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Age-specific differences in Asian Elephant defecation, dung decay, detection and their implication for dung count

Citation

Ashokkumar, Mohanarangan (2022), Age-specific differences in Asian Elephant defecation, dung decay, detection and their implication for dung count, Dryad, Dataset, https://doi.org/10.5061/dryad.1vhhmgqvs

Abstract

In vertebrate population estimation, converting faecal density into animal density requires information on faecal production rate, decay rate, and faecal density.  Differences in the above factors for long-lived species across age classes were not evaluated.  We have evaluated these factors associated with the dung count of Asian elephants (Elephas maximus) in the tropical forest of Southern India.   

The defecation rate of elephants was determined in semi-wild elephants at the Mudumalai elephant camp. The relationship between dung bolus diameter and age was determined to estimate the age of the elephant. Total and age-specific elephant density based on dung bolus diameter were estimated. A total of 24 transect lines of 2-4 km (125 km) were sampled in the study area. An experiment was conducted to assess detection probability across the age classes of dung piles. The dung decay rates across age classes and seasons were determined by marking fresh dung piles (n=1551). The dung-based age structure assessment and its limitations were evaluated.    

The mean defecation rate was 13.51±0.51 per day. The defecation rate was significantly lower for the younger age class and increased with the age of elephants. Defecation rates were significantly lower in the wet season than in the dry. The dung-boli diameter positively increased with the age of elephants, and the growth curve can be used to predict the age and age structure of the elephant population. 

The disparity in the dung production rate results in the lower availability of younger age class (Juvenile and calf) dung in the transect for counting, that results in lower dung abundance.  The detection probability of dung piles of younger age classes was low (0.58). The survival rates of dung piles of younger age classes were lower and increased with the age of elephants in the wet season. Hence, demography assessment of the population based on dung needs to consider age-specific differences in dung production, decay, and detection probability. Through demography assessment using dung provides insight into population age structure, it has limitations in predicting age structure for young elephants. 

Methods

i. Defecation rate

The defecation rate of semi-wild Asian elephants was gathered for different age-sex classes at the forest elephant camp at Mudumalai. 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, respectively (Table-1).  Each elephant was followed for three consecutive days to quantify the defecation rate, thus resulting in a sampling effort of 504 and 612 hours of observation in the dry and wet seasons, respectively.  Elephants were observed by the focal animal sampling method (Altmann, 1974) during the daytime for twelve hours, from 6:00 hours to 18:00 hours. All the activities were recorded with ten minutes of observation and a five-minute interval.  But the defecation occurring in the observer’s interval time was noted, but other activities were not recorded. To determine the defecation rate, the interval between two defecations was averaged, i.e., If a total of 10 defecations were observed over 10 hours (observation duration from first defecation to last defecation), then an average of 9 intervals (n-1) was calculated (10hr/9 intervals = average defecation interval, x). The daily defecation rate was calculated by 24/x. Thus, the defecation rates were calculated based on daytime (12 hour) observations.

ii. Dung bolus size and age relationship

Measurements of dung bolus circumference were collected while observing elephants for estimation of defecation rate in the above-mentioned methods. A total of 17 males and 10 female elephants’ dung-boli measurements were used. At each defecation, the largest intact boli circumference was measured.  The mean dung-boli diameter is calculated from the dung-boli circumference with the equation dung-boli diameter (d)=circumference (c)/p.

The Von Bertalanffy growth equation (VBGM) is useful for fitting vertebrate growth data (Ebert, 1999) and has been used in modelling elephant growth (Lee and Moss, 1995; Reilly, 2002; Morrison et al. 2005). The VBGM was used to construct the growth models and is defined by the growth parameters L, K and t0 and the length measurement L (Von Bertalanffy 1938; Eqn. 2) in this study was the mean dung-boli diameter to interpolate the growth of elephant. 

iii.   Age structure

Dung size has been correlated to the size (height) of elephants and consequently, to estimate age class (Reilly, 2002, Morrions et al., 2005). To determine the age from dung bolus diameter (calculated from circumference measured in the large end of dung bolus), the growth parameters were rearranged in the VBGE equation (Eqn. 3) to predict the mean age (t) from dung bolus diameter (L) measured in the dung transect.   

