White-tailed Deer Signpost Photoluminescence

Ecology and Evolution, 2025; 15:e72618 1 of 10
https://doi.org/10.1002/ece3.72618
Ecology and Evolution
RESEARCH ARTICLE OPEN ACCESS
White-tailed Deer Signpost Photoluminescence
Daniel R. DeRose-Broeckert1 | Billy R. Hammond2 | Steven B. Castleberry1 | Gino J. D'Angelo1
1Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia, USA | 2Vision Sciences Laboratory, Department of
Psychology, University of Georgia, Athens, Georgia, USA
Correspondence: Daniel R. DeRose-Broeckert (dd13873@uga.edu)
Received: 7 August 2025 | Revised: 12 November 2025 | Accepted: 24 November 2025
Keywords: function | photoluminescence | signpost | ultraviolet | vision | white-tailed deer
ABSTRACT
Ultraviolet (UV) induced photoluminescence is widespread in Mammalia; however, its function(s) remain unclear. Most of the
research to date has focused on the surface expression of photoluminescence (e.g., pelage), described qualitatively. Here, we report a quantitative assessment of photoluminescence of white-tailed deer (Odocoileus virginianus, herein deer) used for marking
signposts. We analyzed 146 signposts, including 109 antler rubs on trees and 37 scent-marking scrapes. We compared the spectra
of signposts to the spectra of surrounding environmental features elicited by exposure to excitation lights peaking at 365 and
395nm. Signposts showed significant contrast when compared to environmental backgrounds (p<0.001), and the resulting photoluminescence would be visible to deer based on previously described deer visual capabilities. This research is the first quantitative description of the functional use of environmental photoluminescence by a mammal and gives new perspective about how
white-tailed deer perceive their environment and communicate.
1   |   Introduction
White-tailed deer (Odocoileus virginianus, herein, deer) are a
widespread, well-studied, and intensively managed wild ungulate native to the Americas (Hewitt  2015). Many specifics are
unknown, however, regarding how deer perceive their very
complex environments (Newman et al. 2023). Deer rely primarily on hearing and olfaction to function (Müller-Schwarze 1994),
but vision is an essential complement to these primary senses
(VerCauteren and Pipas 2003).
The study of UV-induced photoluminescence in Mammalia has
been ongoing for over a century (Reinhold et al. 2023; Travouillon
et  al.  2023). Photoluminescence results from photons contacting an organic object and causing a change in energy levels of
lumiphores (e.g., porphyrins), resulting in the emission of light
at longer wavelengths as electrons return to a ground state
(Reinhold et al. 2023). To date, most research and hypotheses
related to mammalian photoluminescence have focused on the
presence of photoluminescence on the animal itself (anatomical
photoluminescence) (Sobral and Souza-Gudinho 2022; Toussaint
et al. 2023; Travouillon et al. 2023). Little research has examined
environmental photoluminescence, though theories related to
anatomical photoluminescence as a form of visual camouflage
within the environment have been proposed (Kohler et al. 2019).
Marshall and Johnsen (2017) proposed five criteria when assigning
biological function to photoluminescence: (1) wavelengths causing photoluminescence occur naturally, (2) emitted color contrasts
typical backgrounds, (3) the photoluminescent object is in a visible location, (4) the intended observer has spectral sensitivity that
encompasses the range of photoluminescence, and (5) a correlation exists between photoluminescence and the animal's behavior.
Several explanations have been proposed regarding the biological
function of photoluminescence in Mammalia and sensitivity to
UV light, including predator evasion through Batesian mimicry or
camouflage (Kohler et al. 2019; Anich et al. 2021; Pynne et al. 2021),
visual enhancement in low light (Kohler et al. 2019), conspecific
communication (Pynne et  al.  2021; Cronin and Bok  2016), and
enhanced foraging, especially in snowy conditions (Cronin and
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
© 2025 The Author(s). Ecology and Evolution published by British Ecological Society and John Wiley & Sons Ltd.
2 of 10 Ecology and Evolution, 2025
Bok  2016). Although these hypotheses were often supported by
data, to date, no case has been documented in which all criteria
proposed by Marshall and Johnsen (2017) were met.
Deer use chemicals for olfactory communication by depositing scent at specific locations within the environment known
as signposts (i.e., rubs, scrapes). Signposts facilitate both visual and olfactory communication between deer (Moore and
Marchinton  1974). Scents on signposts include glandular secretions, urine, and feces (Gassett et  al.  1996, 1997; Osborn
et  al.  2000). Male deer make rubs on vegetation by vigorously
raking their antlers, removing outer bark layers (Atkeson and
Marchinton  1982). Rubbing is seasonal and serves to remove
antler velvet, and for adult males to advertise their presence.
