Ontogenetic scaling of the gastrointestinal tract of a marsupial foregut fermenter, the western grey kangaroo Macropus fuliginosus melanops

Seasonal and sex-specific changes in the gastrointestinal tracts of Peromyscus maniculatus

Functional traits are phenotypic characteristics that contribute to fitness of individuals in dynamic and changing environments. In mammals, both categorical and continuous (e.g., quantitative) functional traits have been extensively utilized as proxies for diet, locomotion, and other aspects of species ecology, but there has been less focus on form and function of soft tissues. This is particularly true for the digestive system, which varies in size and complexity across Class Mammalia and plays a major role in the energetics of species. To guide more effective utilization of gastrointestinal (GI) morphology as a functional proxy in small mammal ecology, we examined how GI tracts (lengths and masses of four GI sections) varied within a population of deer mice (Peromyscus maniculatus) in the Southern Appalachian Mountains of North Carolina, United States. We collected samples of adult P. maniculatus monthly for 1 year and measured GI tracts to quantify variation with respect to seasonality and trophic level, providing insight into plasticity in this soft tissue trait over time. We found that season had a significant effect on the total length and wet mass of the GI tract, with January mice having the longest GI tracts and lengths being shortest in the summer. The relative shortening of the GI tract in summer corresponded with a partial trophic increase detected by stable isotope signatures. GI length and wet mass also were affected by reproduction, but males and females responded in sex-specific ways to demands of reproduction, with reproductively active males having shorter and lighter GI tracts than nonreproductively active males. Our study provides proof-of-concept for understanding population-level plasticity in a rarely collected soft tissue trait, which may also be complementary to standard craniodental measurements as a functional dietary proxy to understand mammalian ecology and community assembly.

Scaling at different ontogenetic stages: Gastrointestinal tract contents of a marsupial foregut fermenter, the western grey kangaroo Macropus fuliginosus melanops

Prominent ontogenetic changes of the gastrointestinal tract (GIT) should occur in mammals whose neonatal diet of milk differs from that of adults, and especially in herbivores (as vegetation is particularly distinct from milk), and even more so in foregut fermenters, whose forestomach only becomes functionally relevant with vegetation intake. Due to the protracted lactation in marsupials, ontogenetic differences can be particularly well investigated in this group. Here, we report body mass (BM) scaling relationships of wet GIT content mass in 28 in-pouch young (50 g to 3 kg) and 15 adult (16-70 kg) western grey kangaroos Macropus fuliginosus melanops. Apart from the small intestinal contents, in-pouch young and adults did not differ in the scaling exponents ('slope' in log-log plots) but did differ in the scaling factor ('intercept'), with an implied substantial increase in wet GIT content mass during the out-of-pouch juvenile period. In contrast to forestomach contents, caecum contents were elevated in juveniles still in the pouch, suggestive of fermentative digestion of milk and intestinal secretion residues, particularly in the caecum. The substantial increase in GIT contents (from less than 1 to 10-20% of BM) was associated mainly with the increase in forestomach contents (from 25 to 80% of total GIT contents) and a concomitant decrease in small intestine contents (from 50 to 8%), emphasizing the shifting relevance of auto-enzymatic and allo-enzymatic (microbial) digestion. There was a concomitant increase in the contents-to-tissue ratio of the fermentation chambers (forestomach and caecum), but this ratio generally did not change for the small intestine. Our study not only documents significant ontogenetic changes in digestive morpho-physiology, but also exemplifies the usefulness of intraspecific allometric analyses for quantifying these changes.