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demogr viability of the largest population of greater one-horned rhinoceros in Nepal
Demography and viability of the largest population of greater one-horned rhinoceros in Nepal

2.4. Survival and annual mortality rate
Most individually known rhinos were located two or more times in a month. We could therefore determine the birth week of most calves with more intensive observations on pregnant females nearing parturition. We monitored radio-collared females on a daily basis and could observe birth and growth of their calves in more detail. Based on these observations and the experience of some of our field technicians who had been working with rhinos for over three decades, age of calves whose birth week was not known with certainty was determined using criteria of size relative to its mother, skin folds and texture, pigmentation, body hair, coordinated movements, suckling and foraging behaviour (Fig. S1). We checked the accuracy of our aging criteria on known aged calves and found that our team could accurately estimate birth date of calves less than two months old to a week. By the end of the study period, age of over 75% of the individually known rhinos was known since birth.
Due to intensive monitoring of the study area by the park staff, army personnel, and rhino monitoring teams we were reasonably certain that all mortalities of rhinos were detected within a couple of days. Post-mortem examinations were undertaken on all rhino carcasses by a veterinary officer. Adult and sub-adult rhino mortalities were categorized into natural, humancaused and unknown causes. Calf mortality causes were categorized as tiger predationinfanticide, human caused and unknown. Mortality data from the study period was combined with mortality recorded since 1998 across Chitwan National Park for analyzing causes of death.
We used six stage categories (Dinerstein and Price, 1991Kandel and Jhala, 2008) that were biologically meaningful in terms of rhino demography (Law and Linklater, 2014): calf (<1 year), juvenile (1–4 years), sub-adult (4–6 years), young adult (6–12 years), prime adult (12–20 years) and old adult (>20 years) since field aging all rhinos to exact years was unrealistic. We estimated stage-specific annual and span survival probability of rhino through known-fate models (Skalski et al., 2005) in program MARK (White and Burnham, 1999Cooch and White, 2009) using the staggered entry design(Pollock et al., 1989; see Table S1 and Table S2). This technique can be used where the fate of an individually identified rhino is known with certainty and independently. We grouped encounter histories of rhinos into time intervals of six months and created a live-dead matrix where ‘10’ meant the individual lived through the interval, ‘11’ meant the individual died during the interval and ‘00’ meant censoring the individual for that specific interval (when unaware about the fate due to non-sighting; Cooch and White, 2009). Rhinos monitored before adulthood, and that lived sufficiently long to enter the next stage, were included in all appropriate stages with the assumption that survival rates in different stage class of an animal were independent. We assumed total life span of rhino as 35–40 years in the wild for span survival analysis based on the long-term records made in Chitwan National Park (Subedi, 2012). We estimated rhino survival by monitoring 80 adult and sub-adult rhinos (nine radio-collared and 71 non-collared but individually known), and calf survival by monitoring 30 rhinos (Tables S1 and S2). We tested for effect of stage groups and gender on survival rates in program MARK. During the course of our study factors likely affecting mortality were similar across years; no extraordinary events of flooddrought, or poaching occurred. We therefore, do not test the effect of individual time intervals on rhino survival and assume a constant survival rate within each gender for each stage across years. Also our sample size was relatively small for a meaningful analysis of complex models that included time and its interactions with sex and stage. We lumped the five stage groups (excluding calves) further when survival was similar between stages and gender for a more parsimonious and robust estimate of survival and compared models using AICc in MARK. Calve survival was estimated with time interval of months in a separate analysis. Models that included effect of sex and months on calf survival were compared using AICc. We report AICc weighted model averaged estimates of survival for all stages (Burnham et al., 2011) for biologically meaningful models whose likelihood estimates converged so as to provide robust estimates of survival. Annual, stage span and cumulative survival were reported.

3.3. Mortality

A total of 374 rhino deaths were recorded in Chitwan National Parkbetween 1998 and 2015, of which 49% were male, 35% female and 16% unsexed. Of the total mortalities 83% were adult, 4% sub-adult and 13% dependent young. Average annual recorded mortality was 21.83 ± 0.66 SE individuals of which poaching caused mortality was 10.05 ± 0.73 SE individuals and was the main cause of rhino mortality (Fig. 2.) in Chitwan National Park. Amongst the 47 calf deaths recorded, majority (29.8%) were due to tiger predation, disease was responsible for 10.6% of the deaths, while infanticide, natural calamities and unknown causes each contributed 19.1%, poaching of the mother and subsequent death of the calf accounted for only 2.13% of calf mortality. Natural calamities included floods, drowning in muddy swamps and death by tree fall.

Megaherbivores exhibit typical K-selected and slow life history traits (Owen-Smith, 1992). Calf mortality amongst mammals with high parental care is usually low. Rhino mothers are extremely protective of their calves and the only threat to their survival is from tiger predation, infanticidal males, catastrophic events like floods, and death of mother (Laurie, 1982Dinerstein and Price, 1991). The estimates from this study are not directly comparable to Dinerstein (2003) since that study did not distinguish between yearling and juvenile age groups. However, after combining the survival estimates of yearling and juvenile rhinos, the estimates of this study are almost identical to those reported by Laurie (1982; 73% compared to 75% this study). It is interesting to note that rhino calves were most vulnerable during their first year of life (23% mortality), after which survival of even juvenile rhinos was high (94–98%). The variability associated with our estimate of survival across all age groups was small. Our mortality estimates included poaching which is additive to the natural mortalitycauses and primarily targets adults. Despite inclusion of poaching caused mortality Chitwan rhinos exhibited a Type I survivorship curve which suggests that current rate of poaching mortality of 5.5 rhinos per year was not a major cause of concern since it did not cause a linear decline in the survivorship curve (Type II curve). Over a longer term an average of 10 rhino deaths were attributed to human caused mortality, this is about 1.8% of the current rhino population (Subedi et al., 2013) and 47% of all recorded mortality. From 28 mortality events Dinerstein and Price (1991) estimated mortality due to poaching as 15%. Laurie (1982) recorded 27 rhino mortalities between 1972 and 1975 of which 30% were due to poaching. Currently poaching is responsible for nearly half of the total observed rhino mortality in Chitwan while other causes like intraspecific fight among males and floods had less impact.

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