General patterns of the attacks

Most of the attacks occurred in California (n = 66, 37%), followed by Colorado (n = 16, 9%), British Columbia (n = 13, 7%) and the other jurisdictions (47%) (Fig. 1 and Supplemental Fig. S1a). The number of attacks recorded in urban areas has increased over time, with similar trends for the different species (Supplemental Fig. S2a). Spring and summer were the seasons showing the highest rates of attack (Supplemental Fig. S2b). Coyotes attacked uniformly throughout the year, with a slight peak during the spring, bear attacks were rare during winter and cougars attacked more often during spring and summer. This seasonal pattern conforms to the species’ biology and confirms what was previously shown in other studies17,19,20,21. Indeed: (a) bears are generally hibernating during winter; and (b) coyotes are rearing their pups during spring, when we observed a slight increase in attacks, and thus they might be in search of additional food and defending their dens during this period1, which makes them more likely to be involved in aggressive encounters with humans and pets22.

Figure 1 Map of the spatial distribution of wild carnivore attacks on humans recorded in North American urban areas between 1980 and 2016. As an example of the landscape structure analyzed in this study, two 1 km2 maps centered at the point of the attack are also shown. The map was created using QGIS software50. Boundary layers for Canada and U. S. were obtained from the Statistics Canada Catalogue51 and United States Census Bureau52 respectively. The two satellite images were obtained from the Google Earth Pro application53. Left image: Google Earth 7.3.1. (Imagery Date: April 24, 2014). California, U.S. 33°53′16.00″N, 117°48′40.04″W. Landsat/Copernicus. https://www.google.com/earth/ [September 20, 2017]. Right image: Google Earth 7.3.1. (Imagery Date: September 20, 2013). Connecticut, U.S. 41°47′24.87″N, 72°45′32.29″W. Landsat/Copernicus. https://www.google.com/earth/ [September 20, 2017]. Full size image

Most of the attacks occurred during the day, especially those involving coyotes and cougars (Supplemental Fig. S3a). This outcome is likely the result of the daily activity of humans in urban areas. Moreover, although coyotes in urban environments have been shown to change their activity patterns to crepuscular and nocturnal to avoid humans15, in many cities they have become habituated to people and, consequently, they might have lost their avoidance behavior and returned to being active during the day15. On the other hand, black bears tend to be mostly active at night to avoid humans23,24.

In general, children (<13 y. o.) were attacked less often than adults (Supplemental Fig. S2c), with a trend towards younger individuals (n = 34 attacks between 0 and 3 years old, n = 16 between 4 and 7 years old and n = 14 between 8 and 11 years old; no attacks were recorded on 9-, 10- and 12-year-old children). Coyotes and cougars attacked children and adults almost equally, while bears attacked considerably more adults than children (Supplemental Fig. S2c). This difference is probably related to the reasons triggering an attack. Indeed, brown and black bears were mainly involved in attacks related to dog presence and anthropogenic food (related to food and trash handling), two scenarios that primarily involve adults, whereas most of the predatory attacks in which victims were prevalently children25 were carried out by cougars and coyotes. Additionally, bears attacked more frequently at night, when children are less likely to be found outside than adults”. These patterns also reflect differences in the species’ ecology. While bears are omnivores, cougars are strictly carnivore and coyotes, although they are known to forage on other resources as well26,27, are also mainly carnivore. Consequently, we can expect cougars and coyotes to be involved in predatory attacks (and, therefore, attack children) more likely than bears.

The presence of dogs at the moment of the attack was the most prevalent scenario, followed by attacks related to anthropogenic food, predatory motivation and other kinds of scenarios (Supplemental Fig. S2d). Cougar and polar bear attacks were all predatory. Victims of predatory attacks were mainly children (84%), and coyotes were responsible for the majority (63%) of these attacks. People involved in attacks related to dog presence were all adults, which represented the majority of the victims of food-related attacks as well. The high incidence of night attacks when the presence of a dog is involved is probably linked to the late walks that dog owners take in urban areas due to their work schedules and locally hot temperatures during the day28,29.

There was only a slight difference between the number of male and female victims (Supplemental Fig. S3b). Most of the victims of black bear attacks were alone, while coyotes attacked unaccompanied people and children in a party nearly equally (Supplemental Fig. S3c).

