Philippa Kaur

Among the many groups of terrestrial species, our understanding of mammal visual signalling might be hampered by the fact that most research on mammals has focused on chemical (e.g., scat, urine, and glands) and acoustic (e.g., howling) signalling1,2. Instead2,3, visual communication might be an overlooked communication channel2,4, despite being perhaps as important as the others, if we consider that: (1) mammal coloration has evolved for inter- and intraspecific communication2,4,5,6,7, which means that mammals use visual signals to communicate; and (2) visual signalling through physical marks (e.g., bites and scratches) is permanent and, thus, has the obvious advantages of (a) being long-lasting, i.e., environmental factors such as rain or snow are less likely to affect the detectability of visual marks as compared to, e.g., chemical signalling8, although mammals have found strategies to make chemical signalling last as long as possible9, and (b) functioning remotely, i.e., even when the signaller is away from the marked location2. Visual marking may also allow individuals to reduce repeated visits to strategic marking points, and thus save time and energy, which would otherwise detract animals from other activities, like foraging and reproduction10. Therefore, visual signalling may represent a reliable and advantageous communication channel8.Solitary species like bears may benefit from advertising their location, size, and reproductive status to expedite mate selection during the breeding season. Moreover, brown bears usually occur at low densities across their range, making direct interactions with one another infrequent11,12. Thus, long-lasting visual signalling may be particularly effective and considerably time saving. To date, studies on bear communication have highlighted two main forms of communication10,13,14,15,16,17: (1) olfactory communication, i.e., the marking of focal trees by rubbing the body against the trunk and/or by urination and deposition of anogenital gland secretions; and (2) pedal marking, by which bears mark the ground with their scent by grinding their feet into the substrate. Auditory communication, e.g., vocalizations used as threats during agonistic encounters, to advertise sexual receptivity, or for communication between females and their cubs, is considered as the least important channel through which bears signal, whereas visual communication has always been considered limited to different forms of body postures or behavioural displays (but see18).Since the beginning of the 1980s, bear marks on trees have puzzled researchers8. The function of, and motivation behind, tree biting and clawing have prompted a variety of theories related to glandular scent deposition (i.e., chemical signalling), but none of these hypotheses has been considered satisfactory, nor have they ever been tested8.
The debarking behaviour of brown bears Ursus arctos, which leaves bright and conspicuous marks on tree trunks (see Extended Data Fig. 1 and Extended Data Fig. 2), presents a unique yet unexplored opportunity to investigate new ways of visual communication in terrestrial mammals, and to better understand both bear and carnivore communication broadly. The hypothesis behind this experimental work is that brown bears may rely on visual communication via the conspicuous marks that they produce on trees.Figure 1Brown bear response to trunk mark manipulation. The behavioural sequence of an adult male brown bear removing the pieces of bark that we used to conceal the visual markings on an ash tree during the mating season in the Cantabrian Mountains, Spain (12/06/2020, 15h37). The whole sequence is shown in the video footage Extended Data Fig. 5.Full size imageAfter manipulating bear tree marks in the Cantabrian Mountains (north-western Spain), we found that bears removed the bark strips that we used to cover their marks during the mating season (Extended Data Figs. 3 and 4), suggesting that bear debarking may represent a visual communication channel used for intraspecific communication.Brown bear responses to marked tree manipulationsAfter concealing bear marks due to trunk debarking with bark strips from the same tree species (see “Methods”), our manipulations on 20 trees triggered a rapid reaction from brown bears. Between the 16th