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Herpetological Review 52(2), 2021
ARTICLES 221
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Herpetological Review, 2021, 52(2), 221–237.
© 2021 by Society for the Study of Amphibians and Reptiles
Glow and Behold: Biouorescence and New Insights on the
Tails of Pitvipers (Viperidae: Crotalinae) and Other Snakes
Biofluorescence, the absorption of photons by biological
tissues that are then reemitted at longer lower-energy
wavelengths, occurs naturally in a broad range of organisms.
In recent years, biofluorescence in tetrapods has emerged as
an increasingly common phenomenon, with many examples
documented in mammals (Jeng 2019; Kohler et al. 2019; Anich et
al. 2020), birds (Pearn et al. 2003; McGraw et al. 2007; Barreira et
al. 2012; Camacho et al. 2019; Wilkinson et al. 2019), amphibians
(Nowogrodzki 2017; Taboada et al. 2017a,b; Deschepper et al.
2018; Goutte et al. 2019; Gray 2019; Thompson et al. 2019; Lamb
and Davis 2020; Whitcher 2020) and reptiles (Hulse 1971; Gruber
and Sparks 2015; Prötzel et al. 2018, 2021; Sloggett 2018; Seiko
and Terai 2019; Eipper et al. 2020; Eto 2020; Top et al. 2020;
Mendyk 2021). The extent of this phenomenon in reptiles and
its ecological and evolutionary underpinnings, however, remain
poorly studied, though various fluorescent emission patterns
have been identified in reptiles including the carapaces of sea
turtles (Gruber and Sparks 2015), bony cranial protuberances of
lizards, (Prötzel et al. 2018), skeletal elements of geckos (Sloggett
2018; Top et al. 2020), and the body scalation of various snakes
and lizards (Hulse 1971; Seiko and Terai 2019; Eipper et al. 2020;
Eto 2020; Prötzel et al. 2021). Taken together, these few examples
suggest that biofluorescence may be more widespread in, and
important to reptiles than previously envisaged.
Snakes offer new opportunities for exploring biofluorescence
in reptiles. To date, biofluorescence has been recorded in several
snake species across various families, including fossorial taxa
such as the leptotyphlopid Rena humilis (Hulse 1971) and various
members of the typhlopid genus Anilios (Eipper et al. 2020), the
marine elapid Laticauda laticaudata (Seiko and Terai 2019),
the terrestrial lamprophiids Limaformosa crossi and Mehelya
poensis (Eto 2020), and rattlesnakes (Klauber 1956). Fluorescent
pteridine-derived substances have also been extracted from the
skin of three colubrids (Elaphe climacophora, E. quadrivirgata,
Euprepiophis conspicillata) and the pitviper Gloydius blomhoffii
(Odate et al. 1959). Given the dramatic differences in body size,
morphology, coloration, ecology, behavior and phylogenetic
relatedness between these taxa and considering that there are
more than 3,800 extant snake species (Uetz et al. 2020) that vary
widely in these characters, it is almost certain that additional
examples of biofluorescence await discovery in snakes.
Recently, a cursory search for ultraviolet (UV) induced visible
fluorescence in a private collection of captive reptiles by one of
us (LP) revealed remarkable tail fluorescence in a captive-bred
sibling group of juvenile Hagen’s Pitvipers, Trimeresurus hageni.
Here, we dramatically increase the number of snake taxa known
to exhibit biofluorescence by describing tail fluorescence in
several genera of pitvipers (Viperidae: Crotalinae) for the first
time.
Materials and Methods
From our initial observations of fluorescence in T. hageni, we
expanded our sampling to include a total of 28 pitviper species
representing ten genera to determine whether tail fluorescence
occurs in additional taxa within the group. To establish whether
the character is inherent to members of Crotalinae rather than
a potential artifact of captivity, a combination of living captive
and field specimens were examined, and several frozen and
fluid-preserved specimens of both wild and captive origins were
analyzed. Fifteen snake species representing the outgroups
Boidae, Colubridae, Elapidae, Lamprophiidae, and Viperinae
also were examined for tail fluorescence (Table 1).
For visualizing fluorescent tissues, we scanned the entirety
of each snake’s body in darkness with a 3-watt, 365 nm LED UV
torch (model UV301D; Shenzhen LIGHTFE Lighting Co., Ltd.,
Shenzhen, China) and recorded qualitative data on any tail
fluorescence observed. Because 365 nm LED torches also cast a
faint blue light that could interfere with or obscure detection of
more subtle biofluorescent tissues, torches were fitted with UV
pass filters to reduce the overall residual visible light emitted.
Although not included as part of our original analysis, we also
later examined several specimens using 395 nm LED UV torches
(model KJ-C6404; YMMYP Technology Co., Shenzhen, China;
and model UV301A; Shenzhen LIGHTFE Lighting Co., Ltd.,
Shenzhen, China) and a 100 µW 405 nm blue-violet laser (model
D8-LASER100; Walfront LLC, Lewes Delaware, USA) for evidence
of fluorescent excitation under greater wavelengths.
LAURENCE PAUL
e-mail: laurencepaul@outlook.com
ROBERT W. MENDYK
Department of Herpetology, Audubon Zoo, 6500 Magazine Street
New Orleans, Louisiana 70118, USA
Department of Herpetology, Smithsonian National Zoological Park,
3001 Connecticut Ave NW, Washington, DC 20008, USA
e-mail: rmendyk@auduboninstitute.org
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222 ARTICLES
Captive specimens were examined from Audubon Zoo’s
herpetology department and a private reptile collection while
the animals were inside their terrariums, or during the course of
routine husbandry practices such as feeding, enclosure cleaning,
weight measurements or veterinary procedures. To minimize
the risk of venomous snakebite, handling live captive specimens
including restraint and physical manipulation was minimized or
avoided. Field specimens were encountered opportunistically.
Road-killed individuals were examined in situ at night, and
nuisance snakes were retrieved and examined in darkness before
relocation.
Fluorescent photography.—For photographing fluorescence,
we used a Nikon P950 and Canon Rebel T5i (fitted with an 18–55
mm, 3.5–5.6 IS lens) with apertures of F/8, ISOs of 100–200, and
shutter speeds of 2–4 sec. Specimens were illuminated with two of
the aforementioned 365 nm LED torches.
