Demographic Variables for Wild Asian Elephants Using Longitudinal Observations

Detailed demographic data on wild Asian elephants have been difficult to collect due to habitat characteristics of much of the species’ remaining range. Such data, however, are critical for understanding and modeling population processes in this endangered species. We present data from six years of an ongoing study of Asian elephants (Elephas maximus) in Uda Walawe National Park, Sri Lanka. This relatively undisturbed population numbering over one thousand elephants is individually monitored, providing cohort-based information on mortality and reproduction. Reproduction was seasonal, such that most births occurred during the long inter-monsoon dry season and peaked in May. During the study, the average age at first reproduction was 13.4 years and the 50th percentile inter-birth interval was approximately 6 years. Birth sex ratios did not deviate significantly from parity. Fecundity was relatively stable throughout the observed reproductive life of an individual (ages 11–60), averaging between 0.13–0.17 female offspring per individual per year. Mortalities and injuries based on carcasses and disappearances showed that males were significantly more likely than females to be killed or injured through anthropogenic activity. Overall, however, most observed injuries did not appear to be fatal. This population exhibits higher fecundity and density relative to published estimates on other Asian elephant populations, possibly enhanced by present range constriction. Understanding the factors responsible for these demographic dynamics can shed insight on the future needs of this elephant population, with probable parallels to other populations in similar settings

Strategies of protected area use by Asian elephants in relation to motivational state and social affiliations

Animals’ space requirements may vary according to life-history and social considerations. We observed 516 wild adult Asian elephants from both sexes, over 9 years, to investigate how life-history traits and social behavior influence protected-area (PA) use at Udawalawe National Park, Sri Lanka. Male PA-use, quantified in terms of average between-sightings-interval (BSI), was significantly influenced by the interaction of age class and motivational state (i.e. reproduction vs. foraging). Musth lengthened with age, with a median of 24.5 days for ages 21–30, 32.5 days for ages 31–40, and 45 days for those \u3e 40. A minority (11%) used it exclusively during musth, while others used it exclusively for foraging (44%) or both (45%). Males using it in both states and older musth-only males were more likely to be seen across years. There were 16 social communities containing between 2–22 adult females. Females’ BSI was significantly influenced by social ties, but this relationship was weak, because members of social communities do not necessarily disperse together, resulting in high individual variation in space-use. Inter-annual variability in sightings among individuals of both sexes indicates that around ¾ of the population is likely non-residential across years, challenging the prevailing fortress-conservation paradigm of wildlife management

Female no. 33 (‘Tailless’), reference female for oldest age class.

<p>This unique female, who was distinctly identifiable even after death due to her tail which was broken at the base, was among the oldest individuals observed in this population. Top panels: Tailless in 2008 exhibits the sunken forehead and cheeks as well as reduction in breast mass characterizing the >60 yr age class. Bottom panels: both upper and lower molars exhibit severe wear with no signs of additional teeth to emerge.</p

Fecundity averaged over births observed from 2006–2012 for N = 280 females.

<p>The average ranges between 0.13–0.17 female offspring per individual per year. Data are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082788#pone.0082788.s007" target="_blank">Table S4</a>.</p

Estimated (a) ages at primiparity and (b) inter-birth intervals (IBI).

<p>Curves represent the cumulative probability that a female gives birth to her first calf at a particular age (a), or exhibits an IBI of a specified length (b). Histograms show counts of observed ages at primiparity and birth intervals respectively. Data are included as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082788#pone.0082788.s005" target="_blank">Tables S2</a> & <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082788#pone.0082788.s006" target="_blank">S3</a>.</p

Uda Walawe National Park.

<p>The study was conducted along the road network in the middle portion of the park, represented by the hashed lines (<100 Km²). Images sources: NASA Earth Observatory (<a href="http://earthobservatory.nasa.gov" target="_blank">http://earthobservatory.nasa.gov</a>), the Department of Wildlife Conservation, Sri Lanka, and the Center for Conservation Research, Sri Lanka.</p

Estimates of inter-birth-intervals with datasets ordered by decreasing accuracy.

<p>Estimates of inter-birth-intervals with datasets ordered by decreasing accuracy.</p

Injuries and mortalities by sex and cause.

<p>Anthropogenic causes - <i>Poison</i>: ingestion of toxic substance, usually hidden in food items such as pumpkins. <i>Snare/knife</i>: wounds that encircle or cut into an appendage from wire or rope. <i>Shot</i>: gunshots and swellings from gunshots. <i>Well</i>: abandoned water wells in which animals can fall. <i>Separation</i>: death of a calf due to death or disappearance of the mother after a human-caused injury. <i>Burn</i>: skin lesions caused by fire. Natural causes – <i>Old age/disease</i>: disappearance or confirmed death within the oldest age class, or known illness. <i>Premature</i>: calf born prematurely. <i>Emaciation</i>: extreme loss of body mass. <i>Jaw injury</i>: swelling or distortion of the lower jaw. <i>Separation</i>: separation of a calf from the mother without any sign of injury to the mother. <i>Other/unknown</i>: unknown circumstances, here grouped with ‘natural’ causes but human influence cannot be excluded. Includes the cases of isolated calves, where reasons for the absence of the mother were not known.</p

Annual totals of recorded births and deaths.

<p>Although there are fewer mortalities than births observed, this should not be taken to indicate a growing population. Most carcasses go undetected due to the dense cover. Moreover chances of detection are likely to be even lower for individuals near the age of dispersal, because disappearance through death vs. dispersal are not distinguishable in this study.</p

A comparison of demographic variables across populations.

<p>Fecundity is given in terms of the average number of female offspring per breeding female per year. Adult sex ratios for captive populations are not given since wild-caught individuals are included therein.</p>*<p>Inferred by Sukumar (2003) from age structure in original studies.</p