of May and the end of September 2020 (overlapping part of the brown bear mating period in the Cantabrian Mountains19), brown bears removed the strips of bark that we used to cover the trunk marks in 9 (45%) out of the 20 manipulated trunks (Fig. 1 and Extended Data Fig. 5). However, if we consider that these nine trees were also the ones that we could manipulate (because of field work restrictions due to COVID-19) from the start of the mating season (beginning of May), 100% of the bark strips used to cover tree marks were removed by bears when the manipulation occurred at the commencement of the mating season. In only one case, a bear removed the bark strips covering marks on a tree that was manipulated later in the mating season (end of June). Control bark strips fixed to (a) the same trunk as the manipulated bear mark, (b) the nearest neighbouring tree to the manipulated one showing bear marks, and (c) the nearest rubbing trees with no bear marks, were never removed by bears. In two cases (50%), after the first removal of the manipulated mark by a bear, which was subsequently covered again with new strips (n = 4), a bear removed the strips a second time. Further, camera traps showed that: (1) bears uncovered the manipulated marks the first time they visited the tree after our manipulation; (2) bark strips that were not removed were always the result of bears not visiting the site after tree manipulations; and (3) the shortest lapse of time between a mark manipulation and a bear visiting the tree for the first time and uncovering the mark was seven days. Thus, manipulations always triggered a rapid response from bears when adult males, probably the same individuals that debarked the trunks, came back and check on marked trees.Conspicuousness of brown bear visual marksThe conspicuousness of a visual signal is not only increased by its position in a noticeable location, but also by the contrast between the signal and its background20,21. A remarkable difference (pixel intensity: mean (± SD) = 85.09 ± 26.77, range = 20.27–177.06) exists between bark and sapwood brightness for all tree species (t = 19.07, p =   More

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These authors contributed equally: Anne-Christine Monnet, Kévin Cilleros, Agathe Leriche.Aix Marseille Univ, Avignon Univ, CNRS, IRD, IMBE. Technopôle de l’Arbois-Méditerranée, cedex 4, BP 80, 13 545, Aix-en-Provence, FranceAnne-Christine Monnet, Kévin Cilleros, Frédéric Médail, Manuel Cartereau, Aggeliki Doxa, Daniel Pavon & Agathe LericheInstitut de Systématique, Evolution, Biodiversité (ISYEB), Muséum national d’Histoire naturelle (MNHN), CNRS, Sorbonne Université, EPHE, Université des Antilles, Paris, FranceMarwan Cheikh AlbassatnehDepartment of Plant Biology and Ecology, University of Seville, Seville, SpainJuan Arroyo, Marcial Escudero & Estefania Santos BareaDepartment of Life and Environmental Sciences, University of Cagliari, Viale Sant’Ignazio da Laconi 13, Cagliari, ItalyGianluigi BacchettaNational Research Council, Institute of Biosciences and Bioresources, 50019, Sesto Fiorentino, (FI), ItalyFrancesca Bagnoli, Ilaria Spanu & Giovanni Giuseppe VendraminDepartment of Botany, Hungarian Natural History Museum, Pf. 137, Budapest, 1431, HungaryZoltán BarinaFRB-CESAB, 5 rue de l’Ecole de Médecine, 34000, Montpellier, FranceNicolas CasajusDepartment of Biology, Laboratory of Botany, University of Patras, 26504, Patras, GreecePanayotis DimopoulosDepartment of Agriculture, Food and Forest Sciences, University of Palermo, Viale delle Scienze bldg. 4, 90128, Palermo, ItalyGianniantonio DominaStatistical Learning Lab, Institute of Applied and Computational Mathematics, Foundation for Research and Technology-Hellas (FORTH), Ν. Plastira 100, Vassilika Vouton, GR – 700 13, Heraklion, Crete, GreeceAggeliki DoxaINRAE, UR629, Ecologie des forêts méditerranéennes, Avignon, FranceBruno Fady & Anne RoigINRAE, Univ. Bordeaux, BIOGECO, F-33610, Cestas, FranceArndt HampeMacedonian Academy of Sciences and Arts, Krste Misirkov 2, 1000, Skopje, Republic of MacedoniaVlado MatevskiEcoGozo, Regional Development Directorate – Ministry for Gozo, Flat 6, Sunset Court B, Triq Marsalforn, Xaghra, Gozo, MaltaStephen MisfudDepartment of Botany, Faculty of Science, University of Zagreb, Zagreb, CroatiaToni NikolicBakkevej 6, 5853, Ørbæk, DenmarkArne Strid More

