Concluding Remarks
According to the metabolic theory of ecology (Brown, Gillooly, Allen, Savage, & West, 2004), populations that maintain a sustained (positive) rate of nutrient conversion into new individuals will persist compared to those that do not (Sibly & Calow, 1986; Sibly & Hone, 2002). This could be attained either by maximising the annual reproductive rate, which in mammals is represented by small, short-lived species (Shattuck & Williams, 2010), and in marsupials characterised by the smallest Didelphimorphia (i.e., the “fast” extreme, see Fisher, Owens, & Johnson, 2001). The “slow pace” marsupial extreme is represented by large herbivorous forms such as Vombatidae and Phalangeridae (Fisher et al., 2001). Hence, the small living Microbiotherids, with a reproductive output of two individuals per year (Nespolo et al., 2022), fall in the “slow” extreme. In terms of the allometric predictions for life histories in marsupials, given by the equation: age at first reproduction = 5.75*MB0.10 provided by Hamilton et al. (2011), a 30 g marsupial such as Dromiciopsshould have an age of first reproduction of 243 days (but it attains sexual maturity at 720 days). A similar computation for a maximum lifetime (=0.041*MB0.20, Hamilton et al. (2011) gives 2.5 years (but in Dromiciops , this parameter is above 4–5 years) (Nespolo et al., 2022). Then, the high observed densities of Dromiciops can only be explained by low mortality and an extended reproductive period during their lifetime, an aberrant lif history which can only be achieved in a complex three-dimensional habitat, such as mature temperate rainforests.
Hershkovitz (1999) proposed that Microbiotheriids’ life history is intimately associated with a combination of Nothofagus trees andChusquea native bamboos (i.e., theChusquea -Nothofagus -Microbiotheria association, CNF), which allowed them to build their sophisticated and impermeable nests that, in turn, are fundamental for hibernating in such a humid and cold forest. Thus, (according to Hershkovitz, 1999) what eventually extinguished all other Microbiotheriids was the disruption of the CNF by desertification at the northern edge of their distribution and freezing temperatures at the South (including Antarctica) (Hershkovitz, 1999, p10). Such phylogenetic conservatism (sensu Buckley et al., 2010) of Microbiotheriids niche is consistent with the paleontological evidence, which describes the oldest and largest Microbiotheriid known (Woodbounodon casei ) as “a generalised non-microbiotheriid Microbiotherian” that “resembles other frugivorous marsupials” (Goin et al., 2007).
Contemporary reconstructions also suggest that habitat preference is highly conserved across the marsupial phylogeny, as ancestral trait reconstruction of basal marsupial nodes is assigned to wet-closed environments with large posterior probabilities (i.e., rainforests, see Fig. 1 in Mitchell et al., 2014). These observations are also supported by recent evidence suggesting that mutualistic associations of microbiotheriids with aerial mistletoes dates back to the Cretaceous (Liu et al., 2018; Watson, 2020). The ancestral marsupial that colonised Australia from Antarctica was probably little different from the present-day Microbiotheriid, Dromiciops —an arboreal, nest-building, social, omnivorous-frugivorous mammal with adaptations to the cold, seasonal and humid canopy of the rainforest. This generalised all-purpose animal had the potential for adapting and specialising to the new ecological niches opened by the isolation of Australia and would explain the success of colonisation and posterior diversification of Australasian marsupials.