Although lateralisation has been observed in many vertebrate species, marsupials have been neglected in the study of lateralisation. We investigated the behavioural responses of the stripe-faced dunnart (Sminthopsis macroura) to a mechanical model of a snake approaching into the monocular (left and right) or the binocular visual field. The snake model was presented to 30 adult subjects. The behavioural responses and the latency to react to the stimulus were scored. Reactivity was calculated by pooling scores for retreat, startle, ears back, and orientation. Retreat tended to be the most common of these responses. Approach of the snake into the dunnarts' left monocular visual field elicited a significantly higher reactivity compared to approach into the right or binocular visual field. Half of the animals did not respond in the 60 seconds allocated when the stimulus approached on their right side, whereas only seven did not respond when the stimulus approached on the left, and ten when the stimulus was presented binocularly. These results are consistent with the right hemisphere's known specialisation for controlling fear and escape responses. Our results suggest that marsupials are lateralised in a way similar to other vertebrates.

The physiological signal for torpor initiation appears to be related to fuel availability. Studies on metabolic fuel inhibition in placental heterotherms show that glucose deprivation via the inhibitor 2-deoxy-D-glucose (2DG) initiates a torpor-like state, whereas fatty acid deprivation via mercaptoacetate (MA) does not. As previous studies using inhibitors were limited to quantifying body temperature in placentals, we investigated whether inhibition of glucose or fatty acids for cellular oxidation induces torpor in the marsupial hibernator Cercartetus nanus, and how the response of metabolic rate is related to body temperature. Glucoprivation initiated a torpor-like state in C. nanus, but animals had much higher minimum body temperatures and metabolic rates than those of torpid food-deprived animals and arousal rates were slower. Moreover, 2DG-treated animals were thermoregulating at ambient temperatures of 20 and 12 degrees C, whereas food-deprived torpid animals were thermo-conforming. We suggest that glucoprivation reduces the hypothalamic body temperature set point, but only by about 8 degrees C rather than the approximately 28 degrees C during natural torpor. Reduced fatty acid availability via MA also induced a torpor-like state in some C. nanus, with physiological variables that did not differ from those of torpid food-deprived animals. We conclude that reduced glucose availability forms only part of the physiological trigger for torpor initiation in C. nanus. Reduced fatty acid availability, unlike for placental heterotherms, may be an important cue for torpor initiation in C. nanus, perhaps because marsupials lack functional brown adipose tissue.

Altricial mammals and birds become endothermic at about half the size of adults and presumably would benefit energetically from entering torpor at that time. Because little is known about torpor during development in endotherms, we investigated whether after the establishment of endothermic thermoregulation (i.e. the ability to maintain a high body temperature during cold exposure), Sminthopsis macroura, a small (approximately 25 g) insectivorous marsupial, is capable of entering torpor and whether torpor patterns change with growth. Endothermic thermoregulation was established when the nest young reached a body mass of approximately 10 g, and they were capable of entering torpor early during development at approximately 10-12 g, lending some support to the view that torpor is a phylogenetically old mammalian trait. Torpor bout length shortened significantly and the minimum metabolic rate during torpor increased as juveniles approached adult size, and consequently total daily energy expenditure increased steeply with age. Relationships between total daily energy expenditure and body mass during development of S. macroura (slope approximately 1.3) differed substantially from the relationship between basal metabolism and body mass in adult endotherms (slope approximately 0.75) suggesting that the energy expenditure-size relationship during the development differs substantially from that in adults under thermo-neutral conditions. Our study shows that while torpor can substantially reduce energy expenditure during development of endotherms and hence is likely important for survival during energy bottlenecks, it also may enhance somatic growth when food is limited. We therefore hypothesize that torpor during the development in endotherms is far more widespread than is currently appreciated.

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