Osphranter rufus (Desmarest, 1822) is a macropod commonly called the red kangaroo and is the largest extant marsupial. Sexually dimorphic in size and coat color, this large macropod is one of four species in the genus Osphranter. In general, males are larger than females, and are reddish-brown in color, whereas females are bluish-gray. O. rufus is endemic to Australia, where it inhabits both arid and semiarid areas with wide habitat preferences that include open plains, open desert, grassland, woodland, or shrubland habitats. Although it is regularly harvested for its meat and hide, O. rufus is an abundant species that is not of special conservation concern and it is listed as “Least Concern” by the International Union for Conservation of Nature and Natural Resources.

Hagfishes are able to squeeze through small openings to gain entry to crevices, burrows, hagfish traps and carcasses, but little is known about how they do this, or what the limits of this ability are. The purpose of this study was to describe this ability, and to investigate possible mechanisms by which it is accomplished. We investigated the hypothesis that the passive movement of blood within a hagfish's flaccid subcutaneous sinus allows it to squeeze through narrow apertures that it would not be able to if it were turgid. To test this hypothesis, we analyzed videos of Atlantic hagfish (Myxine glutinosa) and Pacific hagfish (Eptatretus stoutii) moving through narrow apertures in the lab. We measured changes in body width as the animals moved through these openings and documented the behaviors associated with this ability. We found that hagfishes are able to pass through narrow slits that are less than one half the width of their bodies. Our results are consistent with the idea that a flaccid subcutaneous sinus allows hagfish to squeeze through narrow apertures by facilitating a rapid redistribution of venous blood. In addition, we describe nine distinct behaviors associated with this ability, including a form of non-undulatory locomotion also seen in snakes and lampreys. Our results illuminate a behavior that may be a critical component of the hagfish niche, as a result of its likely importance in feeding and avoiding predators.