As the asymptotic size is reached, the sensitivity of the growth model to changes in diameter increases greatly, such that a small increase in the diameter could indicate a large increase in age.   Therefore, the dung bolus diameters greater than 15 cm, which was an asymptotic size were grouped together and denoted as 20+ years. 

iv.  Detection of dung piles

Detection of various age-class and size classes of dung at a different perpendicular distance was measured using an experimental two belt transect in tropical dry deciduous forest with a length of two kilometers and a width of 25m on either side of the transect. The age class of dung piles was determined based on dung (intact boli circumference measured at the large end) measurements. Initially, dung piles that were visible from the transect were counted and marked with calcium carbonate powder to identify them as detected dung piles.

After completion of the transect count, the dung piles that were present within a width of 25m on either side of transects that were not detected from the transect were recorded as missed dung piles. A 100m rope was kept in the middle of the transect, and then the entire area was searched by four observers walking at a five-meter interval on one side of the transect. All missed dung piles in the transect were recorded with details of perpendicular distance, the extent of dung spread, i.e. length and width of dung spread, the presence of boli, and dung boli circumference were measured. Then the other side of the transect was surveyed using the same method. The proportion of dung piles detected at different perpendicular distances was calculated by dividing the number of dung piles detected by the sum of the number of detected and non-detected dung piles at a particular distance. 

Dung pile status i.e., observed or missed were coded as 1 and 0, respectively. Differences in detection were tested against independent predictors, i.e., perpendicular distance, age class (adult, sub-adult, juvenile and calf) of dung were tested using binary logistic regression using ‘glm’ function in R Software

v.  Dung survival rate

The elephant herds were located, tracked and fresh dung piles (less than six hours old) were marked in all three habitat types.  Every month, an average of 125±77 fresh dung piles/month were marked using numbered bamboo stakes from Jan 2007 to Feb 2008. The variables such as geographic location, age class estimated based on dung circumference, grass composition, canopy cover and total length and width of dung spread were noted.  Every month, dung piles were marked in different habitats and revisited every 15 days to assess the status of dung piles (Fig.1).  To estimate the survival rate of dung piles, based on retrospective method dung piles were examined one day before the survey during dry and wet seasons (Laing et al., 2003; Hedges and Lawson, 2006).  

vi. Age-specific elephant density

Dung density was determined using the indirect dung count method. A total of 24 transects of two to four kilometres resulting in a total of 125km (56.5km in dry season and 68.5 km in wet season) distance were walked (Fig.1). Transects were placed randomly in the study area to get adequate spatial coverage and proportional representation of three habitat types, similar to an earlier study (Baskaran, Udhayan and Desai, 2010). Dung piles that are visible from the transect were counted, and the perpendicular distance was measured using a measuring tape. The dungs were categorized into 'S-system’ based on the stage of decay of dung piles (S1-all boli intact; S2- one or more boli intact; S3-No boli intact with coherent fragments remain; S4-only traces of dung fragments remain) of the Mike dung pile classification system (Hedges and Lawson, 2006). All the dung piles encountered (S1 to S3) in the transects used to estimate dung density were measured. The largest circumference of an intact bolus in a dung pile was measured.  If all the dung boli were not intact, the corresponding age class was denoted as unknown.  The survey were completed within a month (July-2007-wet and March 2008 – dry). 

The Distance 7.2 software was used to estimate overall and age-specific elephant densities (Buckland et al., 2001; Thomas et al., 2010). The age of elephant was determined based on the dung bolus diameter encountered in the transect using Eq. 3. The number of detections of dung piles across specific age was less, and hence they were grouped into 1-3, 4-7, 8-11, 12-15, 16-19, and 20+ age categories to estimate densities. The defecation rate estimated using prediction equation 1 and the mean decay rates for specific age classes were entered separately as multipliers to obtain density for specific age class.

Funding

U.S. Fish and Wildlife Service, Award: AAA No. 98210-6-G113