Nespolo, R. | bioRxiv Science

bioRxiv2020

The Biogeography of Dromiciops in Southern South America: middle Miocene transgressions, speciation and associations with Nothofagus

AimSeveral geological events affecting Southern South America during the middle Miocene climatic optimum acted as important drivers of diversification to the biota. This is the case of Microbiotheria, for which Dromiciops is considered the sole surviving lineage, the sister group of Eomarsupialia (Australian marsupials). Three main Dromiciops genetic lineages are known, whose divergence was initially attributed to recent Pleistocene glaciations. Using fossil-calibrated dating on nuclear and mitochondrial genes, here we reevaluate this hypothesis and report an older (Miocenic) biogeographic history for the genus. LocationSouthern South America. MethodsPhylogenetic reconstruction using sequences from two mitochondrial DNA and four nuclear DNA genes in 159 specimens, from 31 sites across Chile and Argentina. Divergence time estimation using fossil calibration. ResultsOur phylogenetic analysis resolved four well supported clades with discrete geographic distributions. The oldest and most differentiated clade corresponds to that of the northern distribution (35.2{degrees}S to 39.3{degrees}S), which would be a different species (D. bozinovici, sensu Delia et al. 2016). According to our estimations, this species shared a common ancestor with D. gliroides (southern clades) about 13 million years ago (95% CI: 6.4-25.3). The southern clades (39.6{degrees}S to 42.0{degrees}S), showed a divergence time ranging from 9.57 to 6.5 Mya. Strong genetic structure was detected from north to south but not across the Andes, or between Chiloe island/ mainland. Demographic equilibrium is inferred to the northern clade, and recent demographic expansions was detected in the central and southern clades. Main conclusionsThe whole diversification of Dromiciops occurred within the Miocene, being the Middle Miocene transgression (MMT), the massive marine flooding that covered several lowlands of the western face of los Andes between 38-48{degrees} S, the most likely diversifying force. This was the result of an increase in global sea levels due to the Miocene climatic optimum, which shaped the biogeographic origin of several species, including Nothofagus forests, the habitat main of Dromiciops.

evolutionary biology↗

bioRxiv2020

A mesocosm experiment in ecological physiology: adaptive modulation of energy budget in a hibernating marsupial under chronic caloric restriction

During the last sixty years, mammalian hibernation (i.e., seasonal torpor) has been interpreted as a physiological adaptation for energy economy. However -and crucially for validating this idea - direct field comparisons of energy expenditure in hibernating and active free-ranging animals are scarce. Using replicated mesocosms and a combination of energy budgeting approaches (i.e., doubly labelled water, rates of CO2 production and food intake), we experimentally manipulated energy availability and quantified net energy costs of hibernation in a marsupial. We hypothesized that, when facing chronic caloric restriction (CCR), a hibernator should maximize torpor use for compensating the energetic deficit, compared to ad libitum fed individuals (=controls). However, intensifying torpor duration at low temperatures could increase other burdens (e.g., cost of rewarming, freezing risk). In order to explore this trade-off, we followed the complete hibernation cycle of the relict marsupial Dromiciops gliroides, and estimated its total energy requirements, and compared this with a control condition. Our results revealed: (1) that energy restricted animals, instead of promoting heat conservation strategies during hibernation (e.g., social clustering and thermoregulation), maximized torpor use and saved just enough energy to cover the deficit, and (2) that hibernation represents a net energy saving of 51% compared with animals that remained active. This work provides compelling evidence of a fine-tuning use of hibernation in response to food availability and presents the first direct estimation of energy savings by hibernation encompassing the total hibernation cycle.

ecology↗