Abstract

The manatee is an endangered species. To assist in maintaining this species, studies on reproductive biology were performed on the only protected breeding colony in the world. Puberty was found to occur at 7.5 years, gestation period is 13 +/- 1 months, and the normal calving interval was 3.5 years. This and other information can be used to develop a plan including protected, semi-protected, and free-ranging animals to assist the survival of the manatee.

Introduction

The West Indian Manatee (Trichechus manatus or Florida manatee) is an endangered species found in the waters of South and Central Florida during most of the year. It will range as far north as South Carolina and Southern Louisiana during warm weather. The population is estimated to be about 1000, with an average of approximately 100 (10%) verified deaths occurring annually over the 4 year period between 1979 and 1982 (1,2). Deaths seem to be evenly distributed over age classes (more than 13% were dependent calves); therefore the death rate is probably the principal reason that the population is in a negative reproductive balance and has decreased in size. A 10% annual mortality rate evenly distributed over age groups would mean that less than 40% of the female calves reach 9 years of age and begin calf production (Table 1). Recently, there has been increased interest in saving the manatee, but these efforts will need an accurate knowledge of its reproductive characteristics in order to make good population projertions.

Establishment of a protected satellite population structured for high reproductive efficiency would also require an accurate knowledge of the reproductive characteristics of this animal in order to produce a maximum number of offspring in the space available. Offspring not needed to sustain the satellite population with a high reproductive efficiency could be used to maintain a free-ranging population in a negative reproductive balance.

Reproductive parameters necessary for estimating reproductive potential of a protected population include age at puberty, gestation period, calving interval, and age of reproductive senility. The incidence of twinning stillbirths, and infertility affect reproductive efficiency, but values for these factors have not been reported and there is probably insufficient data to make even a close approximation. The incidence of reproductive diseases and their effect on reproductive potential is considered to be small.

Age of reproductive senility cannot be determined because of a lack of elderly animals of known age. One female in the colony under study is approximately 33 years old and still producing calves.

Materials and Methods

Manatee Colony

The animals used for the collection of data are members of a colony founded in 1957 and ranging in size from 2 to 9 animals. The animals are a compatible colony in which reproduction takes place and calves show a rapid growth rate (3). The animals are kept in two connected tanks; one 30 ft in diameter and 10 ft deep, and the second tank 20 ft in diameter and 10 ft deep. Flowing filtered seawater (25 ± 2 degrees C and a salinity of 34 ± 3 parts per thousand) supplies the tanks with about 5% of the volume exchanged per hour.

Details of the husbandry and health care programs have been reported (White et al., in press).

Animals

Five animals have been used in the present study:

1. Romeo --- captured in 1957 at approximately 5 years of age (33 ± 5 years of age at the time of this report). He is considered to be a competent male and has sired calves in 1974, 1980, 1981,1982, 1983, and 1984.
2. Juliet --- captured in 1957 at approximately 5 years of age. At the time of this report, she had been in captivity 26 years and is believed to be 33 (+/-5) years of age. She has produced 4 calves in captivity (1975, 1980, 1982, and 1983).
3. Amanda --- rescued and rehabilitated with a suckling calf (Ariel), less than I month old, in 1973. She would have a minimum age of 22 years and is believed to be 26 (+/- 4) years of age. She is known to have had 3 calves -- 2 born in captivity (1973, 1981, 1982).
4. Ariel --- born approximately I December 1973 in the wild. She has produced one calf in captivity (September, 1982).
5. Lorelei --- conceived and born in captivity in 1975. She has produced 1 calf in captivity (June, 1984).

Observations

Experimental animals were observed several times daily to detect sexual interactions and copulation with the adult mate. They were examined and weighed approximately 4 times each year to determine growth.

Blood Samples

Blood samples were collected from the brachial plexus on the medial surface of the flipper. Serum was separated and stored frozen for later analysis.

Hormones

Serum progesterone concentrations of greater than 1 ng/ml were used to indicate the luteal phase of an estrous cycle and also to detect the corpus luteum of pregnancy.

Serum estrogen concentrations were used to indicate a follicular phase of the estrous cycle. Serum estrogen and progesterone concentrations were determined by radioimmunoassay (3).

Puberty

Puberty or sexual maturity for these studies was defined as the time at which the first ovulation occurs. A maximum value was approximated by backdating from the first birth. A minimum value was approximated by detecting the presence of an estrous cycle (follicular and luteal phase). Copulation with the male was also used to indicate sexual maturity

Gestation Period

Period of gestation was also approximated by establishing minimum and maximum values. A minimum value was the longest interval between beginning of a confirmed pregnancy and birth. The maximum value was the shortest calving interval.

