Abstract

In the wild, manatees utilize fresh, brackish, and salt-water habitats and are capable of living in either low or high salinity conditions for very long periods of time. Studies of the mechanisms by which this is accomplished have been very limited. The following study of serum and urine electrolytes was undertaken to better understand how manatees cope with salinity extremes.

A number of factors make routine urinalysis for large sirenians very difficult. These include the large size of the animals, difficulty of access to collect samples during restraint, and danger to both the animals and the handlers during such restraint. In the past, urine was collected either opportunistically when an animal would voluntarily urinate during an exam, or via catheterization or ultrasound-guided cystocentesis.^1,2 One study used a Frisbee placed under the animals' urogenital area to collect urine.⁴ For the current study two adult male captive-born Florida manatees, Trichechus manatus latirostris, that had previously been husbandry trained for routine blood and urine collection³ were utilized to determine routine urinalysis and urine chemistries (including sodium, potassium, chloride, urea nitrogen, creatinine, calcium, and glucose) under several different conditions relating to salinity and diet (Table 1).

Depending on whether the data passed normality and homoscedasticity tests, either paired t-tests, t-tests, or Wilcoxon rank sum tests were used. Depending on which analyses were compared, there were approximately a total of 153 urine samples analyzed. Routine urinalysis results did not differ from other mammals. When urine chemistries were compared between whole urine and supernatant of centrifuged urine, both urea nitrogen and calcium were significantly higher in whole urine (P<0.001 and P=0.022, respectively). When the two animals were maintained in fresh water (salinity 0 ppt), whole urine sodium and chloride of both animals were significantly lower (P<0.001) than when maintained in seawater (salinity >32 ppt; Table 1). Likewise, both animals had significantly lower urine specific gravity and urine osmolality when maintained in fresh compared to seawater (P<0.001).

When the ratios of urine concentration to plasma concentration were compared, both manatees, maintained in seawater, had significantly higher ratios of sodium and chloride (P<0.001). Ratios of other chemistries were variable, either being not significant in both or significant in only one animal. Both plasma and urine creatinine levels rose significantly when the diet was restricted to <40% of normal diet or when the usual captive diet of romaine lettuce was changed to a wild diet of sea grass.⁵ Although the manatees likely get most of their fluid from the diet, there were no other consistent changes that were significant in the urine chemistries when the diet was restricted or modified. The highest specific gravity and osmolality measured for these two animals were 1.022 and 791 mosm/kg, respectively. This would indicate that these manatees have an extremely limited ability to concentrate urine. Further study of the renal physiology of these animals is necessary for an understanding of the significance of this fact.

In conclusion, it appears that sodium and chloride are actively excreted through the urine of the Florida manatee. In addition, when manatees are maintained in seawater, a substantial quantity of these electrolytes are taken in, either actively through drinking the seawater or passively when feeding on plants in the seawater. Finally, the results indicate that the Florida manatee has a very limited ability to concentrate urine.

Table 1. Mean urine chemistry concentrations, specific gravity, and osmolality for two Florida manatees, when maintained in either fresh water or seawater.

Acknowledgements

The authors thank The Columbus Zoo and Aquarium for funding a portion of this study. They also thank Debbie Colbert, Joe Gaspard, Brandie Littlefield, Wendy Fellner, and Gordon Bauer as well as numerous undergraduate college interns for the training and handling of the manatees, and Howard Rhinehart and Lynne Byrd for collection and analysis of samples.

References

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2.  Bossart GD, LA Dierauf. 1990. Marine mammal clinical laboratory medicine. In: Dierauf, L.A.(ed.) CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation, First Edition, CRC Press, Boca Raton, Florida. Pp. 1-52.

3.  Colbert DE, W Fellner, GB Bauer, CA Manire, HL Rhinehart. 2001. Husbandry and research training of two Florida manatees (Trichechus manatus latirostris). Aquatic Mammals 27:16-23.

4.  Irvine AB, FC Neal, PT Cardeilhac, JA Popp, FH White, RL Jenkins. 1980. Clinical observations on captive and free-ranging West Indian manatees, Trichechus manatus, in Florida. Aquatic Mammals 8:2-10.

5.  Manire CA, CJ Walsh, HL Rhinehart, DE Colbert, DR Noyes, CA Luer. 2003. Alterations in blood and urine parameters in two Florida manatees (Trichechus manatus latirostris) from simulated conditions of release following rehabilitation. Zoo Biology 22:103-120.