| Summary: | Digestive efficiencies (D-m) of ex situ and in situ manatees (Trichechus manatus latirostris) were, for the first time, assessed using manganese (Mn2+) as a naturally occurring marker. The D-m of ex situ manatees determined using [Mn2+] did not differ significantly from the D-m assessed using lignin, supporting the efficacy of the manganese approach. Gastrointestinal tract samples, obtained from recently dead animals, showed [Mn2+] concentrations were lowest in the stomach and remained low in the duodenum and small intestine but increased in the cecum, colon, and rectum, consistent with colonic digestion and absorption. In situ manatees consuming marine vegetation had significantly lower D-m (mean +/- SE, 46.9% +/- 1.8%; n = 8) than did in situ manatees consuming freshwater vegetation (77.8% +/- 2.6%; n = 7), which in turn had significantly lower values than did ex situ manatees consuming lettuce (84.0% +/- 0.7%; n = 37). In situ manatees eating seagrasses had significantly higher D-m than did long-term ex situ animals consuming seagrass for short periods of time (46.9% +/- 1.8% vs. 36.2% +/- 1.2%, respectively), suggesting potential modification of gut flora over time. One significant ramification of our results is that manatees consuming seagrasses would require a greater standing biomass to support their needs than would be required if they were eating freshwater vegetation. This reinforces the critical need to implement habitat conservation and protection before considering downlisting or delisting manatees as an endangered species.
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