EFFECTS OF WATER RESTRICTION ON DIGESTIVE FUNCTION IN 2 MACROPODID MARSUPIALS FROM DIVERGENT HABITATS AND THE FERAL GOAT

The effects of water restriction on digestive function in the euro (Ma cropus robustus erubescens) found in the arid zone of inland Australia , the eastern wallaroo (M. r. robustus) from more mesic regions of eas tern Australia, and the feral goat (Capra hircus) found throughout the range of M. robustus, were compared in order to examine some physiolo gical adaptations required by herbivores for the exploitation of arid environments. Eight animals of each species were held in individual me tabolism cages in temperature-moderate rooms and given a chopped hay d iet ad libitum. Half the animals were restricted to 40 ml water.kg-0.8 0.day-1. This was 40%, 32% and 57% of voluntary drinking water intake in the euro, wallaroo and goat, respectively. All species responded to water restriction by reducing faecal, urinary and evaporative water l osses in association with reductions in feed intake. All animals incre ased urine os molality and electrolyte concentrations but not to maxim al levels, while packed-cell volume and plasma osmolality and electrol yte concentrations were unaffected by water restriction. The euro disp layed a suite of characteristics that separated it from the wallaroo i n terms of physiological adaptation, including lower voluntary water i ntake, an increase in fibre digestibility and maintenance of nitrogen balance during water restriction, and lower faecal water efflux associ ated with a consistently lower faecal water content (54% versus 59% wa ter in the wallaroo during water restriction, P < 0.05). The euro's co lon was 37% longer (P < 0.01) than that of the wallaroo. The goat had the lowest faecal water efflux (P < 0.05) and the longest colon (P < 0 .001). Water restriction did not affect water content in digesta, nor short-chain fatty acid concentrations or production rates in vitro. To tal body water, as a proportion of body mass, was depressed (P < 0.05) in the macropodids, but not in the goat. The reduction in dietary nit rogen intake, which accompanied water. restriction, was partially comp ensated by an increase in urea degradation in the gut from 68% to 76% of urea synthesis water-restricted macropodids. These responses to wat er restriction are discussed in relation to those reported in other ma cropodid and ruminant species.