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

The form, function and fibre-type profiles of the ilio-marsupialis muscles, branches of which insert on to the skin of the nipples and pouch, have been investigated in the small dasyurid marsupial Sminthopsis douglasi. Single fibres from the branches of muscles associated with unsuckled nipples in non-lactating females and with both unsuckled and suckled nipples at four stages during the 70-day suckling period were typed according to their sensitivity to the activators strontium (Sr(2+)) and calcium (Ca(2+)) into fast-twitch, slow-twitch and composite types. An unusual finding was the predominance of composite fibres in the resting state (unsuckled nipples). Changes in fibre-type composition were observed during the suckling period and these changes correlated with events in the development of the suckling young. Composite fibres declined during the suckling period and, at the stage when the young can no longer be accommodated in the pouch but must still be carried by the mother while she is foraging, an increase in fast-twitch fibres that are associated with dynamic muscular activity was seen. Later in the suckling period, when the mammary tissue is greatly enlarged but the mother does not carry the young while out feeding, there was an increase in the proportion of slow-twitch (fatigue-resistant) fibres. The high proportion of fast-twitch fibres present late in the suckling period may be associated with vibratory movements that result in the young relinquishing the nipples.