Hot days, angry dogs: How environmental factors influence canine aggression

By Dr. Chinta SidharthanReviewed by Benedette Cuffari, M.Sc.Jun 19 2023

In a recent study published in Scientific Reports, researchers explore the association between an increase in dog bite incidents in eight cities in the United States and environmental factors such as temperature, ozone, and air pollution with particulate matter below 2.5 microns (PM_2.5), while adjusting for seasonal factors, ultraviolet (UV) irradiation, precipitation, and calendar.

 Study: The risk of being bitten by a dog is higher on hot, sunny, and smoggy days. Image Credit: Nitiphonphat / Shutterstock.com

Background

Aggression is a behavior that can be observed across species and often serves as an adaptive advantage in protecting oneself or members of one’s pack or tribe and in competing for resources, territory, or mates.

Aggression is believed to be linked to an imbalance between the hyper-responsivity brought about by anger-provoking stimuli in the limbic regions and top-down control in the prefrontal regions. External factors have also been known to influence aggression across species, with aggression in humans often rooted in complex sociological and psychological factors.

Research indicates that high temperatures correlate with increased aggression among species such as mice, rats, Rhesus macaques, and humans, as well as between species such as dogs and humans. Analyses of air quality and crime rates indicate that exposure to PM_2.5 and ozone is also linked to increased violent crimes among humans.

However, the link between various environmental factors, including air pollution, and the incidence of inter-species aggression, such as dog bite incidents, remains unknown.

About the study

In the present study, researchers used public records from eight U.S. cities on incidents of dogs biting humans. Statistics indicate that dig bites comprise 0.3% of the total emergency department visits and often result in trauma, cosmetic disfigurement, and amputation of fingers, with severe dog bites sometimes causing craniofacial injury and fatality.

The identified risk factors for dog bites include factors specific to dogs such as breed, spaying or neutering status, sex, and victim-related factors such as gender, age, the behavior of the victim, and familiarity with the animal.

The researchers also obtained daily averages of PM_2.5 and eight-hour maximum ozone (in parts per million) from all the Air Quality System monitors belonging to the Environmental Protection Agency in all the cities included in the study.

Maximum daily temperatures and precipitation data were gathered from the Climatology Network, which is part of the National Oceanographic and Atmospheric Administration. UV index data were also obtained.

The values for precipitation, daily maximum temperature, ozone, PM_2.5, and UV index were standardized to homogenize for exposure effects. The zero-inflated Poisson generalized additive was also applied to the data to account for the day-to-day variations in these exposures, given that the dog bite incidents were zero-inflated since there were days with no incidents reported. The analyses were also stratified by non-winter and winter months to examine the effect of seasons on the incidence of dog bites.

Results

The number of incidents involving dogs biting humans increased with increasing ozone and temperature; however, air pollution involving PM_2.5 did not appear to influence aggression in dogs. Comparatively, high levels of UV irradiation were linked to an increase in the incidence of dog bites. The association between dog bite incidence, ozone, and UV radiation remained consistent across the non-winter and winter months.

The sensitivity analyses also indicated that the individual associations between the incidence of dogs biting humans and factors such as temperature, precipitation, ozone, and UV levels were stable and not influenced by the co-variance between these variables. The findings on increased aggression in dogs with increasing exposure to UV irradiation align with other studies that showed an increase in the steroid sex hormones in men and mouse models after exposure to UV light.

The link between aggression and ozone could be due to ozone entering the airways and subsequently triggering oxidative stress and impaired pulmonary function. In humans, this results in the activation of various messenger pathways, such as those involving interleukins and serum amyloid A, thereby activating the hypothalamic pituitary adrenal axis and influencing behavior.

Since neural circuits involved in aggression are conserved across the mammalian species, ozone exposure in dogs may result in dopamine turnover in the midbrain and striatum, similar to humans, subsequently leading to aggression.

Conclusions

Aggression in dogs appears to be influenced by environmental variables such as temperature, precipitation, UV radiation, and ozone; however, it does not appear to be affected by air pollution involving PM_2.5.