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.