Species and Evolutionarily Significant Units (ESU) recognitions have been increasingly based on molecular data, while the identification of ecologically meaningful phenotypic variations that can
separate biological entities has been largely neglected. We suggest that in lieu of global environmental changes, adaptive physiological traits, especially those related to arid survival,
should be identified and used to better differentiate between species and subspecies. We used the broad-toothed mouse (Apodemus mystacinus) as a model species to examine the concept of
different biological entities that may be manifest within the same species. We examined the water restriction abilities of two populations of A. mystacinus, separated by the Great Rift Valley, under short and long day photoperiod regimes. The two allopatric populations exhibit opposing physiological traits in response to simulated aridity conditions. Under both photoperiod regimes,
increased urine production was followed in the high-altitude inhabiting “Hermon” population by a decreased urine osmolarity, whereas in the low-altitude “Carmel” population it was followed by
an increased urine osmolarity. In a response to increasing dietary salt loads, the Carmel group increased whilst the Hermon group decreased their urinary sodium and chloride concentrations.
Potassium levels of both groups increased during winter, but remained stable during summer, in a response to increasing dietary salt loads. The results show that two populations of woodmouse in
the eastern Mediterranean exhibit striking physiological differences related to aridity adaptation, which typify and separate them. We suggest that the case of the woodmouse should set an
example to a functional approach to the designation of an ESU.
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