Statistical analysis.—A two-tailed Fisher’s Exact Test was
used to compare the presence or absence of tail fluorescence in
taxa with potentially relevant physical or behavioral characters.
Statistical significance was assessed at p < 0.05.
results
We recorded tail fluorescence in 22 species of pitvipers
representing eight genera and 78.6% of the total number of
crotaline taxa examined in this study (Table 2). Tail fluorescence
occurred in two principal tissue types: tail scalation including the
distal tail tip and the rattle. The color of observed fluorescence
varied between species and their age groups from white to blue to
bluish-green, and to greenish-yellow. Fluorescence was observed
in both wild and captive individuals, confirming a natural
presence in pitvipers. The character was also detected in frozen,
formalin-, and alcohol-preserved specimens, although the latter
two experienced fading. Although this study focused primarily
on fluorescent excitation under 365 nm UV light, fluorescence
was also observed under longer lower energy wavelengths (395
and 405 nm) in all of the species that were examined under these
additional wavelengths (Table 2).
Tail scalation.—The proportion of the tail that fluoresced
(and intensity) varied across taxa and between age groups,
ranging from roughly the distal ⅓–½ of the tail in several species
including Agkistrodon piscivorus, A. laticinctus, Bothriechis
marchi, Crotalus aquilus, C. lepidus, C. morulus, Protobothrops
cornutus, Trimeresurus hageni and T. sumatranus (Fig. 1), to just
a few terminal scales on the distal tail tip in P. mangshanensis, P.
mucrosquamatus, P. tokarensis and Tropidolaemus wagleri (Fig. 2).
Examination of three successive years of captive-bred T.
hageni siblings and their wild-caught parents revealed a gradual
reduction in tail fluorescence over time. Fluorescence became
considerably reduced dorsally by the age of three. While some
yearling individuals still exhibited their conspicuous white juvenile
tail coloration, others had already transitioned to mostly reddish
tails more characteristic of adults; both groups still fluoresced
under UV light. A similar difference in tail fluorescence was
also observed between juvenile and adult A. piscivorus. We also
observed a noticeable difference in the degree of tail fluorescence
between the adult male and female T. hageni examined, with
fluorescence much more prominent in the lighter-colored ventral
scalation of the male than the reddish-pink scalation of the female.
Similar differences were also observed in adult T. sumatranus.
In our study, 81.3% of the species that exhibited fluorescent
tail scalation are also known to have conspicuous tail coloration
(usually as neonates and juveniles) (N = 13; p = 0.0497), excluding
species such as P. mangshanensis, P. mucrosquamatus, P. tokarensis,
P. xiangchengensis, and T. wagleri that did not have conspicuously
colored tails but rather slightly lighter colored distal tail tips (Fig.
2). Fifty percent of the species with fluorescent tail scalation are
also known to, or are suspected to perform caudal luring behavior
(N = 8; p = 0.2530; Table 2). Because it is likely that the behavior
simply has not yet been recorded in some poorly-studied species,
we expanded this cohort to include taxa that belong to genera
in which caudal luring has been documented or is suspected to
occur (Table 2). This number increased to 100% of species with
fluorescent tail scalation (N = 16; p = 0.4286). Of the thirteen
species with fluorescent tail scalation that are also known to
have conspicuous tail coloration, all belong to genera that contain
table 1. Snake taxa examined for evidence of UV-induced tail fluorescence.
Family Subfamily Taxa (N)
Boidae Boinae Corallus annulatus (9), C. caninus (2), C. hortulanus (3)
Colubridae Colubrinae Heterodon kennerlyi (1), Masticophis flagellum (2), Pituophis catenifer (1), P. ruthveni (15)
Dipsadinae Conophis lineatus (3)
Natricinae Nerodia erythrogaster transversa (1), N. fasciata confluens (1)
Elapidae Elapinae Acanthophis laevis (2), Dendroaspis angusticeps (1)
Lamprophiidae Pseudoxyrhophiinae Langaha madagascariensis (4)
Viperidae Crotalinae Agkistrodon contortrix (2), A. laticinctus (2), A. piscivorus (6), Bothriechis marchi (2),
B. lateralis (2), B. schlegelii (10), Crotalus adamanteus (1), C. aquilus (8), C. atrox (2),
C. horridus (3), C. lepidus klauberi (10), C. l. lepidus (6), C. morulus (13), C. polystictus (2),
C. ravus (2), Ophryacus smaragdinus (2), Lachesis muta (2), Mixcoatlus melanurus (3),
Protobothrops cornutus (5), P. mangshanensis (2), P. mucrosquamatus (8), P. tokarensis (3),
P. xiangchengensis (2), Sistrurus tergeminus edwardsii (2), Trimeresurus hageni (18),
T. mcgregori (6), T. sumatranus (3), T. trigonocephalus (1), Tropidolaemus wagleri (1)
Viperinae Bitis nasicornis (3), Vipera ammodytes ammodytes (5)
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table 2. Breakdown of pitvipers examined in this study with descriptions of the tail fluorescence observed. Abbreviations used: N = neonate; J = juvenile; S = subadult; A = adult; C = captive specimen; F = field specimen;
PC = preserved captive specimen; PF = preserved field specimen; KTS = known to species; ATF = accompanied tail fluorescence; KTG = known to genus; S = suspected.
Specimens examined Presence of tail Extent of tail Conspicuous Caudal
fluorescence fluorescence tail coloration luring
Genus Species N Age class Type/Origin N J S A Tail Rattle KTS ATF KITS KTG Comments
(N.J.S.A) (C.F.PC.PF) scalation
Agkistrodon contortrix 2 0.2.0.0 0.1.0.1 - Y - - Y n/a Y Y Y Y Distal 1/3–1/2 of tail scalation fluoresced strongly in
juveniles.
laticinctus 2 0.1.0.1 1.1.0.0 - Y - Y Y n/a Y Y S Y Distal 1/3–1/2 of tail scalation fluoresced strongly in
juveniles, but less intensely in adult.
piscivorus 6 1.2.1.2 2.3.0.1 Y Y Y N Y n/a Y Y Y Y Faded in juveniles and sub-adults, absent in adults
Bothriechis marchi 2 0.0.0.2 2.0.0.0 - - - Y Y n/a Y Y S Y Distal 1/3–1/2 of tail scalation fluoresced.