Rubbing is concurrent with increased sudoriferous gland activity as plasma androgen concentrations (i.e., testosterone) rise
(Mirarchi et al. 1977; Kile and Marchinton 1977). Scrapes consist
of a licking branch 1–2m above ground and a depression pawed
in the soil beneath the licking branch (Miller et al. 1991). Deer urinate onto their tarsal glands and onto the overturned soil (Moore
and Marchinton 1974). Scent left on the licking branch is thought
to originate from forehead, preorbital, and nasal gland secretions
and saliva (Miller et  al.  1991). Deer chew the licking branch,
exposing inner wood layers. The olfactory function of signposts
for conspecific communication has been investigated (Mirarchi
et  al. 1977; Hirth 1977; Atkeson and Marchinton 1982; Gassett
et al. 1996), and the basic visual components of signposts have
been described (Kile and Marchinton 1977). No study, however,
has incorporated knowledge of deer vision relative to signposts.
Deer vision is adapted for heightened sensitivity in lowlight conditions (Newman et al. 2023), with short-wavelength (SWS) cones
maximally sensitive to 450–460nm and middle-wavelength
(MWS) cones sensitive to 537nm (Jacobs et al. 1994). The optical lens of deer lacks substantial pigments capable of filtering
UV light (D'Angelo et  al. 2008), and deer have been hypothesized to be visually sensitive to short wavelengths that predominate during crepuscular hours (Kieffer and Stone 2005; Cohen
et al. 2014). Deer forehead glands produce various phenols and
terpenes (Gassett et al. 1997), and similar compounds have been
shown to photoluminesce (Aleixandre-Tudo and du Toit 2019;
Lee et al. 2013). Terpenes are produced by plants, and the cambium layer of some trees photoluminesce when exposed to UV
light (Meier 2015). Lastly, deer urine contains porphyrins (Lim
and Peters 1984; Neves and Galván 2020) and amino acids, both
of which exhibit photoluminescence (Frohlich 2020). Therefore,
we hypothesized that (1) UV light would increase the visibility
of the signposts and (2) rubbed trees, urine, scraped earth, and
licking branches would exhibit photoluminescence when exposed to UV light. To investigate the potential photoluminescence of deer signposts, we aimed to quantitatively measure the
spectra of signposts exposed to UV wavelengths.
2   |   Methods
2.1   |   Study Site and Search Description
The study was conducted in Whitehall Forest in Athens-Clarke
County, Georgia, USA (33.89686, −83.36222). Whitehall Forest
is a 337-ha research forest located in the northeastern portion
of the Georgia Piedmont physiographic region and is owned
and managed by the Daniel B. Warnell School of Forestry and
Natural Resources, University of Georgia.
We systematically searched for deer signposts during two
time periods: September 8–October 2, 2024, and October 14–
November 15, 2024. The period of September 8–October 2, 2024,
represented peak rubbing activity and early scraping activity for
deer in the region (Kile and Marchinton  1977). The period of
October 14–November 12, 2024, represented late rubbing activity and peak scraping activity (Kile and Marchinton 1977; Alexy
et al. 2001; Hearst et al. 2021). We marked deer signposts with
flagging tape and a GPS point and recorded the vegetation species associated with the signpost.
2.2   |   Materials and Data Collection
Signpost spectra were collected within 1–5days after discovery. We conducted all measurements of the spectral qualities
of signposts at night and under an opaque tarp to block any
incident light. Each component of a signpost (i.e., rub, licking
branch, scraped earth, and urine) was exposed to a 365nm light
(Way Too Cool LLC, Glendale, Arizona, USA) and a 395nm
light (Morpilot, Milpitas, California, USA). The utilization of
two wavelengths (365 and 395nm) accounts for variable excitation of potential lumiphores, and both 365 and 395nm are
present in the atmosphere during crepuscular hours (Kieffer
and Stone 2005; Théry et al. 2008). We completed two scans per
component under each lighting condition (i.e., four total scans)
using a PR-650 Telescoping Spectrometer (Photo Research Inc.,
Chatsworth, California, USA). A scan consisted of positioning
the PR-650 35.6–45.7 cm from the target (e.g., rub, bark, scraped
earth, forest floor), adjusting the focus so that the sensor was
clearly centered on the target, exposing the target to UV light
(395 or 365nm), and recording irradiance values reflected/emitted by the target at each wavelength. The PR-650 was mounted
on a tripod with a custom shroud affixed to the side to reduce
UV light source interaction with the sensor. Following the four
scans of each signpost, we completed four scans of the untouched
tree bark of the rubbed tree, or for scrapes, we scanned the surrounding substrate external to the scrape site under the same
lighting conditions. We visually searched scrape sites for deer
urine and scanned the urine under the same lighting conditions.