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African swine fever has breached the island of Borneo, where it is wiping out populations of the wild bearded pig Sus barbatus. First confirmed in early February, the outbreak has driven a precipitous decline in this species in less than two months. Field sites in the east of the Sabah region are reporting a complete absence of live pigs in forests. Local extinctions across swathes of Borneo are a realistic prospect.Bearded pigs are listed as vulnerable by the International Union for Conservation of Nature. They are seen as ‘ecosystem engineers’ in the Bornean rainforest, where they are one of the most abundant species of mammal. Bearded pigs can be legally hunted under permit, and are an important source of animal protein for many communities.The African swine fever virus is already island-hopping across southeast Asia, threatening 11 species of endemic pig, including the Sulawesi warty pig (Sus celebensis). Opportunities to control the disease in wild-pig populations are limited. Vaccines for domestic pigs are still in development, so the best hope for stemming loss of the wild animals could be to protect isolated populations in geographically defensible locations. More

Morphological changes of the leavesThe influence of NO2 stress on the plants was first reflected by the morphological changes of the leaves (Fig. 1)20,22. Slight NO2 injury was manifested by slight green deficiency and light color. Moderate NO2 injury was manifested by irregular watery spots between leaf veins, which gradually developed into yellow necrotic spots followed by lesions at the leaf stalk and margins. When the exposure time extended to 72 h, the leaves turned yellow, and irreversible injury occurred, which led to leaf death. The damaged areas of the leaves of the two species at different time points of NO2 exposure are summarized in Table 1.Figure 1Leaf injury symptoms of Carpinus betulus (A) and Carpinus putoensis (B) under different NO2 exposure time and after recovery.Full size imageTable 1 The damaged areas (percentage) of the leaves of Carpinus betulus and Carpinus putoensis at different time points of NO2 stress.Full size tableChanges in MDA contentThe changes in the MDA content of C. betulus and C. putoensis at different time points of NO2 stress are shown in Fig. 2. With the prolongation of NO2 stress, the MDA content of C. betulus showed an increasing tendency with the variation range from 0.016 to 0.029 µmol g−1 fw. However, no significant differences were observed at different time points of NO2 exposure.Figure 2Changes in the MDA content of C. betulus and C. putoensis at different time points of NO2 stress and after self recovery. Letters or letter combinations containing the same letter indicate no significant difference between the corresponding NO2 exposure time points in the same plant species according to ANOVA or nonparametric Kruskal–Wallis ANOVA followed by Bonferroni tests. Capital letters for C. putoensis and lower letters for C. betulus.Full size imageAs NO2 fumigation time extended, the MDA content of C. putoensis also showed an increasing tendency, with the variation range from 0.015 to 0.034 µmol g−1 fw. Compared with the control group, a significant difference was observed in the MDA content from 24 h, which peaked at 72 h. In C. putoensis, although the recover group and the control group did not show a significant difference, the MDA content of the former lay between 0.015 (6 h) and 0.019 µmol g−1 fw (12 h), which suggests that the plant did not recovered completely from the stress damage.Compared with C. putoensis, C. betulus exhibited a smaller variation amplitude in the MDA content under NO2 stress. The MDA content of C. betulus did not show noticeable changes at 1, 6, and 12 h, and it was till 24 h when a rapid increase occurred. These findings indicate a delayed injury response of C. betulus. In contrast, with the prolongation of NO2 stress, the MDA content of C. putoensis noticeably increased, which indicates an increase in the membrane lipid peroxidation activity of C. putoensis under NO2 stress.Changes in POD activityThe changes in POD activity of C. betulus and C. putoensis at different time points of NO2 stress are shown in Fig. 3. With the