Studies Investigating the Reproduction, Energetics, and Nutrition of the Sirenia

Manatees and dugongs (Order: Sirenia), while often referred to as “marine mammals”, have no evolutionary relationship with any of the other marine mammal groups (Orders: Cetacea and Carnivora).  The four extant sirenian species are the only megaherbivores that are completely aquatic and are found in fresh and salt-water habitats of subtropical and tropical West Africa, Central and South America, Florida, the Caribbean, Australia, and the Indian Ocean basin.  There is currently little known about the physiological ecology of any of these endangered species and this limitation is potentially limiting recovery plans. 

The Physiological Ecology and Bioenergetics Laboratory has a specific on-going commitment to address questions pertaining to the endangered West Indian manatee.  Through the SIRENS program (Studies Investigating the Reproduction, Energetics and Nutrition of the Sirenia) we have begun to document basal metabolic rate and how it changes with maturation and growth, to assess seasonal changes in thyroid hormone production, to monitor changes in the seasonal limits of the thermoneutral zone (TNZ), the relationship between body mass and TNZ, and to measure the composition and insulative quality of the blubber layer.  We have completed a study on whether manatees can drink seawater and assessed their capabilities for living in both fresh and salt water and the implications of them moving from one environment to the other.  We are examining the relationship between assimilation efficiency and food composition and are using the stable isotopes of carbon and nitrogen to examine current and historical trophic interactions of wild manatees and their relative dependence on fresh and salt water habitats.  We have also started to examine their specific nutritional requirements, cost of reproduction, cost of lactation, calf metabolism and growth, and milk composition and to attempt to better understand what physical criteria manatees use to select acceptable warm water refugia.

We still have much to learn about the physiological ecology these endangered animals.  To this end, PEBL has on-going research interests in various facets of sirenian physiology with the goal of better understanding how they cope with their natural environment. We are also initiating collaborative projects with the University of Queensland in Australia to apply some of our methodologies to the study of dugongs.

Manatees: When is even Florida not warm enough?

Cold-related deaths have been documented in West Indian manatees on many occasions over the past 25 years.  Initially the cause of such large numbers of deaths during the winter was unclear, so carcasses were categorized as “undetermined cause of death”.  However, starting in the mid-1980s, descriptions of cold-related deaths began to appear in the records.  There have been at least four major cold events since 1977 that have resulted in large numbers of deaths.  They include the winters of 1976-77, 1980-81, 1983-84, and most recently 1989-90.  In each of these events, animals died rapidly due to hypothermia.  Cold-related deaths of manatees rank as one of the leading causes of death attributed to natural (non-human related) causes.  The only other natural cause that is responsible for large numbers of deaths is exposure to red tide.  Death from exposure to cold is an unfortunate by-product of the fact that these animals are living at the northern most extreme of their geographic range.  These extreme cold fronts can also potentially debilitate, but not immediately kill, manatees that later die from other non-related circumstances. 

Death from cold exposure is a tragic by-product of the fact that Florida manatees live at the northern-most extreme of their range and that even in Florida there are occasional cold periods.  Sadly, studies indicate that juvenile manatees appear to be the most frequently affected by hypothermia.  These young animals are apparently not experienced enough to realize that they need to move to warm water refugia.  This response is learned from their mothers who introduce them to these areas when they experience cold fronts during the long period of maternal care.  In contrast, more experienced adults are well aware of the location of warm water refugia, are better able to monitor temperature cues, and have sufficient fat to carry them through periods when they do not feed.  Dependant calves are not as affected by the cold because they are still supported by an experienced adult. 

Previous data suggested that adult manatees possess metabolic rates that are only 25-30% of predicted values, resulting in a lack of cold tolerance.  The immediate result of having a low metabolic rate and a high thermal conductance, or poor insulation, is an inability to deal with cold conditions.  This results in manatees congregating in specific areas during cold weather.  This response to cold weather conditions is a learned response.  Mothers introduce their offspring to warm water refugia during the prolonged period of maternal dependence common to the species. 