2.3   |   Analysis
We averaged the two scans under each lighting condition (i.e.,
365 and 395nm) for each signpost component (i.e., rub, licking
branch, scraped earth, and urine) for direct comparison to the
average of the scans for the surrounding environment component (i.e., rub versus untouched tree bark of rubbed tree) using
an Excel spreadsheet (Microsoft, Redmond, WA). Because
spectral data are continuous, positive, and skewed, we used
a generalized linear model (GLM) with a Gamma log link (Rcore Team  2025). This model allowed for comparison of the
spectra (i.e., irradiance and wavelength) of all rubs and licking
branches to the spectra of all untouched bark and compared the
spectra of all urine and scraped earth to all forest floor spectra.
Specifically, the model allowed for treating the irradiance as a
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Ecology and Evolution, 2025 3 of 10
response variable to several predictor variables (i.e., rubbed/
scraped=1, unrubbed/forest floor=0, date range (September
8–October 2 and October 14–November 12), tree species, and
wavelength) to determine their individual effects on irradiance.
Most of the spectra consisted of reflectance, which is visually
evident by the large irradiance peak corresponding to the light
source used (i.e., 365 and 395nm). Any peaks at wavelengths
longer than 400nm were considered photoluminescence and
not reflectance. To determine the statistical significance of any
potential photoluminescence, we analyzed irradiance values at
wavelengths between 400 and 554nm. By limiting our analysis
to 400–554nm, we avoided reflectance peaks influencing statistical results while still remaining within deer visual sensitivity
(Jacobs et al. 1994; D'Angelo et al. 2008).
3   |   Results
We scanned 109 rubs for analysis (September 8–October
2=57, October 14–November 12=52), which occurred on 20
tree species (see Table S1). We scanned 37 scrapes for analysis
(September 8–October 2, 2024=10, October 14–November 12,
2024=27) with licking branches occurring on 8 tree species (see
Table S2). Urine was present at 20 scrape sites, and only during
the October 14–November 12 time frame. Early in the study,
we determined that the PR-650 sensor could not accurately
detect licking branches because of their small size, so we did
not include them in the analysis. Additionally, all scans of the
overturned earth in the center of the scrape did not exhibit photoluminescence and exhibited lower average irradiance values
under both lighting conditions, so we excluded overturned earth
from further analysis.
Rubbed trees had greater average irradiance values than unrubbed trees when exposed to 365nm (p<0.001) (see Table S3)
and exhibited photoluminescence peaks at 450 and 550nm
(Figure 1). Rubs created during the September–October period
had lower irradiance than rubs created during the October–
November timeframe (p<0.001) (see Table S3) (Figure 2). Our
GLM model identified a single loblolly pine (Pinus taeda) as an
outlier and failed to converge, even after adjusting convergence
criteria and rescaling the response variable (irradiance) (Gelman
and Hill 2007). We determined that the outlier was not the result of user or equipment error and therefore wanted to retain it
for analysis. In order to retain the outlier, we log-transformed
the response variable and fit our data to a linear model (LM)
using ordinary least squares (OLS) (Gelman and Hill  2007).
The log-transformed LM (outlier present) yielded the same statistically significant results as the GLM (outlier absent) in that
rubs had greater average irradiance values than unrubbed trees
(p<0.001). Since the significance of results was not altered by
the presence of the outlier, we retained it in the analysis and
present the results of the GLM.
Rubbed trees had greater average irradiance values than
unrubbed trees when exposed to 395 nm (p < 0.001) (see
Table  S4) and exhibited photoluminescence from approximately 500–590 nm (Figure  3). Rubs created during the
September–October period had lower irradiance values than
rubs created during the October–November period (p = 0.207)
FIGURE 1    |    The average irradiance of rubs (N=109) made by white-tailed deer (Odocoileus virginianus) in Athens-Clarke County, Georgia, and
bark exposed to 365nm ultraviolet (UV) light from September 8–October 2, 2024, (N=57) and October 14–November 12, 2024, (N=52). Rubs and
tree bark were exposed to 365nm UV light and scanned with a PR-650 spectrophotometer. A generalized linear model was used to compare the
spectra of rubbed portions of trees to their bark. Rubs had greater average irradiance values (p<0.001), and exhibited photoluminescence at approximately 450nm and 550nm, which aligns well with white-tailed deer short-wave sensitive (SWS) (450–460nm) and middle-wave sensitive (MWS)
(537nm) cones (Jacobs et al. 1994). This indicates that rub visibility was increased in low light conditions, and that the resulting emission was visible
to white-tailed deer.
