Photo by Canon_Shooter - Creative Commons License
Source: Photo by Canon_Shooter - Creative Commons License

It was an unusually hot day and I was feeling uncomfortably warm. I rang the bell and as the door opened I could see the form of my colleague standing next to a large German Shepherd. The tall man said hello and motioned me into the house, which like most homes in my region, had no air conditioning. As I entered I bent down to offer a greeting to the dog, however instead of approaching he gave me a hard stare, his ears slid down a bit, and the skin on the top of his muzzle began to wrinkle a bit, suggesting that he was not feeling friendly and that I should back off and not try to touch him. My colleague shooed the dog back a bit and then observed, "Sergeant does not do well in the heat. When it becomes warm enough he becomes really edgy and snappish.

"You know before I got my doctoral degree and became a university faculty member, I was a cop in Cleveland. Whenever the weather would get hot we would brace ourselves because we knew that the number of calls for assaults, domestic incidents, and even heavy-duty violence involving weapons, was likely to be exceptionally high that day. It's not just some kind of a myth among police, but there is supposed to be some good data suggesting that hot weather brings out violent tendencies in people. I always wondered whether that was the case for dogs. It certainly seems to be that way for Sergeant."

My colleague was correct about the fact that there is evidence that hot weather is associated with increased levels of violence in humans. One way to study such a relationship is to compare hotter and colder geographical regions. The results show that violent crime is substantially higher in the hotter countries. Even confining the data collection to regions within a single country shows a similar pattern. One study looked at 260 US cities and found that average annual temperature was a strong predictor of violent crime such as murder, rape, and assaults.

Another way to study the relationship between temperature and violence is to look at temperature variations with in a region. Thus one study looking at regions in the United States found that violent crime rates were 35 percent higher in the summertime.

Perhaps the most graphic illustration of the relationship between temperature and crime was not provided by a scientist, but rather by a Chicago blogger and web designer, Eric Van Zanten. He was motivated to look at the question, not as scientific research, but to simply see how well the open access databases worked in his city. He took all of the data between 2001 and around 2012 which was available from the Chicago Police Department. This resulted in him scoring around 5 million crime reports. He then grouped them by the reported high temperature for that day. Ultimately he was able to gather information about the number of crime reports (for many different crimes) and to show how it related variations in temperature. I have re-plotted his data and that graph appears below.

Based on data from Eric Van Zanten (2013) - SC Psychological Ent. Ltd.
Source: Based on data from Eric Van Zanten (2013) - SC Psychological Ent. Ltd.

Basically the result shows that the total number of crimes increases with rising temperature up to somewhere around 90° F (32° C) and then the rate drops off. This seems to be the pattern that a number of other researchers have found, with only the exact point of the high temperature drop-off tending to vary. A recent study looking at assaults in New Zealand concluded that for every 1° C of rise in temperature there is approximately a 1.5 percent rise in the number of assaults. The argument is that increased temperature results in greater discomfort and irritability which then lowers the threshold for violence.

There is now evidence that dogs are not immune to these same temperature effects. The data was provided by a group of investigators headed by Yongming Zhang of the Department of Respiratory and Critical Care Medicine at the China-Japan Friendship Hospital in Beijing, China. This team used the number of dog bites as their measure of canine aggression. China is a good place to conduct such an investigation because according to a 2016 World Health Organization report, China has recently become the country with the largest number of homeless dogs in the world. This increase in dog population has been associated with a large increase in the number of dog bites. In this recent research the team collected data on the frequency of emergency room visits for dog bite injuries from the Navy General Hospital. This is a large hospital and over that time period they managed to gather a huge sample of data representing 42,481 dog bites.

Given the size of this data bank it is not surprising that the investigators conducted a substantial number of complex statistical analyses. A number of these may be quite difficult to plow through for the average reader who might not have a lot of education in statistical procedures. However the most important question that we started out trying to answer, namely, "Does the aggressiveness of dogs rise as the temperature gets hotter?" produced an unambiguous answer.

First it is important to note that Beijing has a temperate climate with four distinct seasons which produce large variations in temperature over the year. July is the hottest month in Beijing with an average temperature of 27°C (81°F), so it is likely that there will be only a few days during the hot months where the temperature will rise to such a point that we might expect the reduction in aggression that we see in humans at extremely high heat levels. Still there is a lot of day-to-day variability in the temperature and a simple data graph (that I have re-plotted from this experimental report) demonstrates that there is a strong relationship between daily temperatures and people reporting to hospital for dog bites.

Based on data from Yongming Zhang et al (2017) - SC Psychological Ent. Ltd.
Source: Based on data from Yongming Zhang et al (2017) - SC Psychological Ent. Ltd.

So once again the similarity between the behavior of dogs and the behavior of humans seems to be demonstrated (even for some of our less desirable behavior tendencies). Just as we humans get edgy and are easily annoyed when the temperature climbs, so do dogs. We humans can try to reduce the annoyance level by trying to find a cooler place, or failing that at least to have something like a cold beer — but of course alcohol is bad for dogs. So I suppose that when the weather starts to reach the higher levels of discomfort perhaps the best thing to do is to toss your dog a few ice cubes to chew on and then just leave him alone, keeping kids or strangers away from him until the evening, when things will naturally cool down a bit and canine tempers are less likely to flare.

Stanley Coren is the author of many books including: Gods, Ghosts and Black Dogs; The Wisdom of Dogs; Do Dogs Dream? Born to Bark; The Modern Dog; Why Do Dogs Have Wet Noses? The Pawprints of History; How Dogs Think; How To Speak Dog; Why We Love the Dogs We Do; What Do Dogs Know? The Intelligence of Dogs; Why Does My Dog Act That Way? Understanding Dogs for Dummies; Sleep Thieves; The Left-hander Syndrome

Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission


Eric Van Zanten (2013). As the temperature in Chicago rises, so does the crime rate?

Yongming Zhang, Qi Zhao, Wenyi Zhang Wo, Shanshan Li, Gongbo Chen, Zhihai Han, Yuming Guo (2017). Are hospital emergency department visits due to dog bites associated with ambient temperature? A time-series study in Beijing, China. Science of The Total Environment, 598, 71–77.

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