No news from old drawings? Stomach anatomy in muroid rodents in relation to body size and ecology

• Published in: Journal of morphology (1931) 2022-09, Vol.283 (9), p.1200-1209
• Main Authors: Steiner, Natalie, Clauss, Marcus, Martin, Louise F., Imper, Corina, Meloro, Carlo, Duque‐Correa, Maria J.
• Format: Article
• Language: English
• Subjects: Animals; Body Size; Epithelium; forestomach; gastrointestinal tract; Myomorpha; Phylogeny; Rodentia; Stomach - anatomy & histology
• ISSN: 0362-2525; 1097-4687; 1097-4687
• DOI: 10.1002/jmor.21496

Muroid rodents mostly have a complex stomach: one part is lined with a cornified (nonglandular) epithelium, referred to as a “forestomach”, whereas the rest is lined with glandular epithelium. Numerous functions for the forestomach have been proposed. We collated a catalog of anatomical depictions of the stomach of 174 muroid species from which the respective nonglandular and glandular areas could be digitally measured, yielding a “stomach ratio” (nonglandular:glandular area) as a scale‐independent variable. Stomach ratios ranged from 0.13 to 20.15, and the coefficient of intraspecific variation if more than one picture was available for a species averaged at 29.7% (±21.5). We tested relationships of the ratio with body mass and various anatomical and ecological variables, including diet. There was a consistent phylogenetic signal, suggesting that closely related species share a similar anatomy. Apart from classifying stomachs into hemiglandular and discoglandular, no anatomical or ecological measure showed a consistent relationship to the stomach ratio. In particular, irrespective of statistical method or the source of dietary information, dietary proxies did not significantly correlate with the stomach ratio, except for a trend towards significance for invertivory (insectivory). Yet, even this relationship was not convincing: whereas highly insectivorous species had high but no low stomach ratios, herbivorous species had both low and high stomach ratios. Thus, the statistical effect is not due to a systematic increase in the relative forestomach size with invertivory. The most plausible hypotheses so far associate the muroid forestomach and its microbiome with a generic protective role against microbial or fungal toxins and diseases, without evident correlates of a peculiar need for this function under specific ecological conditions. Yet, this function remains to be confirmed. While providing a catalog of published depictions and hypotheses, this study highlights that the function of the muroid rodent forestomach remains enigmatic to date. The ratio of nonglandular to glandular stomach area in muroid rodents shows no evident relationship to any biological characteristic. Highlights Muroid rodent forestomachs vary in dimension as measured from literature drawings. The ratio of forestomach to glandular stomach is not linked to body mass. The ratio is not linked to ecological proxies. The role of the muroid forestomach remains elusive.