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4 of 10 Ecology and Evolution, 2025
(see Table  S4) (Figure  4). A post hoc visual check of model
assumptions revealed a single Chinese privet (Ligustrum sinense) and a single winged elm (Ulmus alata) as potential outliers (Gelman and Hill 2007). However, excluding them from
our model did not alter the significance of the results, and we
retained them in the analysis.
Urine associated with scrapes exposed to 395nm had greater
average irradiance values than the surrounding forest floor
(p=0.005) (see Table  S5) and exhibited photoluminescence
within approximately 480–600nm (Figure  5). Urine exposed
to 365nm also had greater average irradiance values than the
surrounding forest floor (p<0.001) (see Table S6) but exhibited
greater photoluminescence from 420 to 460nm (Figure 6).
4   |   Discussion
Signpost visibility was increased by deer interactions with natural components (e.g., tree, earth via urine), and rubs created
closer to the breeding season had greater irradiance values than
those created earlier in the fall season. Rubs and urine found
on scrapes exposed to 395 and 365nm had greater average irradiance values (i.e., brighter) than the surrounding environment, and exhibited photoluminescence. Under both lighting
conditions, the photoluminescent spectra overlapped strongly
with deer short-wave sensitive (SWS) (450–460nm) and middlewave sensitive (MWS) (537nm) cones, reaffirming that deer are
visually adapted to low-light crepuscular conditions (Jacobs
et  al.  1994; D'Angelo et  al.  2008; Cohen et  al.  2014; Newman
and D'Angelo 2024). Rubs exposed to 365nm had two distinct
peaks that closely aligned with deer SWS and MWS cones, and
urine exposed to 365nm had one distinct peak aligned with
deer SWS cone (Jacobs et al. 1994), indicating that signposts are
uniquely enhanced for increased visibility by deer during crepuscular hours.
There are several possible causes for photoluminescence of
rubs. Lignin in the cambium and inner sapwood layers of wood
photoluminesce when exposed to UV light (Donaldson and
Radotic  2013). Plants also produce various terpenes (Gassett
et al. 1997), some of which have been shown to photoluminesce
(e.g., limonene) (Lee et  al.  2013). The photoluminescence of
wood is so characteristic that it can be used for identification
purposes (Meier  2015). Glandular secretions of the forehead
gland of deer may also be responsible for the photoluminescent spectra of rubs. Gassett et al. (1997) identified 57 volatile
compounds on male deer forehead glands, seven of which occurred exclusively on the forehead, including various phenols
and terpenes. Both classes of compounds have been shown to
exhibit photoluminescence (Lee et  al.  2013; Aleixandre-Tudo
and du Toit 2019). Because white-tailed deer do not exhibit wallowing or perfuming behaviors as seen in other cervid species
(moose (Alces alces) (Miquelle  1991); elk (Cervus canadensis)
(Bowyer and Kitchen 1987); fallow deer (Dama dama) (Massei
and Bowyer  1999); red deer (Cervus elaphus) (Gossow and
Schürholz  1974); sika deer (Cervus nippon) (Miura  1985)), we
conclude that rub photoluminescence seen in this study can be
attributed to forehead gland secretions (Gassett et al. 1997) or
wood properties (Donaldson and Radotic 2013; Lee et al. 2013;
FIGURE 2    |    Difference in log-irradiance of individual rub's spectra (380–780nm) created by white-tailed deer (Odocoileus virginianus) from
September 8–October 2, 2024 (N=57) and October 14–November 12, 2024, (N=52) in Athens-Clarke County, Georgia, when exposed to 365nm
ultraviolet (UV) light. Rubs were exposed to 365nm UV light and scanned with a PR-650 spectrophotometer to quantify rub spectral characteristics and investigate photoluminescence. Rubs created from September 8–October 2, 2024, had lower irradiance than rubs made from October 14–
November 12, 2024, (p<0.001). This indicates that as proximity to the breeding season increased (mid-Nov), rub visibility increased.
