Determinants of isotopic variation in two sympatric mouse lemur species from northwestern Madagascar

Introduction - Malagasy habitats undergo vast seasonal changes in temperature and rainfall regimes, which in turn strongly infl uence the biology and life history of their inhabitants (Wright, 1999 ). The western dry deciduous forests show a yearly alternation between a distinct dry season and a hot and humid rainy season (Jury, 2003 ). Correspondingly, the phenology of dry forest trees – i.e., the availability of leaves, fl owers and fruits – changes substantially over the course of a year (Sorg and Rohner, 1996 ; Thoren et al ., 2011 ). As a result, plant food resources are not available evenly throughout the yearly cycle, and a relatively resource-poor dry season can be distinguished from a relatively resource-rich rainy season (Hladik et al ., 1980 ). The length of both seasons varies across Madagascar due to a geographical gradient in minimal temperatures and annual rainfall . Adaptive responses of lemurs to this environmental seasonality include seasonal changes in general activity (Schmid and Kappeler, 1998 ; Wright, 1999 ; Dausmann et al ., 2004 ), shelter usage pattern (Ramanankirahina et al ., 2012 ), body mass (Hladik et al ., 1980 ; Schmid and Kappeler, 1998 ; Atsalis, 1999 ; Randrianambinina et al ., 2003 ), reproduction (Hladik et al ., 1980 ; Wright, 1999 ), and feeding regime (Hladik et al ., 1980 ; Thoren et al ., 2011 ). It is known, for example, that diademed sifakas ( Propithecus diadema ) shift their diet from fruits and seeds to non-fruit foods, such as leaves and fl owers, and decrease food ingestion and general feeding time during the dry season (Irwin et al ., 2014 ). This ability to fl exibly change behavior or resource usage patterns in view of certain environmental changes is the result of adaptive plasticity (Nussey et al ., 2007 ). Adaptive plasticity in seasonal feeding regimes can be expected to be highest in omnivorous species that can, in principle, switch between different sources of energy and protein by consuming different food plant species or varying the proportion of animal prey in the diet. Moreover, omnivores can consume different parts, such as fruits and gum s, of the same food plant species according to the availability of these items.