Do transmitters affect survival and body condition of American beavers Castor canadensis?

One key assumption often inferred with using radio-equipped individuals is that the transmitter has no effect on the metric of interest. To evaluate this assumption, we used a known fate model to assess the effect of transmitter type (i.e. tail-mounted or peritoneal implant) on short-term (one year) survival and a joint live—dead recovery model and results from a mark—recapture study to compare long-term (eight years) survival and body condition of ear-tagged only American beavers Castor canadensis to those equipped with radio transmitters in Voyageurs National Park, Minnesota, USA. Short-term (1-year) survival was not influenced by transmitter type (w_i = 0.64). Over the 8-year study period, annual survival was similar between transmitter-equipped beavers (tail-mounted and implant transmitters combined; 0.76; 95% CI = 0.45–0.91) versus ear-tagged only (0.78; 95% CI = 0.45–0.93). Additionally, we found no difference in weight gain (t₉ = 0.25, p = 0.80) or tail area (t₁₁ = 1.25, p = 0.24) from spring to summer between the two groups. In contrast, winter weight loss (t₂₂ = - 2.03, p = 0.05) and tail area decrease (t₃₀ = - 3.04, p = 0.01) was greater for transmitterequipped (weight = - 3.09 kg, SE = 0.55; tail area = - 33.71 cm², SE = 4.80) than ear-tagged only (weight = - 1.80 kg, SE = 0.33; tail area = - 12.38 cm², SE = 5.13) beavers. Our results generally support the continued use of transmitters on beavers for estimating demographic parameters, although we recommend additional assessments of transmitter effects under different environmental conditions.