Age Determination by Back Length for African Savanna Elephants: Extending Age Assessment Techniques for Aerial-Based Surveys

Abstract

Determining the age of individuals in a population can lead to a better understanding of population dynamics through age structure analysis and estimation of age-specific fecundity and survival rates. Shoulder height has been used to accurately assign age to free-ranging African savanna elephants. However, back length may provide an analog measurable in aerial-based surveys. We assessed the relationship between back length and age for known-age elephants in Amboseli National Park, Kenya, and Addo Elephant National Park, South Africa. We also compared age- and sex-specific back lengths between these populations and compared adult female back lengths across 11 widely dispersed populations in five African countries. Sex-specific Von Bertalanffy growth curves provided a good fit to the back length data of known-age individuals. Based on back length, accurate ages could be assigned relatively precisely for females up to 23 years of age and males up to 17. The female back length curve allowed more precise age assignment to older females than the curve for shoulder height does, probably because of divergence between the respective growth curves. However, this did not appear to be the case for males, but the sample of known-age males was limited to ≤27 years. Age- and sex-specific back lengths were similar in Amboseli National Park and Addo Elephant National Park. Furthermore, while adult female back lengths in the three Zambian populations were generally shorter than in other populations, back lengths in the remaining eight populations did not differ significantly, in support of claims that growth patterns of African savanna elephants are similar over wide geographic regions. Thus, the growth curves presented here should allow researchers to use aerial-based surveys to assign ages to elephants with greater precision than previously possible and, therefore, to estimate population variables.

Genetic connectivity across marginal habitats: the elephants of the Namib Desert

“…The maximum body size for Etosha elephants is the same as those of other savanna elephants (Trimble et al. ). Likewise, the reported phenotypic differences among desert elephants are not well quantified and may be somewhat anecdotal.…”

“…The high gene flow and lack of evidence for isolation by distance in Namibia would suggest that phenotypic differences between desert and Etosha elephants would be unlikely to have evolved or persisted; there has not even been enough time or isolation for mtDNA haplotypes to differentiate between the two populations. The maximum body size for Etosha elephants is the same as those of other savanna elephants (Trimble et al 2011). Likewise, the reported phenotypic differences among desert elephants are not well quantified and may be somewhat anecdotal.…”

Unmanned Aerial Survey of Elephants

“…At a height of 100 m, the width of the sample strip is 120 m according to the camera used. These images can even be used to determine the age of the elephants by measuring back lengths, considering the pixel size (from 3 to 10 cm) [20], [21]. The detectability of other species was disappointing.…”

Making the most of survey data: Incorporating age uncertainty when fitting growth parameters

“…Our model is not just specific to fish or the VBGF but can also be used in any area of ecology where the growth curves are fitted to data when the age of an individual is not known exactly; such as in studies of elephants (Shrader et al, 2006;Trimble et al, 2011), brown bears (Zedrosser, Bellemain, Taberlet, & Swenson, 2007), and birds (Tjørve & Tjørve, 2010), just to name a few.…”