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The ilio-marsupialis muscle in the dasyurid marsupial Sminthopsis douglasi: form, function and fibre-type profiles in females with and without suckling young

P. A. Woolley^1,*, M. F. Patterson¹, G. M. Stephenson² and D. G. Stephenson¹

¹ Department of Zoology, La Trobe University, Bundoora, Victoria 3086, Australia
² School of Life Sciences and Technology, Victoria University, Melbourne, Victoria 8001, Australia

View larger version (85K): [in a new window]   Fig. 1. (A) X-ray of hindquarters of female Sminthopsis douglasi. (B) Diagram of pelvic region showing insertion of the ilio-marsupialis muscle (IMM) on the anterior iliac spine (AIS) and the passage of muscle through the inguinal canal (IC). The epipublic bones (EP) are embedded in the abdominal musculature. After passing through the inguinal canal, the ilio-marsupialis muscle divides into four branches, each passing to one of the four nipples on the same side. (C) Diagrammatic representation of the pouch. The broken line indicates the limit of the pouch area, and the solid line indicates the free edge of the fold of skin at the entrance to the pouch. The nipples, which lie under the pouch skin fold, are shown as small open circles.

View larger version (73K): [in a new window]   Fig. 2. A wild-caught Sminthopsis douglasi with eight young estimated to be 35-40 days old. Before handling, the young were partially covered by the pouch skin fold, part of which is indicated by the arrow below the thumb of the handler.

View larger version (148K): [in a new window]   Fig. 3. Sagittal section through a nipple and the adjacent pouch skin showing strands of ilio-marsupialis muscle (arrows). Scale bar, 0.5 mm. Insert: strands of ilio-marsupialis muscle at a higher magnification. Scale bar, 50 µm.

View larger version (24K): [in a new window]   Fig. 4. The groups of ilio-marsupialis muscle samples taken from the animals used in this study are shown in relation to the schematic appearance of the pouch in both non-suckling adults and in females suckling young, together with events during the suckling period. Arrows indicate the ages of the young in the samples taken from lactating females. Group 1, nipples never suckled; Group 2, nipples suckled for 36-39 days; Group 3, nipples suckled for 51-53 days; Group 4, nipples suckled for 68-70 days; Group 5, unsuckled nipples in the pouch of females with young on other nipples.

View larger version (11K): [in a new window]   Fig. 5. The relative proportions of fast-twitch, composite and slow-twitch muscle fibres in populations of fibres (fT) dissected from muscles associated with the nipples in each of the five groups studied. Group 1, nipples never suckled; Group 2, nipples suckled for 36-39 days; Group 3, nipples suckled for 51-53 days; Group 4, nipples suckled for 68-70 days; Group 5, unsuckled nipples in the pouch of females with young on other nipples.

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