Defensive Behavior in Rhinella bergi and Rhinella mirandaribeiroi (Anura: Bufonidae) y Rhinella mirandaribeiroi (Anura: Bufonidae)

Here we report two cases of defensive behavior known as “stiff-legged”, unprecedented for the species Rhinella bergi and R. mirandaribeiroi . We registered the behavior at localities in Mato Grosso do Sul and Minas Gerais states, Brazil. With our records, there are now five species from open areas that exhibit this behavior. We assume that the species that inhabit these areas exhibit this avoiding predation behavior simulating a dead body on the ground, behavior known as “death feigning”, unlike forest species, which use this strategy to camouflage themselves between the leaves.

Anurans exhibit several defensive behaviors, including sound emission (e.g. release calls and agonistic calls) and visual displays, such as thanatosis and stiff-legged (Duellman andTrueb 1986, Toledo et al. 2011). The last one is characterized by a posture with the legs stiff and back extended, remaining the individual static in this position for a few minutes after the disturbance caused by a potential predator (Sazima 1978, Schlüter and Salas 1991, Bertoluci et al. 2007).
On 23 November 2017, we observed a male individual of Rhinella bergi (Céspedez, 2000) in a Chacoan area, in the Porto Murtinho municipality (21º41 South, 57º44 West), Mato Grosso do Sul state, Brazil. When we handled the specimen, at approximately 21:00h, it exhibited the stiff-legged defensive behavior (Fig. 1a). Right after we placed the individual on the floor, it continued exhibiting the behavior, keeping the hind limbs stretched and back extended, with the body flattened dorso-ventrally. The second record was registered on 19 December 2018, in the São Gonçalo do Pará municipality (19º59 South, 44º51West), Minas Gerais state, Brazil. We collected a male individual of R. mirandaribeiroi (Gallardo, 1965), which exhibited the same stiff-legged behavior, while being handled (Fig. 1b). We collected both specimens and housed at the Zoological Collection of the Federal University of Mato Grosso do Sul (ZUFMS-AMP13336; ZUFMS-AMP13283; collection permits: SISBio 45889-1).
The defensive stiff-legged behavior is usually combined with the anuran cryptic coloration resembling dead leaves (Toledo et al. 2011). Nevertheless, both species recorded in the present study have terrestrial habits and inhabit open areas (Narvaes and Rodrigues 2009). Rhinella bergi occurs in Chaco regions of Paraguay, northwestern Argentina and Mato Grosso do Sul in Brazil (Céspedez 2000), while R. mirandaribeiroi occurs in Cerrado areas   Kolenc et al. (2009) showed that this species also inhabits non-forested environments, suggesting that P. bibroni exhibit the stiff-legged behavior as death feigning function, rather than camouflage.
In total, 29 anuran species from six families show the stiff-legged behavior, with the family Bufonidae having the largest number of species that exhibit this defensive strategy (n=9), from which four species are from open areas (Table 1). Forest species that exhibit the stiff-legged behavior use this strategy to camouflage themselves among the leaves, once they present cryptic coloration, thus avoiding predation (Sazima 1978). With this, we can assume that the species of open areas perform stiff-legged behavior to look like dead body on the ground (death feigning).

AUTHOR'S CONTRIBUTION
IKRN and PSC design and writing; SM and DJS obtaining data, descriptions and photographs. All authors rewiewed drafts of the paper and approved the final draft.

ACKNOWLEDGMENTS
We are grateful to the reviewers of the manuscript, who made detailed observations that contributed to the improvement of the grade. IKRN thanks Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPQ) for his scholarship (CNPq 133940/2020-9). DJS thanks Conselho Nacional de Desenvolvimento Científico e Tecnológico for his research fellowship (CNPq 309420/2020-2). PSC thanks Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES Finance Code 001)