A confirmed pregnancy was determined by progesterone concentrations greater than 1 ng/ml followed by physical changes (the animals become more rounded and the genitalia is swollen) and a birth.

Postpartum Infertility

The interval following parturition when conception did not take place (Calving Interval minus Gestation Period).

Results

Age of Puberty

Serum progesterone and estrogen concentrations (less than 1 ng/ml and 200 pg/ml, respectively) determined on subadult females at monthly intervals indicated that reproductive cycles were not present in animals less than 7.2 years old. The youngest age at conception recorded for the colony was 7.5 years for Ariel. Lorelei conceived at 8 years, but Juliet did not first conceive until she was approximately 20 years of age (17 years in captivity). Amanda was an adult with calf when rescued and rehabilitated; thus, age at puberty cannot be estimated. Minimum determined age at puberty for 3 captive females was 7.5 years.

Period of Gestation

A minimum value (12 months) was established for Amanda by detecting elevated progesterone levels followed by a known birth date. A maximum value or the shortest calving interval was 14 months for Juliet. Gestation was thus determined to be 13 (± 1) months. The error (± I months) in this determination is less than 8%.

Calving Interval and Postpartum Infertility

Calving interval equals the period of gestation plus postpartum infertility (duration of infertility following parturition). There have been 8 calves born in the colony and 5 documented calving intervals for the colony (Table 2).

Table 1. Effect of Annual Mortality Rate of Reproductive Efficiency of 1000 Female Manatess Calving at 3 Year Intervals

Table 2. Calving Interval

The mean determined calving interval was 42 months and the mean determined postpartum period of infertility was 29 months. Calving intervals ranged from 15 to 91 months and the period of postpartum infertility ranged from I to 77 months. The longest period (77 months) was observed for Amanda while she was being rehabilitated from injuries caused by a boat propeller when she was free-ranging.

Discussion

Based on reproductive characteristics determined in the present study, an average protected female should produce about 7 calves by 33 years of age with an annual reproductive efficiency of 0.2 (calves/year/female) which is higher than unprotected animals (Table 1). Calf production in a protected colony structured to have a ratio of 9 females to 1 male would be about 120 per acre per year.

Estimates of some major factors that affect reproductive efficiency in the protected manatee have been made. Age at puberty did not occur before 7.5 years and the calving interval was 3.5 years. This value includes an animal being rehabilitated and others that were net always fed a diet believed adequate for reproduction. Young animals which have been kept under good conditions of husbandry and have not been sick, injured, or stressed should have average calving intervals of about 3 years.

Calving interval is most important to estimations of reproductive efficiency and these studies indicate some of the major factors affecting this value. Postpartum infertility was significantly increased in the female suffering from propeller cuts. The shortest periods of postpartum infertility occurred in animals which had stillborn calves or had lost their calves. This finding suggests that lactation probably adds more than 10 months to postpartum infertility. The early history of the colony also suggests that in inadequate diet increases the. period of Postpartum infertility. The effect of age on postpartum infertility cannot be studied at this tone. Sample size was small in these studies and additional data will allow a better estimation for all of the values.

A system the size of the one used at Miami Seaquarium constantly stocked with 9 mature females and a mature male should produce an average of about 5 calves per year. A colony of fifty adult females and 5 males under these culture conditions would produce 495 calves in 33 years. It would require an unprotected population of 740 manatees (50% males) and suffering 10% annual death rate to equal this calf production (see Table 1). A 10-fold increase in the size of the Seaquarium system (30 calves per year) an less than an acre of land would probably insure that the manatee population of Florida remained in positive reproductive balance at about the present annual death rate (10%), provided that the experimental release of animals born in protected areas is feasible. Subadult animals would be moved to a semi-protected area where they are not fed and in close proximation to free-ranging animals.

Reproductive efficiency wou Id be much higher in a protected population of animals than in the free-ranging Florida population for the following reasons: 1. The death rate of captive subadult animals is much lower than the 10% annual rate now suffered by free-ranging animals. Almost all captive females should reach the age Of first Calving, while less than 40% of the free-ranging animals suffering a 10% annual death rate, would reach this age. The effect of 7%, 10%, and 13% annual death rates evenly distributed by age have been estimated for females with a 3 year calving interval (Table 1). 2. Injuries, sickness, and stress could almost be eliminated in the protected population. 3. Under controlled conditions, the diet can be improved with hydroponically grown grasses, supplemented with desired nutrients that have been found to improve fertility. 4. Composition of the protected population can be adjusted to increase reproductive efficiency.

