Combining studies of elephant movement patterns and overall diet with incidences of …

• Stable isotopes in elephant hair document migration patterns and diet changes
• Materials and Methods
• Results and Discussion
• Acknowledgements
• References
• Figures

Biology Articles » Zoology » Stable isotopes in elephant hair document migration patterns and diet changes » Materials and Methods

Materials and Methods

- Stable isotopes in elephant hair document migration patterns and diet changes

Global Positioning System (GPS) radio collars were fitted to elephants in Samburu National Reserve, Northern Kenya, between 2001 and 2004 (14). Collars were programmed to record positions at hourly intervals, offering detailed records of movement. Tail hairs from 35 elephants were collected over this period during immobilization operations while the collars were being fitted and for some, later, when collar batteries were being changed or when the collars were being removed. Of these 35 elephants, 7 have a secure isotopic dietary chronology over the period from late 2000 and extending through February 2002; GPS spatial information is available for all resident individuals from July 2001 to July 2002 and from February 2002 to July 2002 for the migrant B1013. We focus on comparing the behavior of the migrant B1013 to that of the resident Samburu group during this time interval.

The elephants tracked in this study used both the semiarid region in and around Samburu National Reserve and the mesic Imenti Forest on the flanks of Mt. Kenya. These two distinct ecotones are east, just north of the equator. The elevation of the semiarid Samburu region is 900 m above sea level and dominated by acacia–comiphora savanna and scrub bushland. Rainfall averages 350 mm per year in this lowland region and occurs during biannual rainy seasons, which generally take place in April and November. The elevation of the Imenti Forest is 2,000 m above sea level and is dominated by evergreen and broad leaf deciduous tree species. Rainfall in this region also occurs biannually and averages 900 mm per year.

In this study, we used 10-day composite normalized differential vegetation index (NDVI) data, available through Satellite Probatoire d'Observation de la Terre (SPOT), to determine changes in season across the study area. NDVI is a remote sensing index value calculated as the ratio between red and near infrared reflection that is highly correlated with green (photosynthetically active) biomass (15, 16). Remotely sensed data provide a direct measure of photosynthetic activity over large spatial regions, offering advantages over the classically used point-sampled rainfall data in areas, like the study region, where weather stations are sparse. Isotope profiles for each elephant were compared with longitudinal 10-day NDVI records to determine the impact of seasonality on diet.

¹³C/¹²C and ¹⁵N/¹⁴N ratios of elephant hair and plant material were measured on an isotope ratio mass spectrometer after combustion in a flow-through modified Carlo–Erba system. Values are reported using the conventional permil () notation, where

and an analogous terminology describes ¹⁵N/¹⁴N ratios. Standards are Vienna Pee Dee Belemnite (VPDB) and air for ¹³C and ¹⁵N, respectively. Uncertainties for average ¹⁵N values for plants discussed in the text are reported as the standard error. Isotope enrichment between hair and diet is 3 for both ¹³C and ¹⁵N (7–9).

Elephant hair was serially sectioned in 5-mm intervals before stable isotope analysis. For one hair from B1013, sectioning was performed at 1-mm intervals for detailed isotope profiles over selected intervals. Tail hair growth rates in Kenya elephants were established in three ways: (i) using overlapping stable isotope patterns in hair collected from the same individual two or more times (12); (ii) using overlapping isotope patterns from individuals having similar geographical ranges but collected at different times (13); and (iii) using isotope data along with visual sightings. One individual, B1013, had only a single hair that was collected during collaring operations; its growth rate of hair was calibrated by comparing ¹⁵N values with visual sightings in Samburu National Reserve.

Diet was estimated using the isotope turnover pool model of Ayliffe et al. (10, 12). We use end-member values of -27 and -13 for C₃ and C₄ plants, respectively, which represent average ecosystem values for Samburu National Reserve and nearby regions (7).