DISCUSSION
Our results supported our hypothesis that predation on Texas horned
lizards in town would be lower than in the natural ranch environment,
and are consistent with other studies that have found lower predation in
urban areas (Fischer et al. 2012). Although we did not explicitly
measure predator densities, there is anecdotal evidence that the
predator community in Kenedy and Karnes City differs in both the
abundance and diversity of predators when compared to the ranch. For
instance, birds of prey and snakes are rarely seen in or near our study
plots in the towns but were seen daily at the ranch. Feral and pet cats
and dogs are also common in town but do not occur on the ranch. Altered
predator communities are a consistent result of urbanization (Prange and
Gehrt 2004, Fischer et al. 2012) and relaxed predation pressure in town
by certain types of predators like birds may be a contributing factor to
the high density of Texas horned lizards in Kenedy and Karnes City
(Ackel 2015) and their dietary shift to consuming small prey items
(Alenius 2018)
Although the use of models has proven effective at measuring predation
in other studies (Brodie 1993, McMilan and Irshick 2010, Vignieri et al.
2010, Farallo and Forstner 2012), a common concern is the confidence
with which predation marks left behind on models are accurately
categorized. Marks left behind on models by birds were easy to discern
by the presence of conspicuous “V” or “U” shaped peck marks. Models
that had been decapitated were also categorized as avian predation
because birds tend to attack towards the head of their prey (Smith
1973). Avian predators accounted for 30% of predation events. Bite
marks accounted for 21.7% and were also relatively easy to discern in
models. Bites included obvious chisel marks left behind by rodent
incisors and half-moon bite impressions on the dorsum and venter of
models that were possibly indicative of predatory lizards
(Crotophytus reticulatus. ). All other damage to models not
falling into those categories were scored as unknown, which represented
48.3% of recorded predation events. These unknown predation events
included models that had limbs removed but no definitive bite or peck
marks. Northern grasshopper mice (Onychomys leucogaster ) are
known predators of horned lizards (Munger 1986, Endriss et al. 2007) and
have been shown to chew the limbs off Texas horned lizards in areas
where both are common (Nathan Rains, Texas Parks and Wildlife, pers.
comm.). We also found several live adult lizards on the ranch with
missing limbs, although it is unknown what caused the loss of limbs. The
models with missing limbs may also be the result of a typical greater
roadrunner “centrifugal-slam” attack in which the bird grabs a lizard
by any limb or tail and smashes it on the ground (Sherbrooke 1990).
These results suggest avian predators are a significant threat to Texas
horned lizards, which is similar to findings in other studies that show
that avian predators like shrikes consume large numbers of horned
lizards and may even be responsible for the evolution of increased
cranial horn lengths in flat-tailed horned lizards (Phrynosoma
mcallii ) (Munger 1986, Young et al. 2004).
Birds are highly visual (Fox et al. 1976) and avian predators of horned
lizards may use their visual acuity to find their cryptically colored
prey. Our results showed that when models were painted to background
color-match their surroundings (i.e. they were more cryptic), avian
predation events were significantly less likely than when models were
not painted to color-match. This supports a long-held hypothesis that
crypsis and background color-matching are the primary defensive
adaptations for horned lizards against visually oriented predators
(Norris and Lowe 1964, Pianka and Parker 1975).
There are inherent limitations in using models for predation studies
which include the lack of movement, smell, and lack of appropriate
behavioral responses and therefore models sample a subset of predators
(Bateman et al. 2016). For instance, we did not expect models to be
attacked by snakes because snakes rely on thermal, motion, and olfactory
cues to sense prey (de Cock Buning 1983); all of which are not exhibited
by model lizards. Similarly, we did not expect models to be attacked by
cats since they are often attracted by motion (Ellis and Wells 2008). We
set up motion detection video cameras in the urban areas for some models
and filmed several instances of cats walking by and ignoring the models.
Nevertheless, local residents have told us that sometimes their pet cats
will bring dead horned lizards back to the house. Cats are known to be
very efficient predators of small reptiles, birds, and mammals in urban
areas (Loss and Marra 2017) and stationary model studies may not be well
suited to sample these predation events. Another limitation of models
may be that predators are attracted to the particular material used to
construct the models (Bateman et al. 2016). Our models were attacked
significantly more than the controls however, suggesting predators were
not simply attracted to the urethane foam or paint used to construct our
models. In the future, we recommend that controls be made into shapes
(i.e. pyramids) that would offer no visual cues to encourage predation
events such as may have occurred by Texas tortoises mistaking them for
cactus pads.
Our hatchling models had fewer predation attempts than the juvenile or
adult models. This result may indicate that stationary hatchling lizards
are less vulnerable to certain types of predation. Conversely the
hatchling models were morphologically the least realistic, due to their
small size and difficulty in constructing them and predators may
therefore simply not have recognized them as potential prey.
Texas horned lizards have declined throughout their historic range and
urbanization is often suggested as one of the main drivers of those
declines (Donaldson et al. 1994, Endriss et al. 2007, Wolf et al. 2013).
This study presents the first data comparing predation of Texas horned
lizards in urban and more natural environments, and may serve as a
foundation for future studies. Understanding how some horned lizard
populations, like those in Kenedy and Karnes City, are able to persist
in urban environments may help inform conservation efforts for other
populations. If these human modified environments have suitable
vegetation and food resources they may in some cases provide a refuge
for some prey species from predators (Keehn and Feldman 2018, Law et al.
2020). Our results further suggest that Texas horned lizards in natural
environments experience high levels of predation pressure, and should be
an important conservation consideration when targeting areas for
potential reintroduction. Models may also be used as a conservation tool
in the future by placing them in potential reintroduction areas and in
areas where they are established prior to the release of Texas horned
lizards to gauge the relative predation pressure the reintroduction site
might experience. Knowledge of predation and the predator community at
potential reintroduction sites may help increase the probability of
survival for reintroduced lizards.