We collect metabolic rate measurements by holding individual captive animals in a temperature‑controlled tank.  The water in this tank gets recirculated through a chilling and heating unit which is housed in our Mobile Energetics Lab (MEL).  The surface of the holding tank is covered with a plexiglas dome, except for a breathing space of approximately 8" in height. Oxygen uptake is measured by continually pulling fresh air through this covered volume and analyzing the oxygen content of the exiting air.  Manatees can be measured in water temperatures ranging from approximately 50° to 95°F. 

Our preliminary data show that manatees which weigh more than 750 lb increase their metabolism in water temperatures below 68°F and some animals doubled their metabolic output when temperatures dipped to 55°F.  This suggests that these animals are capable of dealing with cold, for at least some period of time.  This is consistent with what is observed in the wild.  The critical question is how long can this be maintained.  This will be addressed in phase II of this research.  Results for small manatees (<750 lb) are very different.  These younger animals are very susceptible to cold conditions because they are apparently incapable of increasing their metabolic rate at low temperatures.  Even at temperatures as low as 55°F, these animal showed no indication of any increase in metabolic heat production.  This would very quickly result in hypothermia and death if left even for a few hours.  This apparent inability to increase their metabolic rate in response to cold temperatures is puzzling since in most species this response is independent of age. 

The next phase of this research project will extend studies of the metabolic capabilities by determining metabolic expenditure during a prolonged exposure to a temperature below the lower critical temperature.  Manatees will be maintained at this temperature for a period of 3-4 days during which time metabolic rate, food consumption rate, and ambient and body temperatures will be continuously monitored.  This approach will mimic events which occur in the wild and will be of critical importance in the development of release criteria, and the selection of release sites, as well as understanding the possible ramifications of the potential loss of thermal refugia due to habitat destruction or plant closures.  

Manatee Behavior and Habitat Utilization at Warm Water Sites

Although manatees have occasionally been reported as far as north as Rhode Island and as far west as Texas during the summer months, their physiology restricts them to warmer sub-tropical to tropical water temperatures during the cooler winter months.  During the winter, manatees generally migrate south to warmer latitudes, but a significant number of individuals may remain north of their historical range, relying on a network of artificial, industrial warm-water effluents and natural, warm-water refugia to survive colder water temperatures.   With the potential threat of power plant deregulation and the loss of related critical warm-water effluents, the need to identify and qualify additional sources of warm-water refugia not dependent upon industrial outflows has become increasingly important. 

Brevard County is home to a substantial proportion of the states’ manatee population with winter synoptic counts indicating that several hundred manatees are using numerous aggregation sites, with the highest proportion of manatees being counted at two power plants in the northern part of the county. Smaller aggregations have also been documented at several smaller less well known sites.  While most of the state’s primary warm-water sites have been researched and well-documented, some primary sites and several secondary sites still remain unidentified or are at best, poorly documented particularly in Brevard County.

The goals of this study are to 1)  investigate seasonal as well as annual patterns of manatee use of each aggregation site to determine if they serve as winter refugia;  2) to establish daily patterns of  manatee use; 3) to define  the physical and chemical parameters of each site in a manner comparable to studies of other known secondary warm-water sites; 4) to rank these sites in order of importance with respect to usage during critically cold times;  5) to  predict  the sustainability of these sites over time;  and 6) to evaluate the current protection (if any) and the possible need for further protection or enhancement of these potentially critical sites.

Assessing Body Condition of Manatees: Can you really be too fat?

Body fat in marine mammals serves several roles: as a source of buoyancy, as an energy store, and as an insulator.  It is also used as an indicator of overall body condition with high stores indicating good condition. Successful rehabilitation and reintroduction of rescued and released West Indian manatees is dependent upon our ability to assess their body condition. PEBL scientists are currently evaluating some commonly applied methods that are used for body condition assessment and exploring new approaches. 
We have determined that the two different ultrasound instruments, Scanoprobe and Tringa, that are in common use, gave depth readings (mm) that were not significantly different from each other when used on blubber samples collected from dead manatees.  However, both gave readings that were significantly different from actual ruler measurements collected on the same samples.  These deviations from ruler measurements were typically underestimates of the actual depth and we speculated that these “inaccuracies” might be associated with changes in fat content resulting in density differences. 