prolongation of NO2 stress, the POD activity of C. betulus showed an increasing tendency, with a variation range from 323 to 663 U (g * min)−1 fw. After 30-d self recovery, the POD activity returned to 409 U (g * min)−1 fw, which was comparable to that of the control.Figure 3Changes in POD activity of C. betulus and C. putoensis at different time points of NO2 stress and after self recovery. Letters or letter combinations containing the same letter indicate no significant difference between the corresponding NO2 exposure time points in the same plant species according to ANOVA or nonparametric Kruskal–Wallis ANOVA followed by Bonferroni tests. Capital letters for C. putoensis and lower letters for C. betulus.Full size imageAs NO2 fumigation time extended, the POD value of C. putoensis also showed an increasing tendency, with a variation range from 385 to 596 U (g * min)−1 fw. The recovery group did not show a significant difference compared with the control group.In C. betulus, the POD activity value rapidly increased at 72 h of NO2 stress, which showed a significant difference compared with any other group (adjusted p  0.05).Figure 4Changes in the soluble protein content of C. betulus and C. putoensis under NO2 stress at different time points and after self recovery. Letters or letter combinations containing the same letter indicate no significant difference between the corresponding NO2 exposure time points in the same plant species according to ANOVA or nonparametric Kruskal–Wallis ANOVA followed by Bonferroni tests. Capital letters for C. putoensis and lower letters for C. betulus.Full size imageIn C. putoensis, the soluble protein content also showed an increasing trend as the fumigation time prolonged. The variations ranged from 2.61 to 3.27 mg g−1 fw. Compared with the control group, the recovery group exhibited a lower soluble protein content, although no significant difference was observed between them.As shown in Fig. 4, the maximum difference in the soluble protein content of C. betulus was 2.33 mg g−1 fw, which was greatly larger than that of C. putoensis (0.66 mg g−1 fw). Particularly, C. betulus exhibited a rapid increase in the soluble protein content from 12 h of fumigation, which indicates that C. betulus increased protein synthesis when encountered with NO2 stress, whereas C. putoensis showed only weak resistance against the stress.Changes in NRAt 0 h of NO2 treatment, the NR activity of C. betulus was 1.43 ± 0.14 µmol NO2−·g−1fw·h−1. With the prolongation of NO2 exposure, the NR activity of C. betulus exhibited a gradual increase followed by a gradual decrease, and a significant difference (adjusted p  0.05). In C. putoensis, the NR activity of the control group was 0.58 ± 0.06 µmol NO2−·g−1fw·h−1. As the NO2 exposure time prolonged, NR activity exhibited a rapid increase (adjusted p  0.05). The results were shown in Fig. 5.Figure 5Changes in the NR activity of C. betulus and C. putoensis under NO2 stress at different time points and after self recovery. Letters or letter combinations containing the same letter indicate no significant difference between the corresponding NO2 exposure time points in the same plant species according to ANOVA or nonparametric Kruskal–Wallis ANOVA followed by Bonferroni tests. Capital letters for C. putoensis and lower letters for C. betulus.Full size imageChanges in NO3
−NAs the NO2 treatment time extended, the NO3−N content of C. betulus exhibited an increase followed by a gradual decrease, and a significant difference (adjusted p  0.05). In C. putoensis, the NO3−N content also exhibited an increase followed by a decrease after NO2 exposure. However, a significant difference was observed from 12 h. After 30-d recovery, the index returned to a normal level (adjusted p  > 0.05). The results were shown in Fig. 6.Figure 6Changes in the NO3−N content of C. betulus and C. putoensis under NO2 stress at different time points and after self recovery. Letters or letter combinations containing the same letter indicate no significant difference between the corresponding NO2 exposure time points in the same plant species according to ANOVA or nonparametric Kruskal–Wallis ANOVA followed by Bonferroni tests. Capital letters for C. putoensis and lower letters for C. betulus.Full size imageChanges in mineral elementsThe