Table 1 shows that a 10% annual mortality rate is probably too severe for an animal with the reproductive characteristics of the captive manatee. The animal could possibly withstand a 7% annual death rate and remain in a positive reproductive balance without supplementation from a protected breeding population. At a 10% annual death rate, the population would require supplementation of approximately 20 calves per year to remain in positive reproductive balance. If the average mortality rate rose to 13%, the wild population would need to be supplemented with about 35 calves per year, and most of the births needed to keep the population in positive reproductive balance would occur in the protected population. At a death rate of 16%, only 20% of the female calves would reach puberty and consideration should be given to protecting or semi-protecting all manatees.

If death rates cannot be lowered to 7%, we propose the creation of a small satellite population with a high reproductive efficiency (up to 8 times that of one suffering a 10% annual death rate) supplementing a free-ranging population with a low reproductive efficiency caused by high death rates, injuries, and stress. The overall effect is a population of manatees (free-ranging, semi-protected, and protected) that is in positive reproductive balance. A plan such as this should be further developed and initiated before the population is reduced to critical levels.

In summary, reproductive characteristics of the Florida manatee were determined on a captive colony ranging in size from 2 to 9 animals. Age at puberty for 2 subadult animals was found to be approximately 7.5 and 8.0 years based on colony history, serum estradiol and progesterone concentrations, and conception dates. A range for the gestation period was determined by establishing minimum and maximum values. A minimum period (12 months) was determined by establishing the beginning of pregnancy using physical signs confirmed by a significant elevation in serum progesterone concentration followed by a date for the observed birth. A maximum value for gestation period was the shortest determined calving interval (14 months). Gestation period was thus determined to be 13 +/- 1 months.

Calving intervals for 5 captive births ranged from 15 to 91 months with a mean value of 42. Postpartum infertility ranged from I to 77 months with a mean value of 29 months. Short periods of postpartum infertility were observed when nursing of the dam was short or did not occur. Long periods of postpartum infertility occurred in injured animals or animals having a low plane of nutrition. Based on these reproductive characteristics, the average captive female should produce 7 calves over a 33 year period. Calf production could be up to 120 per acre using high intensity production techniques (9 adult females with one male).

The effect of increased deaths and injuries occurring in free-ranging populations of manatees under pressure from increasing human population densities is a lower reproductive efficiency. It is estimated that death rates less than 7% are required to maintain positive reproductive balance for an animal with the reproductive characteristics of the manatee.

Addition of 30 animals per year to a population of 1000 animals suffering 10% annual death rates would probably maintain a positive reproductive balance. It is proposed that, in order to maintain the total manatee population of Florida in positive reproductive balance, either a reduction of the animal death rates to less than 7% or a small protected satellite population be created and structured to obtain a high reproductive efficiency (8 times the free-ranging population). Animals from the protected population would be used to supplement the free-ranging population.

Acknowledgements

We acknowledge contributors to the Manatee Fund, the College of VLterinary Medicine, University of Florida, the Miami Seaquariurn and its staff for making this study possible, and to Mrs. Michelle King-Vincek for assistance with this manuscript. Radioimmunoassay was performed by Dr. C.L. Chen.

References

1. Tovar, E. Florida 'Mermaids' being killed (source U.S. Fish and Wildlife Service), Florida Times Union, Sept. 1983.
2. Buergelt, C.D. Observations on manatee mortality in Northern Florida --- A Necropsy Survey. in-First Annual Proceedings of the International Association of Aqustic Animal Medicine, P.T. Cardeilhac, ad. 1(1): pp. 28-29 (1984).
3. White, J.R.; Francis-Floyd, R.; and Waterstrat, P. Growth rate of West Indian manatees (Trichechus manatus) raised in captivity. In: First Annual Proceedings of the International Association of Aquatic Animal Medicine, P.T. Cardeilhac, ad. 1(1): pp. 30-34 (1984).
4. Chen, C.L.; Kumar, M.S.A; Willard, M.D.; and Liao, T.F. Serum hydroxycortisol (cortisol) values in normal and adrenopathic dogs as determined by radioimmunaasiay. J. Vet. Res. 39(1): 179-181 (1978).