DISCUSSION
Our isotopic approach revealed the spatial distribution pattern of crop-foraging female sika deer. Female deer in proximity to agricultural crop fields during the winter and spring were more likely to be crop-foraging individuals (Fig. 2). Possible reasons why crop-foraging deer distributed in proximity to agricultural crop fields as follows. Although seasonal migration is well-known in cervids inhabiting middle- to high-latitude regions (Mysterud, 1999; Igota et al., 2004; Monteith et al., 2011; Takii et al., 2012a, b), sedentary behavior has also been observed in accordance with food availability (Kufeld et al., 1989; Igota et al., 2004). In general, the availability of both crops and natural food resources is low during the winter and early spring. However, crops do not completely disappear. In fact, deer aggregate in sown grasslands to consume pasture grasses under snow during the snowy season (Hata et al., 2019), and many leftover vegetables remain in agricultural crop fields for long periods (Hata personal observation). Therefore, deer that are more likely to consume crops might tend to distribute in proximity to agricultural crop fields for crop consumption, even during the winter and spring. In fact, it was reported that two female sika deer tracked with Global Positioning System (GPS) inhabited areas near agricultural crop fields throughout the year (Ishizuka et al., 2007).
The likelihood of crop consumption by females decreased by half as the distance to agricultural crop fields increased to 10 km and fell to essentially zero at a distance of approximately 40 km (Fig. 3). Accordingly, crop-foraging female deer are more likely to distribute within this range. The width of the range may depend on behavioral variation among deer, such as migration. To elucidate the determinants of behavioral differences among crop-foraging individuals, further research is needed with consideration of additional factors, such as the response to hunting risk (Kilgo et al., 1998; Kamei et al., 2010; Lone et al., 2015; Little et al., 2016), genetic inheritance (Mueller et al., 2013), and social learning (Hopkins, 2013). Nevertheless, our findings can be useful to allocate management efforts such as culling at the appropriate spatial scale to mitigate conflicts that arise from crop consumption by deer.
In contrast to our results for female deer, there was no significant trend in the spatial distribution pattern of crop-foraging male deer (Fig. 2). As with many mammals (Pusey 1987), male deer show a greater tendency to disperse from their natal area than female. In fact, dispersal movement has been observed in juvenile male sika deer (Yamazaki & Furubayashi, 1995; Takii et al., 2012b). Takii et al. (2012b) reported that some males dispersed at 1-2 years old with distances of 3.0 to 40.3 km. In our data, eight males were 1-2 years old (Table S1). Even if they inhabited nearby agricultural crop fields with their mothers and consumed agricultural crops when they were young, they might disperse distant other habitats. The differences between sexes in movement behaviors might affect spatial distribution of crop-foraging deer. The small sample size might also have resulted in the large variation in males.
Our results for the spatial distribution patterns of crop-foraging female deer provide a basis for management at an appropriate spatial scale to mitigate conflicts. Because crop-foraging female deer were more likely to distribute within 10 km of agricultural crop fields, the establishment of management areas, including zonation, based on this result might be effective. Intensive culling in this zone would allow for effective removal of crop-foraging female deer. Culling crop-foraging female deer is expected to suppress deer population growth because crop consumption induces precocious maturity and may promote population growth (Hata et al., 2021). Moreover, intensive culling using zonation can prompt deer to flee from nearby agricultural crop fields because deer avoid hunting activity by altering their behavior and habitat utilization (Kilgo et al., 1998; Kamei et al., 2010; Lone et al., 2015; Little et al., 2016). In the future, it is necessary to evaluate the effectiveness of intensive culling in the management areas to decrease crop-foraging deer and subsequent crop damage and ecological impacts.