Fluorescence also observed under 395nm and
405 nm light.
lateralis 2 0.0.0.2 2.0.0.0 - - - N N n/a Y n/a S Y No fluorescence observed
schlegelii 10 0.0.0.10 10.0.0.0 - - - N N n/a Y n/a Y Y No fluorescence observed in adults of multiple color
phases (yellow, green, mixed)
Crotalus adamanteus 1 1.0.0.0 0.0.0.1 N - - - N - N n/a N Y No fluorescence observed
aquilus 8 5.0.0.3 8.0.0.0 Y - - Y Y Y Y Y N Y Rattle fluoresced; distal 1/3–1/2 of tail scalation
fluoresced moderately in neonates, but only dully in
adults. Fluorescence also observed under 395 and
405 nm light.
atrox 2 0.0.0.2 0.1.0.1 - - - Y N Y N n/a N Y Rattle fluoresced
horridus 3 1.0.2.0 2.1.0.0 N - Y - N Y N n/a N Y Rattle fluoresced
lepidus klauberi 10 4.0.0.6 10.0.0.0 Y - - Y Y Y Y Y Y Y Rattle fluoresced; distal 1/3–1/2 of tail scalation
fluoresced moderately in neonates, but dully in
adults. Fluorescence also observed under 395 and
405 nm light.
lepidus lepidus 6 0.3.1.2 6.0.0.0 - Y Y Y Y Y Y Y Y Y Rattle fluoresced; distal 1/3–1/2 of tail scalation
fluoresced moderately in juveniles, but dully in
adults. Fluorescence also observed under 395 and
405 nm light.
morulus 13 7.2.0.4 11.0.2.0 Y Y - Y N Y Y Y N Y Rattle fluoresced; distal 1/3–1/2 of tail scalation
fluoresced moderately in neonates, but dully in
adults. Fluorescence also observed under 395 and
405 nm light.
polystictus 2 0.0.0.2 2.0.0.0 - - - Y N Y N n/a N Y Rattle fluoresced; fluorescence also observed under
395 nm light
ravus 2 0.0.0.2 2.0.0.0. - - - Y N Y Y N N Y Rattle fluoresced
Ophryacus smaragdinus 2 2.0.0.0 0.0.2.0 Y - - - Y n/a Y Y N S Distal 1/3–1/2 of tail scalation fluoresced strongly.
Fluorescence also observed in supraoccular horns.
Lachesis muta 2 0.0.0.2 2.0.0.0 - - - N N n/a Y n/a S S No fluorescence observed.
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table 2. Continued.
Specimens examined Presence of tail Extent of tail Conspicuous Caudal
fluorescence fluorescence tail coloration luring
Genus Species N Age class Type/Origin N J S A Tail Rattle KTS ATF KITS KTG Comments
(N.J.S.A) (C.F.PC.PF) scalation
Mixcoatlus melanurus 3 0.0.0.3 3.0.0.0 - - - N N n/a N n/a N N Possible fluorescence of supraocular horns.
Protobothrops cornutus 5 1.0.0.4 4.0.1.0 Y - - Y Y n/a Y Y N Y Distal 1/3–1/2 of tail scalation fluoresced strongly in
neonate. In adults tail fluorescence was less intense
and restricted dorsally to the distal tip, and ventrally
to the last several distal scales. Fluorescence also
observed under 395 nm light.
mangshanensis 2 0.0.0.2 2.0.0.0 - - - Y Y n/a Y N Y Y Fluorescence observed in last few scales on distal tail
tip in adult male and female.
mucrosquamatus 8 8.0.0.0 5.0.3.0 Y - - - Y n/a N N N Y Intense fluorescence observed in last 1–3 scales on
distal tail tip in both sexes. Entire body fluoresced
dully. Fluorescence also observed under 395nm light.
tokarensis 3 3.0.0.0 3.0.0.0 Y - - - Y n/a N N N Y Fluorescence observed in last 1–3 scales on distal tail
tip in both sexes. Fluorescence also observed under
395 nm light.
xiangchengensis 2 0.0.0.2 0.0.2.0 - - - Y Y n/a N N N Y Fluorescence observed in last 1–3 scales on distal tail
tip in both sexes.
Sistrurus tergeminus edwardsii 2 0.1.1.0 2.0.0.0 - Y Y - Y Y Y N Y Y Entire body and rattle fluoresced. Tail scalation
fluoresced more intensely than the rest of the body
and with a slightly different hue. Fluorescence also
observed under 395 and 405 nm light.
Trimeresurus hageni 18 7.7.1.3 16.0.2.0 Y Y Y Y Y n/a Y Y N Y Distal 1/3–1/2 of tail scalation fluoresced strongly in
neonates. In adults tail fluorescence was less intense
and restricted dorsally to the distal tip, and ventrally
to the last several distal scales. Fluorescence in adults
more intense in the male than the female.
Fluorescence also observed under 395 and 405 nm
light.
mcgregori 6 0.0.0.6 6.0.0.0 - - - N N n/a N n/a N Y No fluorescence obser ved in adults of multple color
phases (yellow, white, brown, black).
sumatranus 3 0.0.0.3 2.0.1.0 - - - Y Y n/a Y N N Y Fluorescence was restricted dorsally to the distal tip,
and ventrally to the last several distal scales.
Fluorescence was more intense in the male than the
female. Fluorescence also observed under 395 and
405 nm light.
trigonocephalus 1 0.0.0.1 1.0.0.0 - - - N N n/a N n/a N Y No fluorescence obser ved.
Tropidolaemus wagleri 1 0.0.0.1 0.0.1.0 - - - Y Y n/a Y N S Y Fluorescence restricted to extreme distal tail tip.
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Fig. 1. Tail fluorescence in: A) a live juvenile Trimeresurus hageni; B) a preser ved neonatal Protobothrops cornutus; and C) a live neonatal
Agkistrodon piscivorus.
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confirmed or suspected caudal luring species (p = 1.0000; Table 2).
In addition to tail fluorescence, P. mucrosquamatus and S.
tergeminus edwardsii appeared to exhibit blue fluorescence
of the body scalation which was not seen in congeners.