Landscape structure and artificial light at the site of the attacks

The exact location of the attacks was available for coyotes and black bears only (n black bear = 22, n coyote = 47) and, of these attacks, 15 occurred at night (n black bear = 7, n coyote = 8). Our results were consistent with our initial hypothesis. Indeed, the PCA (Supplemental Tables S1A and S1B) showed a clear difference between attacks by coyotes and black bears in terms of landscape structure (Fig. 2). On one hand, black bear attacks occurred in areas with specific landscape conditions, i.e. (a) few buildings and roads, and (b) dense vegetation cover, which is in line with the ecology of the species both in wildlands and urban areas11,30,31, as well as with previous studies which have analyzed the spatial distribution of other types of human-black bear conflicts2,4,7,32. These studies suggested that the probability of conflicts with this species was correlated with proximity to large forest patches and intermediate housing densities. This is probably related to the fact that black bears are predominately a forest obligate species30,32, although they have been shown to increase selection for human developments during poor food years and the hyperphagia period (summer-fall)21,31.

Figure 2 (A) Outcome of the PCA run on the 5 landscape variables for black bears (n = 22) and coyotes (n = 47), the species for which the exact location of the attacks was available. Each point represents one attack and arrows show the direction of the variables considered, with variable values increasing according to the direction of the arrow. PC1 and PC2 explained 81.7% and 10.7% of the variance, respectively. (B) Boxplots depicting how values of each landscape parameter differ between the two species considered (the coyote photo was downloaded from 123RF ROYALTY FREE STOCK PHOTOS, www.123rf.com, Image ID 52238509, copyright Koji Hirano, https://www.123rf.com/stock-photo/close_up_image_of_coyote.html?oriSearch=a+portrait+showing+the+expressive+eyes+of+a+large+black+bear++ursus+americanus++in+the+mountians&sti=mshqs5mkzdbtehq90h|&mediapopup=52238509; the black bear photo was downloaded from 123RF ROYALTY FREE STOCK PHOTOS, www.123rf.com, Image ID 69859949, copyright cuttsnaturephotography, https://www.123rf.com/stock-photo/a_portrait_showing_the_expressive_eyes_of_a_large_black_bear_(ursus_americanus)_in_the_mountians.html?&sti=mz65utjpox4ztzb2zg|&mediapopup=69859949). Full size image

On the other hand, coyotes, a species known to be able to adapt well to urban areas and to tolerate high levels of human disturbance33, attacked in a wider range of landscape types than black bears, from areas with high and aggregated cover and few human structures, to extremely urbanized areas with little and fragmented vegetation. Specifically, most of the attacks by this species (~70%) occurred in areas where the vegetation cover was less abundant and more fragmented (i.e., divided into more and smaller patches), and with relatively more buildings and roads, while fewer attacks (~30%) occurred in areas with characteristics similar to those of black bears (i.e., more abundant and aggregated vegetation cover, fewer buildings and roads). Our results are in line with observed patterns related to other types of human-coyote conflicts, which have been shown to be more frequent in developed areas with intermediate housing densities and low vegetation cover than in areas with higher percentages of forest5,14. Additionally, Lukasik and Alexander18 found that more conflicts occurred where small parks, greenspaces and riparian habitats were present in areas with high human densities.

We found that those attacks that occurred at night took place in areas with a relatively low amount of artificial light (radiance always <6.00 W/cm2 * sr), with black bear attacks occurring in particularly dark areas (radiance values lower than 1.00 W/cm2 * sr; Supplemental Table 1C, Fig. 3). This outcome might be related to the recorded abundance of vegetation cover at the locations of black bear attacks. Indeed, we can expect that areas with high vegetation cover are also characterized by lower artificial light than intensely urbanized sectors.

Figure 3 Artificial light conditions for those sites where the attack occurred at night. (A) Frequencies of the different ranges of radiance for the two species for which exact locations were available, i.e., coyotes (n = 8) and black bears (n = 7). (B) Two 1 km2 plots centered at the point of the attack are presented as an example of the artificial light conditions analyzed. Each range of radiance is assigned a different color, from black (radiance <0.25, i.e., no artificial light) to dark-red (radiance >40, i.e., highest amount of artificial light)46 (see the main text for more details). The two images were obtained from the website https://www.lightpollutionmap.info. Full size image

Are there solutions for this increasing conflict?