changes in the mineral elements of C. betulus and C. putoensi under NO2 stress and after self recovery are summarized in Table 2.Table 2 Changes in the mineral element contents of C. betulus and C. putoensis under NO2 stress and after self recovery.Full size tableMacroelements(1) N. At 1 h of NO2 stress, the total nitrogen content of C. betulus increased slightly to 1.68 ± 0.17 g/kg; this value was higher than that of control (1.4 ± 0.13 g/kg), but no significant difference was observed (adjusted p  > 0.05). With the prolongation of the stress, the content decreased, with the variations ranging from 0.84 to 1.68 g/kg and the maximum difference of 0.84 g/kg. The recovery group did not show a significant difference compared with the control group (1.53 ± 0.15 vs. 1.4 ± 0.13; adjusted p = 1.00).Overall, the changes in the total nitrogen content of C. putoensis showed a similar trend with those of C. betulus. At 1 h of NO2 stress, the total nitrogen content of C. putoensis significantly increased compared with that of the control (1.68 ± 0.15 g/kg vs. 1.12 ± 0.11 g/kg; adjusted p  0.05).(4) Ca. With the prolongation of NO2 exposure, the Ca content of C. betulus exhibited an increase followed by a gradual decrease, and the variations ranged from 84 to 243 µg L−1 dw. A significant difference was observed at 72 h of NO2 exposure. In C. putoensis, significant differences in the Ca content were observed during NO2 exposure, except at 12 h. In both species, the Ca content of the recovery group did not show a significant difference compared with the control group. The variation amplitude of the Ca content of C. betulus (159 µg L−1 dw) was noticeably greater than that of C. putoensis (68 µg L−1 dw).(5) Mg. As the NO2 stress prolonged, the Mg content of C. betulus did not show a significant drop, except at 48 h. The variations ranged from 21.4 to 31.3 µg L−1 dw. In C. putoensis, the variations ranged from 12.2 to 32.2 µg L−1 dw. In both species, the Ca content of the recovery group did not show a significant difference compared with the control group. The variation amplitude of the Ca content of C. betulus (9.9 µg L−1 dw) was remarkably smaller than that of C. putoensis (20 µg L−1 dw).Microelements(1) Zn. With the prolongation of NO2 exposure, the Zn content of C. betulus exhibited an increase followed by a gradual decrease. Compared with the control, significant differences were observed at 1, 6, and 12 h. The variations anged from 7.1 to 10.6 µg L−1 dw. In C. putoensis, significant differences in the Zn content were observed at 6 h and 48 h, and the variations ranged from 5.7 to 11.2 µg L−1 dw. The variation amplitude of the Zn content of C. betulus (3.5 µg L−1 dw) was smaller than that of C. putoensis (5.5 µg L−1 dw). In each species, the Zn content of the recovery group showed a significant difference compared with the control group.(2) Mn. At 1 h of NO2 fumigation, a sharp drop was observed, compared with the control. Afterwards, the Mn content of C. betulus exhibited a general increase followed by a gradual decrease. However, at any time point during NO2 exposure, a significant lower Mn content was observed when compared to the control. The variations of the Mn content ranged from 11.2 to 78.1 µg L−1 dw. In C. putoensis, the Mn content during NO2 exposure significantly decreased compared with control, and the variations ranged from 9.4 to 85.5 µg L−1 dw. The variation amplitude of the Mn content of C. betulus (66.9 µg L−1 dw) was slightly smaller than that of C. putoensis (76.1 µg L−1 dw). In each species, the Mn content of the recovery group did not show a significant difference compared with the control group.Correlation analysisThe correlations between the investigated indices and NO2 exposure time were analyzed using the Pearson’s method (Table 3). POD and soluble protein had a strong positive correlation with NO2 exposure time (correlation coefficient: 0.891 and 0.799, respectively), and NR, NO3−N, N, K, and Ca had a strong negative correlation with NO2 exposure time (correlation coefficient: -0.691, -0.805, -0.744, -0.606 and -0.696, respectively). MDA and the Zn content were not correlated with the exposure time.Table 3 Correlations of the investigated indices with NO2 exposure time.Full size table More

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