
estimates were based on 1000 simulations under an ‘all rates different’ model for which the transition
rates were estimated from the data, as was the prior distribution at the root of the tree.
Second, we investigated how the variation in the magnitude of pain was partitioned for three
time periods: immediately (within 1 min of the bite), early (within 5 min of the bite), and throughout
the total duration of the bite (i.e., the maximum level of pain experienced). This gives a measure
of consistency such that if a large proportion of the variance is attributed to snake species then the
pain caused by a particular species should be fairly consistent, but this can vary greatly between
snake species. If venom has been selected for a defensive role then we expect that variance explained
by individual bites should be low (bites by a given species should be broadly consistent in pain
induction). To test this, we constructed three phylogenetic mixed models using MCMCglmm v2.29 [
86
],
one to predict the magnitude of pain (modelled as a Poisson distribution) at each of immediate, early,
and maximum time periods. Note that phylogenetic mixed models are not restricted to one datapoint
per species, so in addition to allowing us to evaluate intraspecific variation we were also able to use
our total dataset of 584 observations for each model. The models included the distraction index as
a fixed effect to control for any effects of pain trajectory on the level of pain experienced at a given time
point, but inference is primarily based on the random effects included in each model. The random
effects were the phylogeny (closely related species cause similar pain levels, distantly related species
differ), snake species (pain levels vary based on the particular species that inflicted the bite, regardless
of phylogeny), and victim (pain levels vary between people who are bitten). Note that the residual
variance in this case can then be considered as an effect of individual bites (pain levels from every
individual bite are unique) after accounting for the snake species and clade as well as the person bitten.
MCMCglmm uses inverse Wishart prior distributions on parameters and we set all priors to have
V=1
and
nu =0.002
. MCMC chains were run for 1.1 million generations, the first 100,000 of which
were discarded as burning, and posterior samples were saved every 1000 generations. The quality of
each model was checked using autocorrelation plots, Geweke plots, and effective sample sizes (all of
which were over 500, mean =852), and in all cases, models ran well.
We note that the two types of comparative analyses are independent in their interpretations.
For instance, even if our mixed models find evidence for much of the variation in pain being due to the
individual person bitten, the ancestral state estimations of distraction index are interpretable (despite
being concerned mostly with snake species and clade levels). This is because they address two different
questions. The mixed models are considering variation in the level of pain experienced, whereas the
distraction index (for which we estimate ancestral states) is a measure of the trajectory of the level of
pain over the bite. One way to think of this difference is that a bite could start offeither mild or very
painful and still gradually get worse (or better) over time. Note again that to guard against systematic
associations between the level of pain and pain trajectory, our mixed models include the distraction
index as a fixed effect (incorporated during the estimation of the variance partitioning of the random
effects under examination).
Supplementary Materials:
The following are available online at http://www.mdpi.com/2072-6651/12/3/201/s1,
Figure S1: Pain in Snakebite Questionnaire, R script for Bayesian stochastic mapping and phylogenetic mixed
models, Table S1: TimeTree database species list with notes on alterations.
Author Contributions:
Conceptualization, H.W.-S. and W.W.; Methodology, H.W.-S., K.A. and W.W.; Software,
H.W.-S. and K.A.; Formal Analysis, K.A.; Investigation, A.N., H.W.-S. and W.W.; Data Curation, H.W.-S.;
Writing—Original Draft Preparation, H.W.-S.; Writing—Review & Editing, H.W.-S., K.A. and W.W.; Visualization,
H.W.-S. and K.A.; Supervision, W.W.; Project Administration, H.W.-S. All authors have read and agree to the
published version of the manuscript.
Toxins 2020,12, 201 16 of 19
Funding: This research received no external funding.
Acknowledgments:
We thank all survey respondents, without whose participation this study would not have
been feasible. We also acknowledge the numerous Facebook group admins, who kindly allowed the survey to be
posted to their pages, and the numerous individuals who engaged with and shared the questionnaire through
social media. For ethics approval, we thank John Latchford of the Bangor University, College of Natural Sciences
Ethics Committee. For his R coding advice, we thank Russel Gray.
Conflicts of Interest: The authors declare no conflict of interest.
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