For many diseases, R 0 is expected to increase with host density [12, 13, 20, 21] . Despite the domestic dog population density in Serengeti (9.38 dogs/km 2 ) being considerably higher than the dog population density in Ngorongoro (1.36 dogs/ km 2 , see Table 3 ), we were unable to detect significant differences in our estimated values of R 0 between the two districts. Nor did we find any conspicuous differences in R 0 estimated from the outbreaks listed in Table 2 , which represent a wide range of population densities. There may, in fact, be no relationship between R 0 and population density for canine rabies. On the other hand, a subtle relationship between dog density and transmission rates might be difficult to detect for a number of reasons. To investigate whether it would be possible to decipher systematic differences in R 0 across the range of values that we estimated, we simulated outbreaks using our epidemiological parameter estimates, but varied R 0 (from R 0 ¼ 1 to R 0 ¼ 2), whilst maintaining individual variance in biting behaviour (same shape parameter k, see Text S2). Although the mean estimates of R 0 from fitting to these simulated trajectories were accurate, they were surrounded by wide confidence intervals ( Figures S2 and  S5 ), suggesting that if only a small number of epidemics were sampled, any underlying relationship might not be apparent.