Marsupials
In Marsupials attachment to the uterine wall occurs at a later stage of embryo development than in placentals. This attachment is loose and only lasts a few days until birth. At birth the front legs, circulatory system, and nervous system have sufficiently developed to allow movement from the urogenital opening to the mammary area, where further development and lactation occurs.
Torpor during gestation has been observed in at least 4 species from the Dasyuromorphia order. When this has been observed the newborns have higher weights during lactation, so torpor may allow greater preservation of stored fat for later use during lactation.
Torpor during gestation was studied by Geiser and Masters (1994) for mulgaras, Dacycercus cristicaudia. Animals were captured in Northern Australia just prior to reproduction studies under temperature-controlled conditions. Pregnant females used torpor most days until 4-5 days before giving birth and did not re-enter torpor throughout lactation. That there is very little temperature variation around the time of birth has been confirmed in a later field study (Kortner et al. 2008)
Somewhat similar behavior was observed for the yellow-footed antechinus, Antechinus flavipes (Stawski and Rojas 2016, Parker et al. 2019). As shown in the figure at right for a typical female, females of this mouse-sized insectivore from the coastal regions of east and south-east Australia utilize torpor frequently in the months leading up to mating in late winter and for the first third of gestation. This was followed by a period without torpor the daily temperature variation was 3-4 degrees C. Temperature variation was much smaller (<2 degrees C) for a period of about 4 days around the time of birth, then was increased during lactation, but without any daily torpor.
Parker et al. (2019) Fig 1A
Selwood (1980) published a timetable for the embryonic stages of what was described as Antechinus stuarti. This species has in the past been lumped with A. flavipes. Since the subjects were trapped near Melbourne and A. stuarti are not found south of New South Wales, they were likely the more-widespread A. flavipes. Early in development there was a four-day period when there was no cell division. On Day 22 embryos were at the primitive streak stage, and by Day 23 the embryos had reached 8-12 somites. A brief period of attachment to the uterine wall began, with birth around Day 27. Based on the figure, if their estimate of the conception day was correct and these developmental dates applied, gastrulation would have occurred well past the time when torpor was observed.
Selwood also reported developmental studies for three other species of marsupials, none of which temperature behavior has been studied. The time between the primitive streak stage and birth was 7.5, 5, and 5 days. If this is typical of all marsupials, the other reported cases of low temperature variability also likely began by the early gastrula stage.
A female kowari, Dasyuroides byrnei, was observed to enter torpor multiple times, and based on later observation of offspring size in the pouch, Kortner and Geiser (2011) concluded that these all occurred while pregnant, but did not give a more precise estimate of when heterothermy ended.
Torpor during pregnancy has also been observed in the striped-face dunnart (Sminthopsis macroura) early in pregnancy, at around 30% through gestation (Geiser et al. 2005), but I have not been able to find any long-term temperature monitoring during pregnancy in this species.
A contrasting pattern has been reported for the sugar glider, Petarus breviceps, of the order Diprotedontia. Daily torpor has been observed, but only during lactation, when the pouch young were greater than 19 days old (Geiser et al. 2008). Torpor with pouch young has also been observed for the feathertail glider, Acrobates pygmaes (Geiser et al. 2008).