Additionally, although closer examination is needed, the horn-
like projections over the eyes of O. smaragdinus and possibly
Mixcoatlus melanurus also appeared to fluoresce. Although we
did not observe fluorescence in the tails of polymorphic adult
B. schlegelii, for which several naturally-occurring color phases
were examined, a small piece of retained shed skin on the
distal tail tip of a yellow adult B. schlegelii following ecdysis did
fluoresce a bluish color under 365 nm light.
Rattlesnake rattles.—Fluorescence was observed in the
entirety of all rattle segments for all rattlesnake species examined
(Crotalus and Sistrurus; Fig. 3) except C. adamanteus, for which
only a preserved pre-button neonatal specimen was available.
Also, fluorescence was observed in the buttons of captive-born
C. aquilus, C. lepidus klauberi, C. l. lepidus, and C. morulus which
were added following their first shed ca. 7–15 days after birth.
Although not examined in this study or included in our analyses,
the rattle segments of wild S. miliarius barbouri have also been
reported to fluoresce (S. Sweet, pers. comm.).
With 62.5% of the rattlesnake species examined in this study
known to feature both rattle fluorescence and conspicuously
colored tail scalation, there was no apparent association between
these characters (p = 1.000; Table 2). Additionally, with only
two of the rattlesnake species examined in this study known to
perform vermiform caudal luring (C. lepidus and S. tergeminus),
there was no apparent relationship between rattle fluorescence
and vermiform caudal luring (p = 1.000; Table 2).
Fig. 2. Fluorescence of the distal tail tips of A) live neonate Protobothrops mucrosquamatus, B) live neonate P. tokarensis, C) live adult
P. mangshanensis.
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Fig. 3. Ultraviolet-induced fluorescence in the rattles of: A) Sistrurus tergeminus edwardsii; B) Crotalus horridus; C) C. morulus; and D)
C. polystictus.
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228 ARTICLES
Additional snake taxa.— In non-crotaline species, we
observed UV-induced visible fluorescence in the extreme distal
tail tips of Langaha madagascariensis (N = 3) and Acanthophis
laevis (N = 2) (Fig. 4), which was similar in appearance to the tail
fluorescence observed in several species of Protobothrops (Fig.
2) and T. wagleri.
discussion
Our discovery of tail fluorescence across eight genera of
pitvipers represents an important further case of widespread
biofluorescence in non-avian reptiles (Prötzel et al. 2018; Eipper
et al. 2020), and the only known examples in tetrapods where
biofluorescence is localized to a specific appendage. Unlike
other reptile taxa which display fluorescence as part of their
general body coloration (Gruber and Sparks 2015; Seiko and Terai
2019; Eto 2020; Prötzel et al. 2021), skeletal elements (Sloggett
2018; Top et al. 2020) or bony cranial protuberances (Prötzel et
al. 2018), the restriction of fluorescence to the tails of various
pitvipers suggests a close association with a specialized behavior
or function of the appendage. We offer several hypotheses
for the ecological relevance and function of this remarkable
phenomenon.
Tail fluorescence in pitvipers.—Aggressive (luring) mimicry
has evolved in many different organisms as a way to manipulate
the behavior of other species, particularly prey (Jackson and
Cross 2013). Caudal luring, the use of the tail as a deceptive lure
to attract prey, is a specialized hunting technique employed by
many snake species across at least seven families (Neill 1960;
Heatwole and Davison 1976; Murphy et al. 1978; Radcliffe et al.
1980; Leal and Thomas 1994; Sazima and Puorto 1993; Antunes
and Haddad 2009; Sheehy 2016). In caudal luring pitvipers,
including rattlesnakes (Schuett et al. 1984; Reiserer and Schuett
2016), the tail is usually conspicuously colored relative to the rest
of the body, especially in neonates and juveniles, and undulated
in a manner that mimics the writhing movements of a worm or
vermiform insect larva (Neill 1960; Greene and Campbell 1972;
Martins et al. 2002; Reiserer and Schuett 2008). Since most of the
species observed with fluorescent tail scalation in the present
study are known to perform caudal luring or belong to genera
in which the behavior has been documented or is suspected to
occur, we suspect that tail fluorescence facilitates or enhances
caudal luring in these species by increasing the conspicuousness
and visual attractiveness of the lure to certain prey species
under certain light conditions. In many cases tail fluorescence
co-occurred with conspicuous tail coloration, a key adaptation
associated with caudal luring in pitvipers and other snake taxa
(Neill 1960; Green and Campbell 1972; Heatwole and Davison
1976; Martins et al. 2002), further supporting our hypothesis.
Since biofluorescence is dependent upon excitation
by external light sources, fluorescent tissues may only be
detectable in certain light environments. At present, there
is little understanding of how species visualize fluorescence
especially in terrestrial environments, but perceptibility is likely
to vary based on species-specific visual sensitivities as well as
the ambient wavelengths present. Ultraviolet radiation reaches
its peak intensity during midday in open environments (e.g.,
Buntoung et al. 2012), but fluorescence may not be perceptible
or as conspicuous under such conditions due to interference
from other wavelengths present, particularly visible light.
Fig. 4. Fluorescence of the extreme distal tail tips in: A) preserved adult Langaha madagascariensis; and B) live adult Acantophis laevis.
PHOTOS BY LAURENCE PAUL
Herpetological Review 52(2), 2021
ARTICLES 229
Instead, fluorescence may be better visualized in low visible
light conditions such as heavily shaded forest environments
or between dusk and dawn when the overall proportion of
shorter-wavelength light present including UV radiation is
much greater (Johnsen et al. 2006). For species with exceptional
visual sensitivities in low light environments, detectability of
fluorescent tissues under such conditions may require very
minimal excitation from UV or other low wavelengths (e.g.,
Kohler et al. 2019), and likely far less intensity than what was
generated by the LED torches used in this study.