 20457758, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ece3.72618 by Readcube (Labtiva Inc.), Wiley Online Library on [24/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Ecology and Evolution, 2025 5 of 10
FIGURE 3    |    The average irradiance of rubs (N=109) created by white-tailed deer (Odocoileus virginianus) in Athens-Clarke County, Georgia,
and bark exposed to 395nm ultraviolet (UV) light from September 8–October 2, 2024, (N=57) and October 14–November 12, 2024, (N=52). Rubs
and tree bark were exposed to 395nm UV light and scanned with a PR-650 spectrophotometer. A generalized linear model was used to compare the
spectra of rubbed portions of trees to their bark. Rubs had greater average irradiance values (p<0.001), and exhibit photoluminescence from approximately 500nm to 590nm, which aligns with white-tailed deer middle-wave sensitive (MWS) cone (537nm) (Jacobs et al. 1994). This indicates that
rub visibility was increased in low light conditions, and that the resulting emission was visible to white-tailed deer.
FIGURE 4    |    Difference in log-irradiance of individual rub's spectra (380–780nm) created by white-tailed deer (Odocoileus virginianus) from
September 8–October 2, 2024 (N=57) and October 14–November 12, 2024, (N=52) in Athens-Clarke County, Georgia, when exposed to 395nm
ultraviolet (UV) light. Rubs were exposed to 395nm UV light and scanned with a PR-650 spectrophotometer to quantify rub spectral characteristics and investigate photoluminescence. Rubs created from September 8–October 2, 2024, had lower irradiance than rubs made from October 14–
November 12, 2024, (p=0.207). This indicates that as proximity to the breeding season increased (mid-Nov), rub visibility increased.
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6 of 10 Ecology and Evolution, 2025
Meier  2015; Aleixandre-Tudo and du Toit  2019). Whether the
photoluminescence is the result of deer forehead glandular secretions or wood properties, the fact remains that rubs visually
contrast the surrounding environment in a way that is uniquely
suited for deer vision.
The porphyrins and amino acids present in urine are the likely
cause of the photoluminescent spectra we observed on scrapes
(Lim and Peters 1984; Neves and Galván 2020; Frohlich 2020).
Although the specific structural or chemical makeup of these
compounds in deer urine is unknown, porphyrins have a range
of excitation wavelengths from 385 to 398 (Reinhold et al. 2023),
which aligns with part of the spectrum of the 365nm light and
the peak wavelength of the 395nm light used in our study.
Tryptophan metabolites (an essential amino acid) are present
throughout the skin and pelage of many mammalian species
(Reinhold et  al.  2023; Travouillon et  al.  2023) and urine (Oh
et al. 2017). These metabolites are one of the hypothesized causes
of photoluminescence on the fur and skin of several mammalian species (Pine et al. 1985; Travouillon et al. 2023; Reinhold
et al. 2023). Regardless of the mode of action, the photoluminescent spectra we observed on scrapes via deer urine were within
deer SWS cone visibility. More research is needed to determine
the specific sources of the photoluminescence of both rubs and
deer urine.
The excitation wavelengths (365 and 395 nm) used in our study
are present during crepuscular hours (Kieffer and Stone 2005)
when deer are most active. During crepuscular hours, ambient
light is enriched with short-UV-blue wavelengths 345–480 nm
(Théry et  al.  2008). Additionally, the results of our analysis
indicated a significant contrast between signposts and typical backgrounds (tree bark and forest substrate). However,
structural characteristics of habitats such as canopy cover
and foliage can affect what wavelengths predominate, the
overall availability of visible light, and contrast levels, all of
which can alter the visibility of objects to potential viewers
(Goldstein 2007; Théry et al. 2008; Cuthill et al. 2019). Deer
signpost activity is generally concentrated on field-forest
edges and forested trails (Kile and Marchinton  1977; Alexy
et  al.  2001; Huang et  al.  2023), which are areas that can be
characterized as being more open than closed-canopy forests. The primary function of signposts is likely for olfactory
communication (Moore and Marchinton  1974; DeYoung and
Miller  2011; Ditchkoff  2011). The effectiveness of olfactory
communication can be determined by levels of surface friction
(i.e., vegetation structure) through the obstruction of airborne
odorants (Conover  2019), indicating that olfactory signpost
communication is maximized in more open locations. Based
on our results, these same conditions also maximize visibility by both wavelength exposure and background contrast
of signposts during UV-blue wavelength saturated crepuscular hours.
