Body size in elephants: influence of resources and poaching

Mikumi National Park in south central Tanzania contains a sub-population of elephant that is part of the Selous elephant population, until recently considered to be one of the largest populations of African Bush Elephant. Elephants in Africa are under threat from poaching for ivory and habitat destruction. Mikumi-Selous elephant population has been the focus of survey and study focused on the consequences of poaching. Over 40 years anecdotal and qualitative reports have indicated that Mikumi elephant are small in body size. Small body size has been attributed to a population age biased towards younger smaller animals. This study determined if the reported small size by age in Mikumi elephant was a consequence of age bias towards young animals or real. Within a well-defined study area, 535 elephant were identified and categorized by age and sex. The relationship between size and age was determined using teeth ages of skulls related to skull size measures compared to a well-documented referent population (Amboseli elephant population). Dung boli measures were used to compare boli size to animals of known ages and also compared to a referent population (Tarangire elephant population). Population parameters were assessed within a standard survey area and related to prior studies. Mikumi elephants were found to be small in size relative to age. Older animals were present but small. Explanations for small body size relative to age were: 1. Mikumi elephants are only apparently small due to a demographic age bias towards younger smaller animals. 2. Mikumi elephants are relatively small by age due to genetic isolation and/or selection against genetically smaller animals 3. Mikumi elephants are growth impaired due to resource or other environmental stressors. Results supported growth impairment due to environmental stress. Genetic isolation was not supported and the demographic profile including old small animals disproved the age bias explanation.