In snakes, vermiform caudal luring primarily targets anuran
and lizard prey (Neill 1960; Heatwole and Davison 1976), which
both factor prominently into the natural diets of many pitviper
species (Orlov et al. 2002a,b; Campbell and Lamar 2004; Martins
et al. 2012). Studies on the foraging abilities of anurans (Larsen
and Pedersen 1982; Aho et al. 1993; Buchanan 1998) and visual
acuities of lizards (Roth and Kelber 2004; Fleishman et al. 2011)
under different light conditions have shown that at least some
species from these groups have exceptional visual sensitivities
in low light environments that are many times greater than that
of the human eye. Moreover, the discovery of biofluorescence
in various anuran and lizard taxa which is suspected to aid
intraspecific signaling or identification (Taboada et al. 2017a,b;
Prötzel et al. 2018, 2021; Sloggett 2018; Goutte et al. 2019; Top et
al. 2020; Whitcher 2020) suggests that at least some species within
these groups are capable of visualizing fluorescent tissues and
that biofluorescence already plays an active role in their visual
and behavioral ecologies. Many terrestrial invertebrate groups
preyed upon by lizards and anurans also include biofluorescent
species including, but not limited to larval lepidopterans
(Messenger et al. 2019; Moskowitz 2018; Sourakov 2017, 2019),
which could serve as a mimicry model for vermiform caudal
luring in some tail-fluorescent pitvipers (Fig. 5).
It appears that at least some of the fluorescent pitvipers
documented in this study experience reductions in tail scalation
fluorescence over time. We have observed this transition in T.
hageni and several species of rattlesnake (C. aquilus, C. lepidus, C.
morulus) in captivity and suspect that other fluorescent pitvipers
follow a similar trend. Here, the reduction in fluorescence appears
to parallel the loss of conspicuous tail color in various pitviper
species which has been linked to ontogenetic shifts in diet (Neill
1960; Heatwole and Davison 1976). Although our limited sample
sizes preclude statistical analysis, we also observed differences in
fluorescent emission patterns between adult male and female T.
hageni and T. sumatranus, with males displaying a greater degree
of tail fluorescence than females. If tail fluorescence in these
species is linked to caudal luring, such differences in emission
patterns could be reflective of intraspecific differences in dietary
preferences or foraging habits. While dietary studies for T. hageni
and T. sumatranus appear to be lacking, several other pitviper
taxa have been shown to exhibit intraspecific sexual variation in
diet (Daltry et al. 1998; Vincent et al. 2004; Lin and Tu 2008). A
greater sampling of adult males and females in these species will
be needed to confirm whether these differences are inherently
dimorphic.
Several species of rattlesnake (Crotalus, Sistrurus) are also
known to use vermiform caudal luring to attract ectothermic
prey such as lizards and frogs (Kauffeld 1943; Jackson and Martin
1980; Schuett et al. 1984; Starrett and Holycross 2000; Reiserer
and Schuett 2008, 2016). However, with the possible exceptions
of S. miliarius (Jackson and Martin 1980) and C. cerastes (Reiserer
and Schuett 2008; Clark et al. 2016), this behavior appears to be
mostly limited to neonates and juveniles with very limited or no
rattle segmentation, and is rarely observed in larger specimens
with fully-developed rattles. Unlike the prehensile tail in other
caudal-luring pitvipers, rattle segments are autonomous from
the tail musculature and therefore may have reduced capacity
for undulation in the same vermiform manner. Moreover, since
the natural diets of many rattlesnake species tend to shift away
from ectothermic to endothermic prey with ontogeny (Campbell
and Lamar 2004; Ernst and Ernst 2012; Schuett et al. 2016a), there
may be less of a need for a vermiform lure in larger individuals
and species. Thus, even though the structure itself does bear
physical resemblance to segmented invertebrates (see Reiserer
and Schuett 2016), some of which may even biofluoresce,
vermiform caudal luring might not account for fluorescence of
the rattle. Instead, fluorescence of the structure, as first noted
by Klauber (1956), might serve a slightly different deceptive
function.
Rattlesnakes are often associated with ecosystems that
support grass communities (Campbell and Lamar 2004; Ernst
and Ernst 2012; Schuett et al. 2016a), and grasslands may have
played a key role in the origin and diversification of rattlesnakes
(see Reiserer and Schuett 2016). In addition to providing refuge
and other resources that may be utilized by rattlesnakes, grasses
are also important for sustaining small mammal communities,
particularly granivorous rodents that are heavily dependent on
harvesting and caching their seeds throughout the year (e.g.,
Monson 1943; Chapman 1972; Brown and Heske 1990; Hesk
et al. 1993, 1994; Longland 1994). Through our observations
of tail fluorescence in captive rattlesnakes, we noticed that
the stems and spikelets of grasses planted in their terrariums
(Chasmanthium latifolium, Bromus catharticus [Poaceae;
Pooideae]) also fluoresced at a similar color and intensity as
the snakes’ rattles under the same 365 and 395 nm UV torches
(Fig. 6), which corroborates Baby et al.’s (2013) discovery of
widespread UV-induced fluorescence in the reproductive
structures of several subfamilies of grasses with a peak excitation
wavelength of 366 nm. We also could not help but notice the
resemblance of the rattle in terms of its general morphology and
appearance to the paired spikelets of various pooid grass genera
(e.g., Chasmanthium, Eragrostis, Glyceria, Uniola, Hordeum,
Fig. 5. UV-fluorescent lepidopteran larvae may serve as a model for the fluorescent vermiform caudal lure in some pitvipers. Examples
of biofluorescent North American taxa: A) Hemaris thysbe; B) H. diffinis; and C) Manduca sexta.
PHOTOS COURTESY OF DAVID MOSKOWITZ
Herpetological Review 52(2), 2021
230 ARTICLES
Bromus, Distichilis), especially under UV light (Fig. 6); such
morphological similarities have apparently long been recognized
by botanists and reflected in the vernacular names assigned to
several pooid species distributed throughout the world (e.g.,
Rattlesnake Grass [Briza maxima], Rattlesnake Mannagrass
[Glyceria canadensis], Rattlesnake Brome [Bromus briziformis]).
Given these considerations and since granivorous rodents
comprise an important dietary component of many rattlesnakes
(Campbell and Lamar 2004; Reiserer et al. 2018; Ernst and Ernst
2012; Schuett et al. 2016a), we consider the possibility that the
fluorescent rattle may function as a deceptive lure that mimics
the biofluorescent spikelets of grasses and possibly the seeds of
other biofluorescent plants to attract seed-harvesting rodents.
Biofluorescence is well known in plants (Buschmann et
al. 2000), but its apparent usage by grasses as a visual signal
for insect pollinators and possibly seed predators represents a
newly recognized phenomenon (Baby et al. 2013). Although it
has not yet been tested experimentally, with retinal receptors
that are highly sensitive in low-light environments (Jacobs et al.