Much of the debate pertaining to the function of photoluminescence observed in Mammalia hinges on whether the
FIGURE 5    |    The average irradiance of white-tailed deer (Odocoileus virginianus) urine found at scrapes (N=20) and surrounding forest floor
when exposed to 395nm ultraviolet (UV) in Athens-Clarke County, Georgia, from October 14–November 12, 2024. Urine and surrounding forest
floor were exposed to 395nm UV light and scanned with a PR-650 spectrophotometer to quantify spectral characteristics of white-tailed deer scrapes
and investigate photoluminescence. A generalized linear model was used to compare the spectra of urine to the surrounding forest floor. Urine
had greater average irradiance values than the surrounding forest floor (p=0.005). Urine exhibited photoluminescence when exposed to 395nm
from approximately 480–600nm. Although the difference in irradiance was significant, it was not visually striking since the urine spectra did not
deviate far from normal distribution. However, the urine exposed to 395nm on scrapes is likely visible to deer during low light conditions since the
photoluminescence spectra does align with deer short-wave sensitive (SWS) (450–460nm) and middle-wave sensitive (MWS) (537nm) cones (Jacobs
et al. 1994).
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Ecology and Evolution, 2025 7 of 10
emitted spectra are visible to the target observer (Marshall and
Johnsen  2017; Travouillon et  al.  2023; Reinhold et  al.  2023).
Deer are dichromats adapted to low-light crepuscular conditions as evidenced by the presence of SWS and MWS cones
(Jacobs et al. 1994), an even distribution of rods (Staknis and
Simmons  1990), and SWS cones (D'Angelo et  al.  2008), and
deer have been shown to be visually sensitive to shorter wavelengths (< 400 nm) (Cohen et al. 2014). The spectral signature
of signposts we documented was within a range visible to deer.
Rubs and urine exposed to 365 nm likely would be particularly
visible to deer because the peaks of these spectra closely align
with deer SWS cone types, further suggesting a possible visual relevance of signpost visibility in low-light crepuscular
conditions.
Although our methods did not directly investigate deerrelated behavioral changes associated with photoluminescence, there is evidence to suggest that a relationship exists.
In Georgia, rubbing begins in late August–mid September
(Kile and Marchinton  1977) as deer antlers mineralize and
harden (Demarais and Strickland  2011). Early rubs are made
through the process of removing the dried velvet (Moore and
Marchinton 1974; Pierce et al. 2022), resulting in a combination
of blood and glandular secretions remaining on the structure.
Later rubs serve as both a visual and olfactory dominance display
to rival males (Miller et al. 1991) as testosterone levels increase
(Atkeson and Marchinton  1982; Kile and Marchinton  1977;
Mirarchi et al. 1977). Our analysis indicated that rubs made earlier in the season had lower irradiance values than rubs made
temporally closer to the breeding season under both lighting
conditions. This suggests that rub visibility increased in tandem
with increased hormone levels and known behavioral changes
associated with the breeding season. Overall, the photoluminescence of rubs and urine at scrape sites could be a form of cryptic
communication like that of nocturnal mammals (Olson et  al.
2021; Newar et al. 2024).
It should be noted that many lumiphores are prone to photobleaching, resulting in decreased photoluminescence (Reinhold
et al. 2023). Tryptophan and porphyrins in particular are known
to degrade due to light exposure, with tryptophan degrading over
the course of months and porphyrins within minutes (Reinhold
et al. 2023; Toussaint et al. 2023). Many other lumiphores used
in commercial applications have been shown to degrade at varying rates, with overall photostability depending on a wide array
of environmental factors (Giroux et al. 2023). This is a likely explanation for the occurrence of outliers we observed in our study
because, in a wild setting, it would be nearly impossible to know
with any certainty the moment a signpost was created and therefore just as difficult to measure it within minutes of creation. A
possible solution for future research targeted at controlling for
temporal degradation of lumiphores present on signposts could
be to observe deer in a captive setting and collect spectral measurements of signposts upon creation of the signpost.
FIGURE 6    |    The average irradiance of white-tailed deer (Odocoileus virginianus) urine found at scrapes (N=20) and surrounding forest floor
when exposed to 365nm ultraviolet (UV) light in Athens-Clarke County, Georgia, from October 14–November 12, 2024. Urine and the surrounding
forest floor were exposed to 365nm UV light and scanned with a PR-650 spectrophotometer to quantify the spectral characteristics of scrapes and investigate photoluminescence. A generalized linear model was used to compare the spectra of urine to the surrounding forest floor. Urine had greater
average irradiance values than the surrounding forest floor (p<0.001). Urine exhibited photoluminescence from approximately 480–600nm with a
distinct peak occurring from 420 to 460nm. The resulting emission of white-tailed deer urine exposed to 365nm aligns with white-tailed deer shortwave sensitive (SWS) (450–460nm) cone (Jacobs et al. 1994), indicating that urine on scrapes is visible to white-tailed deer in low-light crepuscular
conditions.
