Abstract

Dolphins urinate in the water in which they swim, making it difficult to measure their urine output. While 24 hour urine collection studies have been conducted in the past, a comprehensive review and summary of these studies, with regard to volume production, has not been published. From the standpoint of clinical evaluation of dolphins, treatment of uric acid kidney stones, insight into the relation of nutrition, hydration, and feeding strategies to renal function, and even to studies related to enclosure water sanitation, urine volume may be an important consideration. We reviewed the dolphin experiments of Ridgway² which followed and elaborated on the earlier studies of Malvin and Ryner¹ with respect to hourly urine volume produced over 24-hours.

The dolphin subjects were six female Tursiops truncatus (CYN, WAV, SHE, SQQ, ABF, ABG) and one Lagenorhynchus obliquidens (ATH). In the experiments reviewed here, dolphins were catheterized with a retention catheter, and suspended in a padded transport sling immersed in a container of water so that the lower half of the animal, below eye level, was immersed. The upper half of the animal was kept wet with water sprays. The catheter from the bladder was connected to tubing that led to a urine collection container so that the volume of urine excreted could be measured hourly. In some experiments feces were collected using a separate, larger catheter placed in the rectum. Inflatable cuffs on the retention catheters were filled with saline and were monitored and adjusted frequently to prevent irritation.

There were 18 different treatments among the seven animals (Table 1). All animals fed fish showed a marked diuresis during hours 1 to 10 following the feeding. Fasting produced no diuresis with low urine production during the entire 24 hours of collection. Giving fresh water produced more variable results, ATH showed no significant diuresis during the 24 hours after a dose of 2 L (just under 2.5% of body weight) while CYN showed a moderate diuresis after a dose of 4 L (2.2% of body weight). SHE showed a marked diuresis beginning in the first hour after receiving 3 L of seawater. WAV showed a very slight diuresis after 4 L of fresh water but much less diuresis after a dose of 2 L of 10% dextrose plus 2 L of water. When SQQ was given 2 L of sea water plus fish a rapid and extreme diuresis followed for 7 hours of the 24 hour study. Sea water ingestion always produced a diuresis with a very rapid onset. In each case, body water was almost certainly used to help bring electrolytes to within homeostatic values. When carbohydrate was given (10% dextrose solution) diuresis was not as marked even when fish were given in addition to the dextrose. Representative graphs demonstrating the effects of different treatments on the same animal are provided in Figures a-d.

Table 1. Total 24-hour urine & feces volume, dolphins.

+ Local salinity was 34 parts per thousand
*Collection stopped after 20 h
** Catheter out 1 hr
*** All fish was Spanish Mackerel Trachius symmetricus.

Figures a-d. Hourly urine volume over 24 hours in an adult female bottlenose dolphin (Tursiops truncatus), by treatment.

Click on the image to see a larger view

	Figure a. Hourly urine volume: WAV, Tursiops truncates. Last 24 hours of a 40 hour fast.
	Figure b. Hourly Urine Volume: WAV, Tursiops truncates. 2 L water + 2 L 10% dextrose
	Figure c. Hourly Urine Volume: WAV, Tursiops truncates. 4L water
	Figure d. Hourly urine volume: WAV, Tursiops truncates. 5.5 kg Spanish mackerel

Conclusions

Aminogenic stimulation from a fish meal (versus content volume or carbohydrate stimulation) causes an 8 to 10 hour diuresis in healthy bottlenose dolphins. While low numbers did not allow for statistical comparisons of urine output by fish dose, a dose-response relationship appeared to exist during hours 1-10. Sea water always produced a rapid and marked diuresis even when only small amounts were given. Dextrose administration appeared to suppress diuresis somewhat.

Follow-Up Questions

 What is the clinical relevance, if any, of a repeated diuresis that may result from multiple feedings per day?

 Does carbohydrate administration suppress glucagon release from the pancreas and therefore reduce the diuresis that ordinarily occurs with a fish meal?

 If the frequency or magnitude of the diuretic response to fish ingestion is dose-dependent and clinically relevant, then what are the implications of either 1) feeding small meals throughout the day, or 2) increasing the duration of time between feedings?

References

1.  Malvin RL, Ryner M. (1968) Renal function and blood chemistry in Cetacea. Amer. J. Physiol. 214, 187-191.

2.  Ridgway SH. (1972) Homeostasis in the aquatic environment. In: Mammals of the Sea: Biology and Medicine., ed. by S.H. Ridgway. Charles C. Thomas, Springfield, 590-747.