Stable isotope ecology of the koala (Phascolarctos cinereus)

Abstract

Australia has undergone significant climate change, both today and in the past. Koalas, due to their restricted diet of predominantly eucalyptus leaves and limited drinking behaviour may serve as model organisms for assessing past climate change via stable isotopes of tooth enamel. Here, we assess whether stable carbon and oxygen isotopes from tooth enamel record known climate variables, including proxies of relative aridity (e.g. mean annual precipitation, mean annual maximum temperature, and relative humidity). The results demonstrate significant negative relationships between oxygen isotope values and both relative humidity and mean annual precipitation, proxies for relative aridity. The best model for predicting enamel oxygen isotope values incorporates mean annual precipitation and modelled oxygen isotope values of local precipitation. These data and the absence of any relationship between modelled oxygen isotope precipitation values, independently, suggest that koalas do not track local precipitation values but instead record relative aridity. The lack of significant relationships between carbon isotopes and climate variables suggests that koalas may instead be tracking the density of forests and/or their location in the canopy. Collectively, these data suggest that koalas are model organisms for assessing relative aridity over time – much like kangaroos.

Leaf water δ18O, δ2H and d‐excess isoscapes for Australia using region‐specific plant parameters and non‐equilibrium vapour

“…Thus, modelled leaf water values provide a more accurate representation of isotope ratios of biological materials that integrate over longer periods of time than they do for instantaneous leaf water samples. Materials that would be best represented by the isoscapes developed here would include plant tissues as well as herbivore tissues of organisms that use plant water as a substantive source of body water, such as kangaroos, koalas and emus (Ayliffe & Chivas, 1990; DeSantis & Hedberg, 2016; Miller & Fogel, 2016; Murphy et al, 2007).…”

Invasive species, not environmental changes, restrict the population and geographical range of the quokka (Setonix brachyurus)

“…The lower incisor was drilled using a variable speed dental tool with a 1‐mm carbide dental burr. The lower incisor was used to avoid destruction to molars and to follow similar methods employed in DeSantis & Hedberg, , and Fraser, (see DeSantis & Hedberg, for the rationale for selecting incisors; see Fig. a).…”

“…DMTA can assess the most recent diet of food consumed over the past few days, weeks or potentially months (Walker, Hoeck, & Perez, ; Grine, ; Teaford & Oyen, ; Teaford & Lytle, ; Scott, Teaford, & Ungar, ; Percher et al , ). Tough and hard food consumption can most readily be distinguished in numerous animal groups including primates, carnivorans, ungulates, sloths, small mammals (e.g., shrews, rabbits and voles) and marsupials using the DMTA metrics of anisotropy ( epLsar ) and complexity ( Asfc ), respectively (e.g., Ungar et al , , ; Scott et al , ; Scott et al , ; Prideaux et al , ; El‐Zaatari, ; Schubert, Ungar, & DeSantis, ; Scott et al , ; Scott, ; Haupt et al , ; Schulz et al , ; Withnell & Ungar, ; Calandra et al , ; Calandra & Merceron, ; DeSantis, ; DeSantis & Hedberg, ; Merceron et al , ; DeSantis et al , , ). Specifically, organisms eating harder and/or brittle foods such as nuts, insects, seeds or bones will exhibit higher complexity and the tooth surface can be characterized as having large gouges (quantified as area‐scale fractal complexity, or Asfc ; e.g., Scott et al , ; Schubert et al , ; DeSantis et al , ; Ramdarshan et al , ; Scott, ; Stynder et al , ).…”

“…Unlike many tropical or low‐latitude temperate ecosystems where δ 13 C enamel values can alone clarify grass or browse consumption, δ 13 C enamel values from Australian mammals often require DMTA to properly distinguish between the consumption of C 3 /C 4 grass and C 3 /C 4 trees and shrubs because these vegetation types can be sympatric in the areas occupied by the species of interest (Prideaux et al , ; DeSantis et al , ). Oxygen isotopes, on the other hand, from kangaroos and koalas are particularly useful at tracking relative climate aridity (e.g., relative humidity and/or precipitation; Murphy, Bowman, & Gagan, ; Prideaux et al , ; DeSantis & Hedberg, ) because these taxa acquire the majority of their water from plant material (i.e., ‘evaporative sensitive’ taxa per Levin et al , ). Higher δ 18 O values indicate conditions where evaporation rates are relatively higher, as a result of increased temperature and/or aridity due to the preferential evapotranspiration of the lighter 16 O isotope contained in leaves.…”

“…Higher δ 18 O values indicate conditions where evaporation rates are relatively higher, as a result of increased temperature and/or aridity due to the preferential evapotranspiration of the lighter 16 O isotope contained in leaves. On the other hand, lower δ 18 O values indicate an environment with higher precipitation and/or relative humidity (Levin et al , ; Murphy et al , ; Prideaux et al , ; DeSantis & Hedberg, ).…”