Apparent Dry-Season Fasting in Amazonian Manatees (Mammalia: Sirenia)

Abstract

Amazonian manatees (Trichechus inunguis), restricted to deep water areas in the larger lakes in the central Amazon during the dry season, do not have any obvious food sources until water levels rise between 1-2 m. Evidence from Lago Amana suggests almost seven months of fasting. Manatees may eat dead vegetative matter of both autochthonous and allochthonous origin, relying on hindgut fermentation to derive nutritive value from such material. It is suggested that the extremely low metabolic rate (36% of standard) and the large quantity of blubber may represent critical adaptations in the survival of this species during prolonged dry seasons. It may be calculated that a fat manatee may survive up to 200 days before exhausting its lipid reserves. RELATIVELY FEW MAMMALS are capable of fasting for long periods of time. Among exceptions are baleen whales during migration (Rice and Wolman 1971, Lockyer 1976), polar seals during molt (McLaren 1958), male otariid seals during the breeding season (Peterson and Bartholomew 1967), and hibernating/estivating rodents, bears, etc. This note reports the probable occurrence of seasonal fasting in the Amazonian manatee (Trichechus inunguis) during prolonged or extreme dry seasons. In the central Amazon Basin, river levels rise and fall on an annual cycle with a 10-15 m amplitude (Schmidt 1973). Generally the highest water levels (flood season) are encountered in June and the lowest levels (dry season) are in November, although there are some differences between individual rivers depending on the rainfall in their catchments. While studying manatees at Lago Amana (2?46'S, 64?39'W), a large permanent terra firme lake (145.3 kM2) in the Brazilian Amazon, I was able to observe the effects of a prolonged dry season on an estimated 500-1000 manatees, that annually congregate in the deeper waters of the lake at this time of year. During the last week of August and the first week of September 1979, the water level was dropping (Fig. 1) and exposing large expanses of clay or mud beaches. Most of the aquatic macrophytes were stranded on these beaches, and the grasses of the genera Panicum, Luziola, Paspalum, Echinochloa, and Hymenachne and the floating aquatics, Pistia, Pontederia, and Eichhornia, normally eaten by manatees, were generally unavailable. The last evidence of manatee feeding (bitten-off leaves and/or uprooted plants) was on a clump of the sedge, Scirpus cubensis, in relatively shallow water on 29 August. Manatee feces, floating at the water surface, were commonly found during August, but were not evident by the end of September. As the water levels continued to drop, there were no aquatic plants available for the I Contribution 24 of the Department of Aquatic Mammal Biology, INPA. A traducao em portugues encontra-se a disposipo na Biblioteca do INPA. BIOTROPICA 15(1): 61-64 1983 61 This content downloaded from 157.55.39.96 on Sun, 02 Oct 2016 05:48:09 UTC All use subject to http://about.jstor.org/terms manatees to eat. The exposed beaches produced luxuriant growths of grasses and leguminous plants as described by Junk (1970). In mid-December, the water levels started to rise, rapidly flooding the beaches and associated vegetation, of which only a small number of species may be considered to be either aquatic or semi-aquatic. Evidence of manatee feeding was noted on 28 December and continued for about two weeks, until the water level started to drop for a second time, reaching a level even lower than the previous one. Manatee feces were collected in quantity for food-habit analyses during the last two weeks of January, but were unavailable thereafter. Signs of manatees feeding were noted again during the first week of March, but only a single fecal mass was found. The water level continued rising until our departure on 17 May. By this time, most of the manatees had migrated out of the lake into the more productive varzea2 lakes or had moved into the igap6 to feed on the recently inundated growth. Thus, during the seven-month period, September to March, the manatees in Lago Amana apparently had access to food plants for a total of approximately two weeks. Although the absence of obvious plant food sources and the lack of feces suggest that the manatee population of this lake was in a fasting state, this may not be completely true. In certain parts of the lake there are deposits of dead vegetative matter of both autochthonous and allochthonous origin that may be used as a supplementary food source by manatees. Such material would be high in fibrous matter and low in soluble carbohydrates (Howard-Williams and Junk 1976). In Florida, manatees (T. manatus) have been occasionally observed feeding on this type of material (Hartman 1979).