1991, 2001), rodents are suspected of visualizing and exploiting
Fig. 6. A) Comparison of fluorescent Chasmanthium latifolium spikelet (left) to the rattles of C. atrox (center) and C. lepidus klauberi
(right) under room lighting and UV light; B) fluorescent Bromus catharticus spikelets under UV light, with a C. lepidus klauberi rattle
(arrow) mounted on a stem to demonstrate similarities in fluorescence and general appearance under UV light. Although neither grass
species depicted here is indigenous to the New World, they illustrate a general spikelet morphology that is conserved across many
pooid genera naturally distributed throughout the world including the Americas; biofluorescence is also widely distributed in grasses
(Baby et al. 2013).
PHOTOS BY LAURENCE PAUL
Herpetological Review 52(2), 2021
ARTICLES 231
biofluorescent grasses to locate their seeds and flowers (Baby et
al. 2013). If this is true, rattlesnakes, in turn, could be exploiting a
foraging tactic of rodents by mimicking a biofluorescent dietary
staple. Such deceptive luring could complement the predatory
repertoires of at least some rattlesnake species that are known to
take up ambush positions along small mammal runways or near
rodent dens (e.g., Tevis 1943; Fitch and Twining 1946; Hennessy
and Owings 1988; Duvall et al. 1990; Reinert et al. 1984, 2011;
Dugan and Hayes 2012), and might also explain why rattlesnakes
are sometimes observed at rest or in ambush with their rattles
exposed and positioned close to the head (Fig. 7; Rabatsky and
Farrell 1996; Theodoratus and Chiszar 2000).
We consider deceptive prey luring to be the most likely
explanations for tail fluorescence in pitvipers, but we recognize
that fluorescence could serve other functions in these species
that may not necessarily be mutually exclusive. For instance,
since various antipredator displays are known to snakes
including those that involve specialized tail movements
and posturing (e.g., Greene 1973; Kochva and Golani 1993;
Rabatsky and Waterman 2005; Melvinselvan and Nibedita 2016),
fluorescence could increase the conspicuousness of the tail as
a visual display to distract or deter attackers. Rattlesnakes are
best known for their use of the rattle in producing an audible
warning signal, but it is unclear whether the shaking rattle
itself, which is typically raised above the body and made visible
to the attacker, might also serve as a visual aposematic display.
Here, in a similar light to Vogel’s (1964) presumption that the
contrasting black and white tail banding of C. atrox enhances
the visibility of the species’ rattling threat display, fluorescence
could enhance the conspicuousness of the vibrating rattle as
a visual aposematic display in low-light environments (Fig. 8),
possibly to crepuscular or nocturnal mammalian predators such
as canids, felids, mustelids, or procyonids.
Many other pitvipers including members of Agkistrodon,
Bothrops, Lachesis, Protobothrops, and Trimeresurus also exhibit
defensive tail shaking or vibrating behavior when threatened or
distressed (Greene and Campbell 1972; Greene 1973; Campbell
and Lamar 2004; Mendyk and Paul, unpubl.). However, unlike
rattlesnakes, the tail in most of these species is not elevated
above the body to increase its conspicuousness or visibility to
the attacker. Instead, the tail is usually kept low and pressed
against the substrate (Sisk and Jackson 1997) and used to rustle
leaf litter or vegetation, where it probably would not benefit from
increased visibility via fluorescence. Additionally, since some
species do not appear to retain tail fluorescence into adulthood
or experience marked ontogenetic reductions in its intensity
yet still perform defensive tail shaking or vibrating as adults, a
defensive role may not account for this character in some taxa.
For some reptiles including chameleons (Prötzel et al.
2018), geckos (Sloggett 2018; Top et al. 2020; Prötzel et al.
2021) and anoles (Mendyk, in review), biofluorescence may
play a role in intraspecific communication by highlighting or
accentuating certain body markings, skeletal elements or bony
protuberances. While snakes rely heavily on chemical cues for
intraspecific communication (Madison 1977; Ford 1986; Mason
1992; Mason and Parker 2010), there is increasing evidence that
visual displays play an important role in their sociality as well
(Carpenter 1977; Putman and Clark 2015; Schuett et al. 2016b).
Such communicative displays can include tail signaling, which
has been recorded in adults of at least two New World pitvipers
shown to exhibit tail fluorescence in the present study—A.
contortrix (Schuett 1997) and C. atrox (Schuett et al. 2016b)—
as well as in C. oreganus (Putman and Clark 2015). In species
with communicative tail signaling, fluorescence could enhance
the visibility of the signal to conspecifics. Yet again, while this
could potentially account for tail fluorescence in rattlesnakes
and other pitviper species that display the character as adults,
we are less inclined to consider this as a possible explanation for
species that do not appear to fluoresce as adults or experience
marked reductions in fluorescence over time.
Phylogenetic and evolutionary implications.—Is tail
fluorescence plesiomorphic to Crotalinae, or did it arise
independently in multiple genera like other shared characters of
the group (e.g., Sanders et al. 2004)? Although our preliminary
sampling of pitviper taxa is incomplete, our data clearly show
tail fluorescence to be phylogenetically widespread within
Crotalinae (Fig. 9). If tail fluorescence tends to co-occur with
conspicuous tail coloration and caudal luring in pitvipers as
has largely been the case in this study, all pitviper genera could
potentially harbor species with this trait (Fig. 9). Clearly, a more
Fig. 7. Rattlesnakes resting with their rattles positioned close to the head; A) Crotalus aquilus under ambient room lighting; B) C. lepidus
klauberi under UV light.
PHOTOGRAPHS BY LAURENCE PAUL.
Herpetological Review 52(2), 2021
232 ARTICLES
robust sampling of taxa, sexes and age classes is needed to
determine the full extent of tail fluorescence within individual
species as well as across Crotalinae.