Out of the 19 samples analyzed for lipid content, 15 had a large decrease in lipid content within the blubber layer.  In one example, the outer portion of the blubber ranged from 85 to 91% fat, whereas the deeper portion of the blubber was only 50% lipid. This unexpected compositional change, corresponded with the depth measured using the Tringa on all 15 occasions and with depth readings from the Scanoprobe on 9 of the 15 samples.  This suggests that the ultrasound unit is identifying the depth where composition changes and not the depth of the blubber.  The primary advantage of the Tringa over the Scanoprobe is that it produces an image which can be interpreted to distinguish both these qualitative (% lipid) differences in blubber composition as well as actual quantitative (depth) differences.  This requires some training by the operator, but the ability to measure both blubber depth and composition will give us a much clearer understanding of the body condition and health of these endangered animals.

Do manatees need to drink fresh water?

Little is known about the ability of West Indian manatees to osmoregulate and maintain water balance, but previous studies have suggested that they should be able to actively drink salt water based on urine data, their renal anatomy, and their occurrence in marine habitats for extended periods. Certainly manatees are frequently seen “drinking” (or are they playing?) with water from hoses. Other sirenian species are strictly fresh water inhabitants (Amazonian manatees – Trichechus inunguis) or marine (dugongs – Dugong dugon).  Because West Indian manatees inhabit both fresh water and marine environments, they provide an ideal species to address questions of water balance and osmoregulation in sirenians.

We worked with captive West Indian manatees at facilities in Florida and Brazil, as well as getting blood samples from free-ranging West Indian manatees in Puerto Rico and Colombia.  We wanted to see how manatees could cope with a variety of experimental conditions: animals living in fresh water eating lettuce (something with a very high water content); animals living in salt water eating lettuce; and animals living in salt water eating seagrasses (high in salt content and low in water content) with and without access to fresh water to drink.  In each case we measured how much fresh water they drank and how much they obtained directly from the food they were eating (measured using labeled water dilution) and the ability of the animal to maintain electrolyte and water balance. 

Captive manatees held in salt water without access to fresh water and fed a diet of sea grass showed significant increases in plasma osmolality and plasma concentrations of sodium and chloride within 9 days.  These manatees eventually refused to eat sea grasses.  These data suggest that wild manatees may require regular access to fresh, or perhaps brackish, water to meet water balance needs.  In captive situations, this need is met by drinking fresh water or by eating food that is high in free water (e.g., lettuce, which is approximately 94% water).  Manatees living in fresh water and consuming lettuce showed the highest rate of daily water intake (145 ± 12 ml kg-1 per day) compared to manatees in salt water on a diet of lettuce (45 ± 3 ml kg-1 per day) or manatees living in salt water on a diet of sea grass (21 ± 3 ml kg-1 per day).  These data suggest that manatees living in fresh water not only get a great deal of water from the food they eat but that they are also drinking large quantities of water.  We were ultimately able to demonstrate that manatees do not actively drink salt water, and that incidental ingestion of salt water during feeding elicited typical mammalian endocrine responses resulting in excretion of the excess salt load.

Assimilation Efficiency

Manatees in captivity are fed a diet consisting predominately of romaine lettuce, which doesn’t vary dramatically in composition over the course of the year.  In the wild, manatees consume numerous freshwater plants and sea grasses, with Southern naiad, hydrilla, and paragrass being the preferred species of freshwater plants.  Turtle grass, manatee grass, shoal grass and clover grass likely constitute the primary food sources in saltwater systems.  To a lesser extent manatees may also eat invertebrates, terrestrial grasses growing close enough to the shore to reach, acorns which have fallen into the water, and the flesh of fish caught in gill nets.

The amount of nutrients derived from eating plants varies according to which part of the plant is consumed, and also varies depending upon the age of the different parts.  Absorption efficiency of nutrients is affected by digestive tract morphology, food type and quality, seasonality, meal size, time between meals, and the nutritional state of the animal.  Manatees occasionally consume only the leaves of plants and at other times dig up rhizomes and consume the entire plant.  When a manatee concentrates its feeding in one area for an extended time, it may eat tunnels through the mass of vegetation.