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8 of 10 Ecology and Evolution, 2025
5   |   Conclusion
Our study is the first quantification of environmental photoluminescence related to mammal ecology and the first that
meets at least four of the five criteria for assigning biological
function to photoluminescence proposed by Marshall and
Johnsen (2017). The excitation wavelengths we used are naturally present, the emitted wavelengths contrasted with typical
backgrounds, the photoluminescent portions of the signposts
were in locations that maximize visibility to deer, and deer have
spectral sensitivity to see the photoluminescence. Though we
did not directly test for a behavioral change in deer as a result
of the presence of photoluminescence, the irradiance of rubs increased at the same time as deer hormone levels increased, and
behavioral changes are known to change with the progression
of the breeding season. Future research investigating behavioral
changes associated with signpost photoluminescence should
directly manipulate photoluminescent portions of the structure
so that the base color remains the same while removing the lumiphore responsible for emission (Marshall and Johnsen 2017).
Future research focusing on the causal agent of the signpost
photoluminescence should investigate the spectral qualities of
trees and other vegetation known to be preferred by deer for
signpost creation, as there could be potential for preferences
related to visual attributes. Research investigating the spectral
qualities of specific glandular compounds produced by deer
would confirm the sources of photoluminescence documented
in our study and would elucidate if signpost photoluminescence
is temporally related to the breeding season. Although our results suggest a foundation for changes in deer behavior associated with signpost photoluminescence, observations of deer
interacting with manipulated signposts would enhance understanding of how spectral characteristics, locations, and timing
of signposts impact deer behavior.
Author Contributions
Daniel R. DeRose-Broeckert: conceptualization (equal), data curation (lead), formal analysis (lead), investigation (lead), methodology
(equal), writing – original draft (lead), writing – review and editing
(equal). Billy R. Hammond: conceptualization (supporting), formal
analysis (supporting), investigation (supporting), methodology (equal),
resources (lead), validation (supporting), writing – review and editing
(supporting). Steven B. Castleberry: conceptualization (supporting),
project administration (supporting), supervision (supporting), validation (supporting), writing – review and editing (supporting). Gino J.
D'Angelo: conceptualization (equal), investigation (supporting), methodology (equal), project administration (lead), resources (supporting),
supervision (supporting), writing – original draft (supporting), writing
– review and editing (supporting).
Acknowledgements
This study was funded by the Warnell School of Forestry and Natural
Resources and the University of Georgia Deer Lab. We would like to
thank the University of Georgia Vision Sciences Laboratory for providing
equipment and guidance as it related to photometry and collecting spectral signatures. Matt Williams of MycoHabitat provided important insights which contributed to the design of our study. We are grateful to the
Warnell School of Forestry and Natural Resources Lands and Facilities
Director Mike Hunter for access to study sites. We are especially appreciative of William Heath, Jack Bohlen, Meagan Adkins, Tatum Money,
Josie Brock, Logan Anderson, Kat Maupin, Caleb Booth, Cole Young,
and Sarah Shayeb for assistance locating signposts, particularly Lilyanne
Callaham for assisting with the collection of spectral signatures.
Conflicts of Interest
The authors declare no conflicts of interest.
Data Availability Statement
All data and information are available in Supporting Information. All rub
and bark irradiance values resulting from 365nm exposure are available
in “CSV_RubData_365Light”, all rub and bark irradiance values resulting from 395nm exposure are available in “CSV_RubData_395Light”,
all urine and forest floor irradiance values resulting from 365nm exposure are available in “CSV_UrineData_365Light”, and all urine and
forest floor irradiance values resulting from 395nm exposure are available in “CSV_UrineData_395Light”. Averages used in analysis are
also available. Average rub and bark irradiance values resulting from
365nm exposure are available in “CSV_AverageRub_365Light”, average rub and bark irradiance values resulting from 395nm exposure are
available in “CSV_AverageRub_395Light”, average urine and forest
floor irradiance values resulting from 365 nm exposure are available in
“CSV_AverageUrine_365Light”, and average urine and forest floor irradiance values resulting from 395nm exposure are available in “CSV_
AverageUrine_395Light”. Supplementary tables with descriptions are
available in “Supplementary Tables_Signposts”.
