Deimatic displays, where sudden changes in prey appearance elicit aversive predator reactions, have been suggested to occur in many taxa. These (often only putative) displays frequently involve different components that may also serve antipredator functions via other mechanisms (e.g. mimicry, warning signalling, body inflation). The Colombian four-eyed frog, Pleurodema brachyops, has been suggested to gain protection against predation through putative deimatic displays where they inflate and elevate the posterior part of their body revealing eye-like colour markings. We exposed stationary artificial frogs to wild predators to test whether the two components (eyespot/colour markings, defensive posture) of their putative deimatic display, and their combination, provide protection from predation without the sudden change in appearance. We did not detect any obvious additive effect of defensive posture and eyespots/colour markings on predation risk but found a marginally-significant trend for model frogs in the resting posture to be less attacked when displaying eyespots/colour markings than when they were not, suggesting that the presence of colour markings/eyespots may provide some protection on its own. Additionally, we found that models in a resting posture were overall more frequently attacked on the head than models in a defensive posture, indicating that a defensive posture alone could help redirect predator attacks to non-vital parts of the body. The trends found in our study suggest that the different components of P. brachyops’ coloration may serve different functions during a deimatic display, but further research is needed to elucidate the role of each component when accompanied by sudden prey movement.

We tested the effect of the defensive posture and eyespots/colour markings present in the putative deimatic display of P. brachyops by exposing 1000 frog models, divided equally in four treatments, to natural predators in four different study areas. Two treatments included the eyespot/colour markings in the posterior dorsal region. These characteristic markings in P. brachyops consist of two rounded lumbar glands in a black colour accompanied by a pair of orange spots. Four different types of models were thus used: defensive posture with visible eyespots/colour markings; defensive posture without eyespots/colour markings; resting posture with visible eyespots/colour markings; and resting posture without eyespots/colour markings. 250 models of each type were placed semi-randomly (no two models of the same type were placed consecutively), distributed in equal numbers in each of the four locations. The model sequence was determined using an Excel-based random numbers generator. 

In each study site, we distributed the models sequentially with an inter-model distance of 5 m in all directions. The models remained in the field for three days, with visits every 24 hours to check for predation attempts on them. Models found with attack marks during the first (24h) or second day (48h) after deployment were removed and replaced with a new model (i.e., models that were found with attack marks during the last session [third day] were not replaced). Missing models were also replaced to keep the number of models within each category constant. Replacement models were treated as new replicates and their time to exposure to predation was counted from the moment at which they were deployed. All missing models were excluded from the data because we were unable to reliably attribute their absence to predation events. Models were exposed to predation for 24–72 hours. Bird attacks were identifiable by V or U -shaped and stab-like marks, whereas mammal attacks were identified by the bite marks, as reported in similar studies. Other attack marks, such as multiple small holes or scratches, were also recorded, but not included in our analyses. We also recorded the area of the body where the model was attacked (head or rest of the body), as attacks in the head could be an additional indication that the predators do recognise them as prey. Alternatively, attacks in the ‘rest of the body’, particularly in models displaying the eyespot/colour markings, could be an indication that these markings might redirect predator attacks away from vital body parts.

Differences among the morphs in attack risk over time were analysed via a survival analysis (Cox Regression) using the probability of bird/mammal attack as a binary response variable, and the eyespots/colour markings (with/without), posture (resting/defensive), and their interaction as fixed factors. To account for the non-independence of models within a study plot we included location as a random factor. We also took the subset of attacked models and tested the effect of eyespots/colour markings and posture (fixed factors) on the probability of attacks in the head using a Generalised Linear Mixed Model (GLMM) with binomial distribution and logit link function, and including location as a random effect. In all cases, we ran separate models for attacks by mammals and birds. All analyses were conducted in R, using the RStudio platform, and the package coxme.

Excel and R.