Little is known about how much energy manatees assimilate from the various types of wild forage they consume.  Naturally occurring, apparently inassimilable minerals have been used before as dietary markers in various lizards, birds, and mammals.  In all of these studies, manganese was determined to be a good marker for use in dietary studies as virtually none of it is absorbed across the gut.  Another advantage of using manganese is that it is potentially useful in determining the assimilation efficiency of free-ranging animals.  This technique basically involves collecting food and fecal samples and comparing the concentration of manganese in each. 

The use of naturally occurring manganese as an inert marker allows us to measure the assimilation efficiencies of manatees in captivity and, theoretically, in the wild and to assess the impacts of varying environmental conditions on their nutritional status.  The goal of the present project was to determine the assimilation efficiency, and thereby how much energy is gained, for manatees consuming a seagrass diet. 

It has long been assumed that the nutritional ecology of megaherbivores is largely a function of their extreme body size and it has always been thought that larger animals can tolerate diets of lesser quality because of the higher digestive efficiency that results from long retention times (due to a longer intestine) and lower relative energy requirements.  There is limited information available for most megaherbivores, but apparently elephants do not make use of the high digestive potential that is theoretically provided by their massive body size.  They tend to have faster passage times and lower digestive efficiencies than might be predicted based solely on body size.   At least some of these attributes are likely due to the surprisingly short (for their body size) digestive tract.  Measured efficiencies range from 22%, for African elephants on a zoo diet to 73% for Asian elephants eating palm leaves.  Most studies indicate efficiencies on hay diets ranging from 33-53% .  Data for wild male elephants, feeding predominantly on browse in Kruger National Park, indicated efficiencies of 29.7% - 44.7%.

Our study suggests that manatees have AE% similar to elephants with efficiencies as high as 85% when feeding on vegetables and as low as 40% when feeding on manatee grass.  In spite of comparable AE%, manatees differ significantly from elephants in that they have one of the longest mean retention times of any mammal – 120-168 hours.  In theory manatees should have very efficient digestive systems but our results indicate that manatees are not very efficient at processing seagrasses and that there are significant differences in the ability of manatees to digest different vegetation types.  Manatees also differ significantly from dugongs which have assimilation efficiencies of between 70 and 95% when eating seagrasses.  Seagrasses generally have high fiber content (up to 63% of dry matter - DM) and low protein content.  Others have found a direct correlation between fiber content and digestibility in dugongs.  In contrast, some freshwater aquatic plants, such as hydrilla, duckweed,and water lettuce, may contain only 5.4 to 26.7% (DM) fiber. Romaine lettuce is also very digestible containing 95% water and only 2% fiber. Freshwater plants are considerably more digestible than seagrasses with preliminary data suggesting an AE% of 67.3 ± 8.9% (n=3) or for a lettuce diet where AE% was 82.4 ± 1.1% (n=33).  

Lactation and Milk Composition

Very little is known about manatee reproductive physiology, in particular the energetic needs of calves and maternal energy transfer.  These are parameters that cannot be easily measured in free-ranging animals and we are therefore forced to examine animals in captivity and extrapolate that information to free-ranging animals.  One technique that has been applied successfully to the measurement of milk transfer and calf energetics is the use of doubly labeled water (DLW). 

This study was the first attempt to quantify the energetics of the suckling period for the West Indian manatee and took place at The Living Seas of Epcot Center.  Over the years, data has accumulated on growth rates of orphaned calves and some captive born animals, yet we had no understanding of transfer of energy and how calves utilized it.  Growth rate of the calf in the present study was almost double that previously published and all of the parameters measured in the present study were unquestionably influenced by this incredible rate of mass gain.  This calf was an example of an animal that was raised under near-ideal conditions, with low foraging costs, no thermoregulatory demands, and a high availability of food.  This preliminary study has expanded our knowledge of this critical period in the life of a manatee and will both improve our capabilities in the captive care of this species and our understanding of their natural history and biology.

