Age-Specific Differences in Asian Elephant Defecation, Dung Decay, Detection and Their Implication for Dung Count

Abstract The various components of the dung count method of population estimation were evaluated such as defecation rate, decay rate, detection probability of dung and age-specific estimates of elephant density based on dung size. The defecation rate of elephants was determined in captive elephants of the Mudumalai elephant camp, Tamil Nadu, India. A total of 14 elephants in the dry season (Dec-Mar 2002) and 17 elephants in the wet season (Jun-Oct 2007) of different age-sex classes were observed for 42 days and 51 days by focal sampling methods and circumference of largest dung pile were measured to determine the growth curve. Total and age-specific elephant density based on dung circumference were estimated using indirect dung count method, 24 transect lines of 2 ~ 4km resulting in a total of 125km distance in Mudumalai Tiger Reserve. The dung decay rate was determined by marking fresh dung piles (n = 1628) every month with an average of 125 ± 77 dung piles/month from Jan 2007 to Feb 2008. The mean defecation rate was 13.51±0.51 (n = 94) per day. The defecation rate varied across age-sex classes and seasons with a lower defecation rate for younger age classes. The dung circumference measurements showed similar growth curves to other Asian and African elephants. Dung size-based age classification significantly overestimates the adult age class, giving an estimate of 82% adults in the population against the true 48.5%. Experimental analysis of detection of dung across perpendicular distance detection of larger and smaller dung size/age classes is equal at a visible distance and declined significantly for younger age/size class at the furthest distance. Decay rate varied significantly according to age classes and across seasons, with lower dung survival rates of younger age classes compared to adults. The minimum required sample size for the dung decay experiment was 250. The age-specific estimate of elephant density based on the indirect dung count method revealed a precise estimate on density, both decay and defecation contribute less than 12% of the variance of the estimate. Age composition based on dung count is highly skewed towards adults, younger age class were low due to lower defecation and faster decay and lower probability of detection in the transect.