Migration

Migration is an important ecological process that significantly affects individual fitness, population demography, and the structure of local ecosystems. It is also defined as an adaptive response to seasonal movement between two non-overlapping regions (Purdon et al., 2018). Elephants are highly mobile mammals that can travel long distances. Purdon et al. (2018) observed the migratory movements of 139 African elephants to understand where and why they migrate. Out of 139 elephants, only 25 migrated (Purdon et al., 2018). African elephants are known to be a partially migratory species, where some individuals migrate opportunistically and not every year (Purdon et al., 2018). It was noted that elephants migrated corresponding to seasonal changes; the onset of rainfall and greening of forage signified the start of the wet season whereas the duration, distance, and timing of dry season migrations varied (Purdon et al., 2018). The authors hypothesized that elephant migration is driven by individual traits, density, the distribution and availability of resources, and seasonal changes (Purdon et al., 2018).

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At the population scale, migration can be separated into 2 parts: partial and complete (Purdon et al., 2018). Partial migration is known to be the more common type within large mammalian species (Purdon et al., 2018). At the individual scale, migration can be obligate, where individuals migrate annually, or facultative, where individuals migrate in response to environmental conditions (Purdon et al., 2018). If seasonal changes are anticipated, migration can evolve to become obligate but in unpredictable seasonal environments, facultative migration is expected (Purdon et al., 2018).

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African elephants are widely distributed, but are most commonly found in areas where there is increased rainfall and photosynthetic producers (photoautotrophs) which can obtain energy and nutrients by harnessing sunlight (Purdon et al., 2018). It is anticipated that possible migratory behavior stems from accessibility of forage (Purdon et al., 2018). Elephants typically migrate during periods of new plant growth to acquire fresh forage and then return to avoid harsh weather conditions or limited resources when seasons change (Purdon et al., 2018). It is expected that the timing and duration of migrations would depend on the distribution of of forage and rainfall since they seek out greener vegetation and have limited water sources (Purdon et al., 2018). I water is not a limiting factor, such as during the wet season, then elephants are able to migrate further with higher primary productivity (Purdon et al., 2018).

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Based on the data collected, the scientists were able to come to the conclusion that elephants are partial and facultative migrators due to seasonal rainfall (Purdon et al., 2018).

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In a more specified setting, scientists used the chronologies of stable isotopes measured from elephant hair to determine a correlation between migration patterns and seasonal diet changes. Stable carbon isotopes record diet changes and portray variation between browse and tropical grasses through the C3 and C4 photosynthetic pathways (Cerling et al., 2006). Stable nitrogen isotopes record regional patterns related to aridity (Cerling et al., 2006). 

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Using stable isotopes is useful in distinguishing varied diets of African elephants due to the sizable differences between plants and their photosynthetic pathways (Cerling et al., 2006). In this study, the authors compare the growth rates and stable isotope ratios found in a nonmigratory population with an individual migrant elephant, (B1013) (Cerling et al., 2006). It is suspected that the individual elephant may be migrating due to seasonal crop raiding (Cerling et al., 2006).

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Elephants typically have a browse dominated diet and consume grass only when it is in new growth during the wet season (Cerling et al., 2006). Both groups of elephants presented equal rates of C4 grass consumption during the wet season (Cerling et al., 2006). However, during the dry season, while the resident herd had a low C4 consumption rate, the migrant elephant presented a slightly increased amount in the C4 isotope that later showed it was crop raiding at nights (Cerling et al., 2006). The data presented further supports the theory that elephants individually migrate for fresh growth of forage and vegetation (Cerling et al., 2006).