Understanding Why Dogs Tend To Mate When It Rains | PLOS ONE

Long term data

Mating related behaviours (MRB) were observed in all four years during the monsoon [56]. In 2013, MRB were first observed in the month of June, while in all the other years the onset of mating was in the month of July. The occurrence of MRB reduced after the month of September. Hence observations spanned from June to September for 2013, and from July—September for the other three years. The frequency of MRB was strongly dependent on the precipitation levels on a given day (Linear Regression: R2 = 0.612, F = 223.694, p < 0.001; Fig 1). There was significant variation in the frequency of MRB across the years when the data for the period July—September was considered (Repeated measures ANOVA, F3,44 = 7.696, p < 0.0001). Post hoc analysis revealed that there was significant difference in the frequency of MRB between 2013 and the other three years, while there was no difference in the levels of MRB between 2010, 2011 and 2012 (Table 2). Interestingly, when the frequency of MRB for first three months after the onset of mating were considered, i.e., June to August for 2013 and July—September for the other years, there was no significant difference between the years within the same precipitation category (Repeated measures ANOVA, F3,44 = 1.670, p = 0.187). This suggests that the mating behaviour in dogs has some pattern that might be correlated with the precipitation pattern. In both the above cases, there was no significant variation in MRB within a precipitation category when the data was considered at a weekly level (Repeated measures ANOVA, F11,36 = 1.151, p = 0.353; F11,36 = 1.472, p = 0.185). For all subsequent analysis, the data for the June—August period of 2013. i.e. for the first three months after the onset of mating was considered.

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Table 2. Results of the post hoc Tukey’s test for Repeated Measures ANOVA carried out to check whether there was significant variation between the years (2010–2013) for the occurrence of mating related behaviours at different levels of precipitation (in mm) for three months from the onset of mating activities (July to September for the first three years, June to August for 2013).

https://doi.org/10.1371/journal.pone.0143501.t002

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Fig 1. The occurrence of mating related behaviours (MRB) was highly correlated with precipitation levels (mm).

The precipitation levels reported here are actual readings of precipitation on the day of observations, as given by IMD. The linear fit is given by the black line, represented by the equation y = 0.4888x + 18.893, R2 = 0.6117.

https://doi.org/10.1371/journal.pone.0143501.g001

The frequency of MRB was highest for the “high precipitation” category– 2703 acts of MRB were observed over the four years on days that received high precipitation, while this number was 778 and 252 in the medium and low precipitation categories. The variation in the frequency of MRB for different precipitation categories was significant (Kruskal-Wallis test: χ2 = 8.540, df = 2, p = 0.014). The full factorial ANOVA was highly significant (F59,660 = 43.082, p < 0.0001), with only the interaction between year and precipitation level being non-significant (Table 3). Post hoc tests revealed significant differences between all behaviours, and between all the three precipitation levels. But variation within the years was not significant (Fig 2, Table 3).

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Table 3. Results of a Full Factorial ANOVA to test for interaction between precipitation levels, occurrence of MRB and years of observation.

https://doi.org/10.1371/journal.pone.0143501.t003

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Fig 2. The frequency of MRB varied with precipitation levels.

The bar chart shows the mean and standard deviation of the frequency of all MRB occurring at different levels of precipitation (as noted during the time of observations), over four years, 2010 to 2013, during the primary mating season. Variation in the frequency of MRB was not significant across the four years within a precipitation category (alphabets), but varied significantly different levels of precipitation within a year (*).

https://doi.org/10.1371/journal.pone.0143501.g002

Since there was significant variation between the behaviours, an ANOVA with post hoc tests at the level of each behaviour was used to test for variations for occurrence of the behaviour at different levels of precipitation. For all behaviours other than MT, the frequency of occurrence of the behaviour was significantly different between all three precipitation levels, while the rate of MT was different only for the high precipitation category (S1 Table; Fig 3).

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Fig 3. All MRB showed variation across precipitation categories.

Mean and standard deviation of the frequency of different mating related behaviours observed at the three precipitation levels noted during the time of observations—the different alphabets represent significant differences within a behaviour category, between precipitation levels.

https://doi.org/10.1371/journal.pone.0143501.g003