The role of human behavior in the attacks

Dog presence: It is noteworthy that in at least 20 (66%) of the 33 attacks related to the presence of dogs, humans were not the first target. In these incidents, either the carnivore targeted the dog first and the owner intervened in its defense (most of the cases, 80%, n = 16) or the dog confronted the carnivore, with the owner being subsequently involved in the encounter. Improved public education by local authorities on how to behave with dogs in areas frequented by wild carnivores would certainly help increase public awareness and thus reduce the occurrence of these incidents. As also previously suggested14,16,17, keeping dogs on-leash while out walking in areas with carnivores would reduce the number of risky encounters. In the case of coyotes, scaring off the animal with the help of objects has been recommended by wildlife services16,19,34. Similarly, keeping dogs inside or in a well-fenced shelter in the yard might help to avoid predatory attempts when the owner is not directly taking care of their dog16. Our results suggest that, while for coyotes these precautions should always be taken, i.e. independent of the landscape structure and light conditions, in areas where black bears are present they might be particularly important when the vegetation cover is high and the density of human buildings is low. Additionally, particular attention should be taken at night, especially in areas where artificial illumination is scarce.

Attractants management: The insufficient management of anthropogenic food such as pet food, bird feeders and garbage, both in private properties and public parks, together with the practice of wildlife feeding, are already known to be among the most common causes of human-carnivore conflicts15,19,20,35. The proper management of attractants is even more important within urban areas, due to the high number of people potentially exposed to a risky encounter with a wild carnivore. Although significant effort has been made to inform and educate the public on how to reduce attractants, and wildlife feeding has been forbidden in many cities19,36, the increasing trend of attacks indicates that current efforts might not be sufficient and more resources should be invested in preventive actions. Additionally, while education and regulations alone might have little effect on changing human behaviour37,38, combining these actions with proactive enforcement (e.g. increased patrolling and application of warnings) might prove to be more efficient in altering human behaviour11,38.

Predatory attacks on children: Lone children are the preferred target of coyote, cougar and black bear predatory attacks. This kind of attack is the most dangerous and has already been documented in previous studies15,19,25. When outside, both in yards and green spaces, children should be continuously supervised by an adult, at a minimum, and never left alone. The presence of an adult may help to reduce the chances of a child being attacked. Additionally, fencing yards and playgrounds in areas where carnivores are present may be an effective precaution to increase child safety.

The role of landscape planning

Assuming that both human and carnivore populations will continue to rise in the future, we should expect an increasing overlap between urban areas and carnivore ranges and, therefore, an increase in the number of attacks. The sprawl of human developments towards natural habitats is rapidly rising and residential housing is expected to increase across the landscape, due to homebuyers’ preferences for single-family detached homes10,39. Moreover, the recent trend towards “greener” and wildlife-friendly urban landscape design is leading urban planners to promote the inclusion of natural patches and wildlife habitat requirements into the urban matrix9,40,41, which may create optimal habitats for some carnivore species. The presence of green spaces and the recent spread of the practice of “wildlife gardening” (i.e., employment of a series of practices aimed at increasing wildlife in gardens) have been shown to provide important benefits to both human health and wildlife biodiversity6,40,42. However, practices such as keeping dense vegetation and fruit-trees in yards and green areas, as well as leaving bird feeders outside, are likely to attract wild carnivores and, consequently, may increase the probability of a risky encounter5,11,32. These practices should then be avoided in urban areas with resident carnivore populations and/or located near carnivore habitats. Instead, reducing thick vegetation (e.g., dense forests or bushes) to increase visibility and prevent carnivores from using it as shelter, as well as the implementation of fences and improved artificial illumination systems in green areas and yards, can effectively result in increasing both human and pet safety (see also5,15,34).

Similarly, in areas scheduled for development, urban planners and homebuyers should be informed of the risk that low-density developments (i.e., sparse housing developments which incorporate large wildland areas) might involve10. These kinds of developments, which also include ex-urban and suburban areas, have already been shown to favor the colonization of urban areas by wild carnivores5,33 and present a higher concentration of human-wildlife conflicts, especially when situated in proximity to natural areas4,6,10,14. In this sense, in terms of land use, our findings support the “land sparing” model, which favors high-density developments in order to preserve wildland43,44. This kind of development might be an effective way not only to minimize habitat fragmentation in general, but also to exclude carnivores from urban areas by separating human developments from wildlife habitats and, thus, reduce the occurrence of negative interactions with these species. Finally, we suggest that further studies should investigate whether the attacks are more likely to occur in specific areas within the areas used by the species. This fine-scale analysis would require radiotagging of urban carnivores, which will allow comparing the characteristics of the urban sites where attacks may occur (our results) vs. the areas selected by these species.