Sirenians are unique among the marine mammals in that they are herbivores. The mammalian order Sirenia has fossil representatives on the order of 50 million years old (Eocene epoch), making them as old as the cetaceans. While there are similarities between the two groups in terms of adaptation to the aquatic environment (e.g., streamlining, loss of hindlimbs, use of the tail for propulsion), the sirenians stayed on the path to herbivory while the cetaceans switched from the herbivorous habits of their terrestrial ancestors to become carnivores. Modern sirenians include three species of manatees (family Trichechidae: Amazonian manatee, Trichechtis inun-guis; West African manatee, T. senegalensis; West Indian manatee, T. manattts; the latter is divided into two subspecies: An-tillean manatee, T. m. manatus, and Florida manatee, T. m. latirostris). The Sirenia also includes the family Dugongidae (dugong, Dugong dugon), and Steller’s sea cow (family Dugongidae: Steller’s sea cow, Hydrodamalis gigas). The manatees, as their common names suggest, are distributed in the coastal tropical and subtropical regions of the Atlantic Ocean. The dugong is found in the South Pacific and Indian Ocean regions. The manatees and the dugong are considered endangered or threatened under various national laws and international conservation schemes. The Steller’s sea cow is the exception, not only is it extinct (eliminated by overhunting about 25 years after its discovery in 1765) but it inhabited the islands in the western reaches of the Aleutian archipelago in the North Pacific Ocean. Definitely not a tropical clime!
I. Methodology
Robert Ricklefs (1973) stated that “The life history of an organism can be described in terms of fecundity, growth and development, age at [sexual] maturity, parental care, and longevity.” These parameters are closely linked and are acted on by natural selection. While these parameters are easy to state, they are not always easy to measure. It is easy to study a colony of mice in the laboratory but not so with an animal like the Florida manatee that may reach a length of over 4 m and a weight in excess of 1500 kg. How, then, does one get this information for sirenians? Approaches to this question involve both field and “laboratory” (i.e., animals in captivity in marine zoological parks and zoos) studies. In the field, one can learn to identify individual animals using shape, size, and various markings and color patterns or one can mark (using various methods) individual animals. For large, long-lived, aquatic mammals like the sirenians, this becomes an extremely expensive and time-consuming operation. However, in many cases, it is the only way to obtain the desired information. Alternatively, one can collect data from animals that die from both natural and unnatural (i.e., human-related) causes. Animals killed for human consumption or those killed as a result of other human activities provide a sample, albeit potentially biased (nonran-dom), of males and females of all age classes. Marine mammal stranding networks collect life history data on animals found dead (i.e., stranded, beach cast, beached). In Florida, the Florida Fish and Wildlife Conservation Commission operates a network dedicated to the collection and examination of all manatees found dead in the state. From these carcasses one can gather information on, among others, reproductive status (immature, mature, pregnant, number of pregnancies) and age. Estimating the age of an animal is critically important in the estimation of life history parameters. Toothed cetaceans and pinnipeds, for example, have permanent teeth with roots that grow continually (but slowly) from birth to death. One can section these teeth and, under the microscope, count growth layer groups (GLGs). If one then knows the frequency with which these GLGs are deposited (e.g., one per year, 2 per year), GLG counts can be directly converted to and estimated age in years. The frequency of GLG deposition can be estimated using known age animals and using chemical markers deposited in the teeth of live animals. There is variability in the reliability of this process from species to species and the only way to know the true age of an animal is to follow it from birth to death.
Growth layers are also often deposited in growing bone. The dugong has tusks that erupt through the gums in the male but not in the female. These tusks grow throughout the life of the animal and GLGs accumulate on an annual (assumed) basis. Manatees, however, do not have permanent teeth. While some growth layers are present in the tooth roots in young animals, these are lost through resorption, as manatees have a continual, horizontal (back to front) tooth replacement throughout life. However, because other growing bones accumulate GLGs, scientists discovered that GLGs in manatee ear bones can be used to estimate age. Studies with chemical markers demonstrated that manatee GLGs accumulate on an annual basis.
Studies with dead animals are very useful and relatively inexpensive, but long-term studies on live animals are necessary to fill in the life history blanks. For example, by following a female from year to year, it is possible to document when she has a calf and how much time elapses between successive calves. While radio and satellite tracking allows scientists to follow individual manatees over long periods of time, it is virtually impossible to watch them from minute to minute. However, studies on manatees in captivity (aka “the laboratory”) can fill some of these life history gaps, particularly in the areas of reproductive biology and growth and development. In captivity manatees can be observed continually if necessary. Blood and urine samples can be collected to document estrous cycling and gestation in females and testicular activity cycling in males. Parental care of the offspring can be documented in great detail.
II. Longevity
Detailed age estimation studies have only been done for the Florida manatee and for the dugong. Using tetracycline marking, we know that Florida manatees deposit one GLG per year. Based on growth layer groups in the ear bone complex, some Florida manatees may five to be 60 years of age. However, due to bone resorption, 60 years may be a minimum estimate. A manatee born in the Miami Aquarium in 1948 is alive and well today (August 2000) at an aquarium in Bradenton, Florida. This animal provides a good measure of validation for the 60-year estimate. The average longevity for Florida manatees has not been well estimated but is probably on the order of 30 years less. This estimate may be biased downward by manatee deaths resulting from human activities (e.g., watercraft collisions). Average longevity may vary between sexes and among year classes. Longevity data do not exist for Antillean, Amazonian, or West African manatees.
