Visual marking in mammals first proved by manipulations of brown bear tree debarking

Abstract

The rather limited human ability to understand animal vision and visual signalling has frequently clouded our expectations concerning the visual abilities of other animals. But there are multiple reasons to suspect that visual signalling is more widely employed by animals than previously thought. Because visibility of visual marks depends on the background in which they are seen, species spending most of their time living in dark conditions (e.g., in forests and/or having crepuscular and nocturnal habits) may rely on bright signals to enhance visual display. Here, as a result of experimental manipulations, we present, for the first time ever, evidence supporting the use of a new channel of intraspecific communication by a mammal species, i.e., brown bear Ursus arctos adult males relying on visual marks during mating. Bear reactions to our manipulation suggest that visual signalling could represent a widely overlooked mechanism in mammal communication, which may be more broadly employed than was previously thought.

Sex-, age-, and time-specific visual communication in brown bears

“…Because hibernation is facultative in the Cantabrian Mountains, with multiple individuals not hibernating ( Ruiz-Villar et al 2019 ), we expected some marking behaviors to occur even during the hibernation period. Following the procedure of Penteriani et al (2021) , and to trigger the reaction of bears using preexisting marks on focal rubbing trees monitored by camera traps, we concealed the existing visual marks, one or more portions of the trunk where the bark has been removed ( Fig. 1 ), with strips of bark from the same tree species ( Fig.…”

“…Although research on mammal visual signaling is scarce and its role in communication has often been regarded as secondary, especially in crepuscular and nocturnal species, recent studies have shown that visual signaling could represent a more common and important form of mammal communication than previously thought ( Caro et al 2017 ; Penteriani and Delgado 2017 ; Negro et al 2020 ; Penteriani et al 2020 , 2021 ). For example, long-lasting and permanent physical marks such as scratches and bites left by felines and ursids on diverse types of natural (e.g., tree trunks, rocks) and artificial (e.g., wooden poles) elements of the landscape have the potential to play an important role in animal communication ( Cornhill and Kerley 2020 ; Penteriani et al 2021 ).…”

“…This possibility was later supported by Thapar (1986) for tigers and Feldman (1994) for domestic cats, and more recently, Hirano et al (2008) reported that mechanical marks made by howler monkeys ( Alouatta guariba clamitans ) could play the nonmutually exclusive function of being both a visual and chemical signal. To date, only one experiment ( Penteriani et al 2021 ) has been conducted to test the possibility that the marks produced by clawing and biting might represent a visual communication signal in a large carnivore, the brown bear ( U. arctos ), via tree debarking.…”

“…The suggestion of a new means of communication such as visual via physical marks on focal tree trunks ( Fig. 1 ; Penteriani et al, 2021 ) has demonstrated that bear communication could be richer than generally believed and deserves further studies to better understand individual-related differences and temporal patterns of visual communication.…”

“…To this end, and following methods already employed in Penteriani et al (2021) that presented the first evidence supporting the use of visual signaling by brown bears, we attempted to move our understanding of mammal visual communication a step forward by: (a) performing an experimental manipulation of bear tree marking behavior in the Cantabrian Mountains (northwestern Spain) during the 12 months of 2021; and (b) videorecording bear marking behaviors using camera traps. Because the previous research ( Penteriani et al 2021 ) was performed during the minimum time period (from the 1st of May to the end of September 2020) required to explore the possibility that brown bears might communicate by visual signals, this new study: (a) took into account the whole set of bear marking behaviors at rubbing trees throughout the entire year; (b) analyzed the entire set of bear sex and age classes interacting at rubbing trees; and (c) focused on yearly and daily temporal patterns of marking behavior. Additionally, while being conceptualized for visual marking, this study also allowed us to collect information on chemical communication behaviors–that is, it was not only possible to describe visual marking behavior throughout the year but also contextualize, describe, and compare visual signaling in relation to other bear marking behaviors at and around rubbing trees.…”