Our observations of fluorescence in the extreme distal
tail tips of Langaha madagascariensis (Lamprophiidae) and
Acanthophis laevis (Elapidae) confirm that this character has
also arisen outside Crotalinae. Since there appears to be an
association between tail fluorescence and both caudal luring
and conspicuous tail coloration in pitvipers, it will be useful to
determine if tail fluorescence occurs in additional taxonomic
groups that also possess these characters, such as true vipers
(Henderson 1970; Heatwole and Davison 1976; Parellada and
Santos 2002; Reiserer 2002), boids (Radcliffe et al. 1980), colubrids
(Leal and Thomas 1994; Tiebout 1997; but see Reiserer and
Schuett [2016] for a refutation of the latter), dipsadids (Sazima
and Puorto 1993; Stender-Oliveira et al. 2016), elapids (Neill
1960; Carpenter et al. 1978; Khan and Tasnim 1986a; Chiszar
et al. 1990; Hagman et al. 2008), pseudoxyrhophiids (Sheehy
2016), pythonids (Murphy et al. 1978; Whittaker and Shine 1999;
McFadden 2005) and tropidophiids (Antunes and Haddad 2009).
The origin of the crotaline rattle and the circumstances
of its evolutionary development have been long-debated
topics in herpetology with many hypotheses proposed over
the last century (reviewed by Reiserer and Schuett 2016). Tail
fluorescence adds vital new information to this discussion,
lending further support to Schuett et al.’s (1984) hypothesis of
a caudal luring origin for the rattle. Since rattlesnakes represent
a more recently derived lineage within Crotalinae (Wüster et
al. 2008; Alencar et al. 2016), tail fluorescence, which appears
to be plesiomorphic to the monophyletic clade encompassing
Crotalus, Sistrurus, and Agkistrodon, would have originated
prior to the rattle (Fig. 9). If tail fluorescence is closely associated
with caudal luring, this would suggest that the long-perceived
primary role of the rattle as an audible aposematic alarm may
have evolved secondarily to its role as a biofluorescent lure (see
Reiserer and Schuett 2016). Such a scenario would account for
incipient stages in the evolutionary development of the rattle
from a “normal” biofluorescent tail tip used for vermiform
caudal luring, such as that seen in the sister group Agkistrodon,
to a possible fluorescent proto-rattle also aimed at vermiform
caudal luring, to the fluorescent present day rattle, which does
not appear to be used in such a capacity but may serve a slightly
different deceptive role.
Implications for future studies.—Tail fluorescence in pitvipers
raises many new questions about the biology of this group and
opens up various avenues for future research. Since it is apparent
that tail fluorescence occurs over a range of wavelengths and
because we were unable to collect spectral data, studies that
determine the optimal excitation and emission wavelengths of
tail fluorescence across these taxa (e.g., Prötzel et al. 2018, 2021)
will be crucial for pairing this phenomenon with specific light
environments where fluorescence would be best visualized
as well as the species that may be best suited for visualizing
it. Information gained through spectral analyses will also be
important for developing behavioral studies aimed at testing the
hypotheses presented in this study.
Much remains to be learned about the visual systems of
reptiles and amphibians, and little is presently known about how
visual sensitivities to certain wavelengths may influence aspects
of their behavioral ecology or how different light environments
may have helped shape the evolution of key physical features
such as conspicuous tail coloration and biofluorescence, or
behavioral innovations such as caudal luring. While some
studies have looked into the effects of light intensity on foraging
behavior in snakes including caudal luring specifically (Neill
1960; Rabatsky and Farrell 1996; Chiszar et al. 1990), none appear
to have focused on specific wavelengths, nor has biofluorescence
been considered a potential influencing factor. Neill (1960)
may have unknowingly hinted a potential connection between
UV-induced biofluorescence and caudal luring in juvenile A.
contortrix, reporting that luring behavior under artificial room
lighting did not occur until the animals were exposed to low
Fig. 8. Fluorescence of the rattle (A) and defensive rattling display of Crotalus lepidus klauberi (B).
PHOTOS BY LAURENCE PAUL
Herpetological Review 52(2), 2021
ARTICLES 233
levels of natural light filtered through a window. This raises
additional questions relating to whether the snakes themselves
are cognizant of, or capable of visualizing their own fluorescence
under certain light conditions and whether this may influence
performance of caudal luring behavior.
Histologically, there appear to be many interesting aspects
of tail fluorescence in pitvipers that warrant further study. For
instance, despite having a uniform appearance under visible
light, there are clearly structural differences between scales on
the tail that fluoresce and more proximal neighboring scales that
do not. Additionally, it is unclear how these fluorescent tissues
might change structurally over time, what mechanisms may be
responsible for reductions in fluorescence, and whether this
coincides with the ontogenetic loss of conspicuous tail coloration
observed in various pitviper species (e.g., Neill 1960; Heatwole
and Davison 1976). It will also be important to compare the
structural and biochemical basis for fluorescence in the crotaline
rattle to that of fluorescent tail scalation in pitvipers to shed light
on the relationship between these tissues and the evolution of tail
fluorescence in the group.
Most studies on fluorescence in reptiles to date have relied on 365
or 395 nm UV torches for exciting, visualizing and photographing
biofluorescent tissues (Prötzel et al. 2018, 2021; Sloggett 2018;
Seiko and Terai 2019; Eipper et al. 2020; Eto 2020; Top et al. 2020).
One drawback to these torches is that they inevitably emit some
residual visible light that could interfere with or obscure detection
of more subtle biofluorescent tissues, especially when examined
on semi-reflective or lighter-colored backgrounds, in faded fluid-
preserved specimens (e.g., Eipper et al. 2020), or when attempting
to examine active, uncooperative specimens. Therefore, our
results should be considered preliminary, and the confirmation
of, but not necessarily the absence of, tail fluorescence from the
species examined. Additionally, due to the potential effects of
this residual light, the color of fluorescence observed with these
torches may not necessarily reflect the true colors visualized by
species in nature. Studies are clearly needed to determine if and
how fluorescence may be visualized by reptiles and amphibians
under natural conditions; here, the important work by Gruber
et al. (2016) on biofluorescence in marine elasmobranchs could
serve as a useful model for developing similar approaches for
herpetofauna.