Based on GLGs in their tusks, the oldest dugong examined to date was a female with 73 GLGs, which translates to 73 years if we assume one GLG per year. Tusks in male dugongs erupt through the gums and are worn down, eliminating the early GLGs in older animals. Minimum maximum longevity estimates for male dugongs are about 35 years.
III. Age at Sexual Maturity
Female Florida manatees reach sexual maturity between 2.5 and 6 years of age. Male Florida manatees reach sexual maturity between 2 and 11 years of age based on the presence of active spermatogenesis in the testes. It is important to note that even if a male manatee is sexually mature at an early age, it may not be physically large enough to compete for mating rights in an estrous (mating) herd. However, under circumstances wherein there are not other competing males, small manatees may be able to mate successfully and produce offspring. There are no data on age at sexual maturity for Antillean, Amazonian, or West African manatees.
Even though the estimated maximum life expectancy for the dugong is similar to that of the Florida manatee, dugongs appear to mature at greater ages, and the age at sexual maturity may vary among populations. Female dugongs in Australia and Papua New Guinea mature at 9.5 to 17.5 years. Male dugongs in the same areas mature at 9 to 16 years of age. These differences could reflect distinct genetic differences between populations or the effects of density-dependent factors.
IV. Parental Care
In both manatees and dugongs the male plays no apparent role in the care of the young. After mating, males and females go their separate ways. Florida manatee calves will stay with their mothers for 1-2 years on average. Most calves (about 70%) stay with their mothers through one winter season and the remainder dirough two winter seasons. Nutritional weaning is a gradual process but there are few data on the length of lactation in the Florida manatee or in the dugong. Lactation may last 1.5 years in dugongs and is probably similar in the manatee. Some wild and captive manatees appear to lactate for several years under certain circumstances. Most Florida manatee and dugong calves start feeding on vegetation at a few weeks of age and may be nutritionally independent before they reach the age of 1 year. However, at least in the case of the Florida manatee, calves probably learn the locations of feeding areas and warm water refugia by following their mothers through at least one winter season. The latter is extremely important for weaned juvenile survival during cold winters.
V. Growth and Development
Florida manatee calves average about 120 cm in length at birth, but viable calves may have a birth length ranging from about SO to 160 cm. This, along with variable individual growth rates, results in a highly variable length at age distribution. For example, 2-year-old manatees at Blue Spring (Florida) may range from 210 to 260 cm total length. From a sample of carcasses of Florida manatees in the age class >1 and <2 years, total lengths ranged from about 120 to over 260 cm. Florida manatees grow rapidly during the first few years, and body length becomes asymptotic at about 300 cm and an age of 8-10 years. The birth weight for Florida manatees ranges from 30 to 50 kg and the average adult body weight is about 500 kg. The Antillean and West African manatees are probably similar to the Florida manatee in these respects. The Amazonian manatee is smaller and has a birth length of 85-100 cm and a birth weight of 10-15 kg. Large adults may reach lengths of 280 cm and weights of480 kg. The dugong has a birth length of 100-130 cm and a birth weight of 25-35 kg. Adult dugongs average 270 cm in length and 250-300 kg body weight.
VI. Fecundity
As with other aspects of sirenian life history, data on fecundity are limited and based primarily on studies of the Florida manatee and the dugong. A key factor in assessing fecundity is the gestation period, which, despite numerous conceptions and births of Florida manatees, is not known for any sirenian species. However, scientists generally agree that gestation for the Florida manatee and the dugong is in the range of 12-14 months and that the other species of sirenians are probably similar. The known interbirth interval for wild Florida manatees averages 2.5-2.6 years. The estimated interbirth interval for Florida manatees ranges from 2.5 to 3.0 years when gestation period estimates of 12, 13, and 14 months are applied. This suggests that the true gestation period may be close to 12 months. Estimated interbirth intervals for the dugong range from 2.7 to 5.8 years depending on the length of the gestation period assumed (12, 13, or 14 months) and the population of dugongs used. We do know that Florida manatees resume estrous cycling within 1-2 months after the loss of a calf and become pregnant shortly thereafter. Whereas manatees and dugongs (both males and females) display seasonal reproductive activity, there is scant evidence to suggest that they have reproductive senescence as many mammals, including humans, do.
Other factors important in the calculation of fecundity are the number of offspring per birth and the sex ratio of offspring at birth. Wild Florida manatees have been documented producing twin offspring (but no more) in about 1.4 to 1.8% of births. Estimates of twinning based on Florida manatee carcass studies are as high as 4%. Limited and/or anecdotal data suggest that twinning occurs in the Antillean manatee and in the dugong. The sex ratio of the offspring at birth appears to be 1:1 for both the Florida manatee and the dugong. It is probably reasonable to assume that both Amazonian and West African manatees have similar patterns of twinning and offspring sex ratio.
Applying all of our knowledge of and assumptions about Florida manatee life history, the average fecundity (number of female births per female per year) for age classes 4-29 years was estimated at 0.189, 0.238, and 0.127 using different sets of data obtained from the examination of carcasses. Similarly, estimates of annual pregnancy rate (APR) and gross annual recruitment rate (GARR) ranged from 0.190 to 0.394 and 0.044 to 0.90, respectively, depending on the set of carcass data and gestation period used. Similar calculations for the dugong based on a smaller data set yielded apparent (annual) pregnancy rates of 0.093 to 0.353 over a series of years.
VII. Summary
Even though the Florida manatee and the dugong have been studied intensively since the 1980s, detailed data on many aspects of their life history are only beginning to be elucidated and there is considerably less information on the other species of sirenians. Given the threatened or endangered status of this unique group of marine mammals, every effort should be made to leani about their biology so that we may ensure their survival.