Suckling duration (114 to 156 sec) and frequency (every 49-62 min) remained relatively constant throughout the ~560 day lactation period.  Milk consumption rose from 4.0 l d-1 (at age 55 d) to a maximum of 12.2 l d-1 (at age 245 d), before declining to 2.4 l d-1 (at age 550 d).  Concurrent with this decline in milk intake was an increased intake of romaine lettuce.  By 550 days, the calf was consuming 18 kg d-1 of lettuce.  These data represent the first attempt to understand and quantify the energetics of the suckling period for this species.  It appears that the combination of low metabolism with a relatively high energy intake (high fat milk and large volume) resulted in the deposition of large amounts of lipid and a sustained high rate of growth (~0.7 kg d-1) through the relatively long nursing period of this species.

Manatees: So much vegetation but what should I eat?

It is well know that manatees feed on aquatic vegetation, including seagrasses and some species of exotic aquatic plants. They have also been observed to occasionally feed on terrestrial grasses and overhanging vegetation such as mangroves.  Historically, the only ways to determine feeding habits has been to watch manatees eat, analyze the feces of living manatees, or by stomach content analysis of recently deceased animals.  These methods all provide information on recent feeding events, however, none is an efficient method by which to look at the manatees’ feeding habits over a long period of time, and it is this level of information that is important if one is trying to determine critical habitats.

Previous studies on a variety of species have shown that the use of naturally occurring stable isotopes of carbon and nitrogen is an effective tool in the study of feeding habits.  This technique has demonstrated that the isotope composition of the food is reflected in the body of the consumer.  Essentially this is similar to the adage – “you are what you eat”.  By analyzing the amounts of these isotopes in a small skin sample, we can gain insights into what types of food were consumed over the past month.  Carbon isotope ratios differ between C3, C4, or CAM photosynthetic plants, and between fresh, brackish, and marine systems.  The application of this approach to manatee skin will thereby allow us to examine the recent feeding habits of manatees and gain insight into whether they were using fresh, brackish, or marine habitats.  

Manatees in different parts of Florida eat very different mixes of vegetation.  In some areas, sea grasses are consumed almost exclusively, while in other areas manatees eat almost all fresh water plants.  The ability to examine both historical and current feeding preferences of manatees could provide invaluable information to those agencies responsible for defining critical habitat and help identify food resources that are most in need of protection and management.

A recently completed study (Christy Alves, M.S. 2007) was the first to calculate stable isotope turnover rate in the skin of any marine mammal.  Stable carbon and nitrogen isotope ratios were examined over a period of more than one year in the epidermis of rescued Florida manatees that were transitioning from a diet of aquatic forage to terrestrial forage (lettuce).  Mean half-life for 13C turnover was about 53 days for skin from manatees rescued from coastal regions and mean half-life for 15N turnover was about 70 days for skin from manatees rescued from coastal and riverine regions, respectively.  These slow turnover rates, mean that carbon and nitrogen stable isotope analysis using manatee epidermis is useful in summarizing average dietary intake over a long period of time but not for assessing recent diet.  In addition to turnover rate, a diet-tissue discrimination value of 2.7‰ for 13C was calculated for long-term captive manatees on a lettuce diet.  Determining both turnover rate and diet-tissue discrimination is essential in order to accurately interpret stable isotope data.

These results were subsequently used to interpret carbon and nitrogen stable isotope data in epidermal samples collected from free-ranging manatees in Florida, Belize, and Puerto Rico.  Regional differences in carbon and nitrogen isotope signatures in manatee skin were consistent with signatures in plant samples collected in those regions.  Signatures in the skin of manatees sampled in Belize and Puerto Rico were indicative of a diet composed mainly of seagrasses, whereas those of Florida manatees exhibited greater variation suggesting possible diets of primarily freshwater, brackish, or marine vegetation. 

Department of Biology, University of Central Florida, 4000 Central Florida Blvd, Orlando FL 32816-2368

gworthy@mail.ucf.edu office: 407/823-4701 fax: 407/823-5769