Finally, our findings, together with other recent discoveries
of biofluorescence in reptiles (Gruber and Sparks 2015; Prötzel
et al. 2018, 2021; Sloggett 2018; Seiko and Terai 2019; Eipper et
al. 2020; Eto 2020) and other tetrapods (e.g., Nowogrodzki 2017;
Taboada et al. 2017a,b; Camacho et al. 2019; Jeng 2019; Kohler et
al. 2019; Wilkinson et al. 2019; Anich et al. 2020) call attention to
the limitations of our own sensory modalities when studying and
interpreting the ecology, behavior and functional morphology of
other species (e.g., Martin 2012). The fact that tail fluorescence
has gone largely undetected for so long in pitvipers including
rattlesnakes, a group that has been intensively kept and studied
in captivity over two centuries (Bennett 1829; Harlan 1830;
Mitchell 1860; Murphy 2017), raises an important question: what
other key biological attributes of species may we be missing due
to our visual biases? For instance, UV-reflectance, which like
biofluorescence also falls outside of the human-visible spectrum
and utilizes wavelengths in the UV range, was only recently shown
to play an important role in the behavioral ecology of reptiles,
namely lizards (e.g., Font et al. 2009; Bajer et al. 2011; Abramjan
et al. 2020). Furthermore, given that both high UV-reflectance and
biofluorescence have been described in various vertebrates and
invertebrates and closely interact with one another (e.g., Lim et al.
2007; Barreira et al. 2012; Finkbeiner et al. 2017), and considering
the widespread fluorescence described in this study, it is likely that
some snake species possess both of these traditionally-overlooked
colorations. In the case of pitvipers, it appears that interpreting
caudal luring behavior and the function of the crotaline rattle
solely through the lens of our human vision may have contributed
to an incomplete understanding of these specialized adaptations.
Placing greater emphasis on the roles that different wavelengths
play in the ecology and behavior of species will be especially
important as further examples of biofluorescence inevitably
continue to be discovered and described.
Fig. 9. Preliminary generic-level distribution of tail fluorescence in
Crotalinae from the current study based on phylogenetic relation-
ships inferred from molecular data (Alencar et al. 2016) and recent
taxonomic revisions by Campbell et al. (2019). Tail coloration and
caudal luring data were derived from multiple published sources
(Ditmars 1907; Steiner 1907; Henry 1925; Smith 1943; Neill 1948; Al-
len 1949; Burger and Smith 1950; Wharton 1960; Antonio 1980; Mur-
phy and Mitchell 1984; Schuett 1984; Tryon 1985; Khan and Tasnim
1986b; Sazima 1991; Strimple 1995; Andrade et al. 1996, 2010; Daltry
et al. 1998; Whitaker and Captain 2004; Farrell et al. 2011; Freitas and
Silva 2011; Martins et al. 2012 [and references therein]; McCleary et
al. 2015; Owens 2016; Barnes and Knierim 2019; da Fonseca et al.
2019; de Plecker and Dwyer 2020; Zhang et al. 2020) and unpublished
data of the authors. Solid line and dashed branches distinguish Old
and New World taxa, respectively. Blue stars denote confirmed tail
fluorescence from this study; solid black stars represent a confirmed
character in the genus; outlined stars denote the suspected presence
of a character in the genus.
Herpetological Review 52(2), 2021
234 ARTICLES
Acknowledgments.—We thank Samuel Sweet for collecting and
sharing useful field data and observations with us, and Clinton Szy-
manski for valuable insight on the subject of caudal luring. Bob
Thomas loaned specimens, Craig Gagne provided access to addi-
tional specimens, and Michelle Hatwood, Richard Dunn, Adrienne
Atkins, Christopher St. Romain, and Karen Ross assisted with secur-
ing field specimens. Melanie Litton, Will Fullerton, and Adam Weisse
provided husbandry support. Wolfgang Wüster, Bree Putman, and the
Smithsonian Institution Libraries provided useful literature and David
Moskowitz generously contributed photographs. Lastly, we thank the
Audubon Nature Institute and Smithsonian National Zoological Park
for institutional support (RWM), and Gordon Schuett, Joe Mendelson,
and Jadyn Sethna for helpful comments on this manuscript.
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© 2021 by Society for the Study of Amphibians and Reptiles
Terrestrial Microhabitats of Plethodontid Salamanders
in the Southern Blue Ridge Mountains
The streams and forests of the southern Blue Ridge Mountains
(USA) support a diverse fauna of stream-dwelling, streamside,
and woodland species of lungless salamanders (Plethodontidae)
(Beane et al. 2010). In the warmer months terrestrial forest-
floor ecosystems are enriched by an influx of streamside species
which co-occur with resident woodland species (Hairston
1981; Petranka and Smith 2005). During daylight hours these
salamanders remain concealed beneath cover objects on the
forest floor, to emerge after dark to forage (e.g., Ash 2020; Hocking
et al. 2021). Cover objects represent an important resource for
salamanders (and other small animals), providing moisture and
a source of invertebrate prey, as well as a refuge from predators
(Caruso 2016). A principal category of cover object is referred
to as down woody debris (DWD), which includes round wood
(trunks and limbs of trees and shrubs), as well as bark and other
woody material, all in contact with the ground and in varying
states of decay. DWD is an important component of biomass
in temperate North American forests (Woodall et al. 2013), and
supports a diverse animal community (Stokland et al. 2012).
Other cover objects include rocks, leaf litter, and moss.
In a study in the Great Smoky and Balsam Mountains, Caruso
(2016) surveyed cover object use by plethodontid salamanders
on two 3 × 50 m forest plots at each of 40 sites. The categories
of cover objects were logs, bark piles, and rocks. Caruso (2016)
observed 624 individual salamanders in 4 genera and 11 species.
Salamanders showed a preference for larger woody retreats,
mainly logs, which Caruso (2016) ascribed to the higher moisture
content and greater prey abundance of such cover objects.
Among four species of Desmognathus the tiny D. wrighti used
smaller retreats than the larger D. santeetlah, D. ocoee, and D.
imitator.
Rossell et al. (2018) examined cover object use by Northern
and Southern Pygmy Salamanders (D. organi, D. wrighti) at 73
sites in the southern Blue Ridge of North Carolina. They recorded
size and type of cover object used by these salamanders, as well
as a number of environmental variables. Both species preferred
woody cover objects rather than rocks, and apparently selected
RICHARD C. BRUCE
Department of Biology, Western Carolina University,
Cullowhee, North Carolina 28723, USA
Current address: 50 Wagon Trail, Black Mountain, North Carolina 28711, USA
e-mail: ebruce1563@aol.com