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Supporting Information
Additional supporting information can be found online in the
Supporting Information section. Data S1: ece372618-sup-0001-DataS1.
zip. Table S1: Rubs (N=109) created by white-tailed deer (Odocoileus
virginianus) from September 8 to October 2, 2024 (N=57) and October
14 to November 12, 2024 (N=52) in Athens-Clarke County, Georgia.
Rubs were exposed to 395 and 365nm ultraviolet (UV) light and scanned
with PR-650 spectrophotometer to quantify spectral characteristics and
investigate photoluminescence. Table S2: Scrapes (N=37) created by
white-tailed deer (Odocoileus virginianus) in Athens-Clarke County,
Georgia from September 8 to October 2, 2024 (N=10) and October
14 to November 12, 2024 (N=27) with the total for each tree species
listed. All scrape sites with urine present (N=20) occurred during
October 14–November 12, 2024, time period. Scrapes were exposed to
395nm and 365nm ultraviolet (UV) light and scanned with a PR-650
spectrophotometer to quantify spectral characteristics and investigate
photoluminescence. The PR-650 sensor was unable to detect licking
branches, and scrapped earth in the middle of the scrape did not exhibit
photoluminescence. Table  S3: Results from generalized linear model
for rubs (N=109) created by white-tailed deer (Odocoileus virginianus)
in Athens-Clarke County, Georgia, when exposed to 365nm ultraviolet
(UV) light comparing irradiance of rubbed portion of trees to bark from
400 to 554nm. Rubs were scanned with a PR-650 spectrophotometer
from September 8 to October 2, 2024, (N=57) and October 14–November
12, 2024 (N=52) to quantify rub spectral characteristics and investigate photoluminescence. The average rub irradiance was greater than
bark (p<0.001), and rubs made from September 8 to October 2, 2024,
had lower average irradiance compared to rubs made from October
14 to November 12, 2024 (p<0.001). Species that had a significant effect (p<0.05) on irradiance relative to the intercept include American
beautyberry (Calllicarpa americana), eastern redcedar (Juniperus virginiana), dead hardwood (spp. unkn.), hawthorn (Crataegus aestivalis),
Hickory (Carya spp.), common persimmon (Diospyros virginiana), loblolly pine (Pinus taeda), Chinese privet (Ligustrum sinense), sweetgum
(Liquidambar styraciflua), and vaccinium (Vaccinium spp.). Table S4:
Results from generalized linear model for rubs (N=109) created by
white-tailed deer (Odocoileus virginianus) in Athens-Clarke County,
Georgia, when exposed to 395nm ultraviolet (UV) light comparing
irradiance of the rubbed portion of trees to bark from 400 to 554nm.
Rubs were scanned with PR-650 spectroradiometer from September 8
to October 2, 2024 (N=57) and October 14–November 12, 2024 (N=52)
to quantify rub spectral characteristics and investigate photoluminescence. The average rub irradiance was greater than bark (p<0.001), and
rubs created from September 8 to October 2, 2024, had lower average
irradiance compared to rubs made from October 14 to November 12,
2024 (p=0.207). Species that had a significant effect (p<0.05) on irradiance relative to the intercept (American beautyberry (Calllicarpa
americana)) include hawthorn (Crataegus aestivalis), Chinese privet
(Ligustrum sinense), and winged elm (Ulmus alata). Table S5: Results
from generalized linear model for scrapes created by white-tailed deer
(Odocoileus virginianus) with urine present (N=20) in Athens-Clarke
County, Georgia, from October 14–November 12, 2024, when exposed
to 395nm ultraviolet (UV) light comparing irradiance of deer urine to
surrounding forest floor from 400 to 554nm. Urine and adjacent forest floor were exposed to 395nm UV light and scanned with a PR-650
spectrophotometer to quantify scrape spectral characteristics and investigate photoluminescence. The average irradiance of white-tailed
deer urine was greater than the surrounding forest floor (p=0.005).
Table S6: Results from generalized linear model for scrapes created by
white-tailed deer (Odocoileus virginianus) with urine present (N=20)
in Athens-Clarke County, Georgia, from October 14 to November 12,
2024, when exposed to 365nm ultraviolet (UV) light comparing irradiance of white-tailed deer urine to the surrounding forest floor from 400
to 554nm. Urine and adjacent forest floor were exposed to 365nm UV
light and scanned with a PR-650 spectrophotometer to quantify scrape
spectral characteristics and investigate photoluminescence. The average irradiance of white-tailed deer urine was greater than the surrounding forest floor (p<0.001).
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