Dolphin Watch
Bigger is better in the water game
By ANN WEAVER
| Article published on Thursday, Jan. 22, 2009 |
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| Photo by ANN WEAVER |
| Bulls probably have it a little easier in cold waters because they have more body bulk than females and calves. |
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How are the dolphins doing in this hideous cold?
For once, the good citizens of Gulf Blvd have nothing over the rest of the country. In the short time since Christmas, we’ve gone from kayaking to sweater weather and beyond. Still, we have it easy. We can stay inside and turn up the heat.
The backwaters have dropped to their lowest temperatures of the year (in the high 50 degree F range). Even the trout are schooling and crouching against muddy seas floors to get warm. Aren’t the dolphins cold? How about the babies? Are little Doodle, Peewee, Qball, Laska, Vice, Juno and Dose big enough to stay warm? Or are they cavorting like colts out there?
I wish I could say. Windy days at sea are called “difficult observational conditions” for good reason. Dolphins stay underwater in high seas, surfacing to breathe only when they have to. Plus, we can’t see them “crouch” against the cold; they aren’t built that way.
Dolphins have the same body temperature as us, which like us, they have to maintain. The lower the water temperature, the greater the battle to retain heat and the greater the stress on a dolphin. And they can certainly die of hypothermia.
But in an effort to reassure myself as much as you, I can review the reasons why I hope they’re managing ok.
Dolphins obviously live in water constantly. Not only is water cooler than the surrounding air. It’s much harder to stay warm in water than air of the same temperature because water has twenty-five times more thermal conductivity than air. Thermal conductivity is the rate at which bodies lose heat. An air temperature of 72 degrees F is comfortable. A bathtub of 72 degrees F makes you chilly in short order.
Because water robs heat faster than air, bigger is better in the water game.
Consequently, large size alone is a requirement for aquatic mammals; whales take this to extremes. Adult Florida dolphins are 8-9 feet long (2.5-2.7 meters). Mature males are up to a quarter heavier than females, weighing up to 570 pounds (260 kg) compared to some 418 pounds (190 kg) for females. Really big males are over 600 pounds! Pound for pound, it’s probably easier on the bulls than the moms out there.
Streamlining, which accounts for the dolphins’ cigar-shaped body, reduces a dolphin’s surface to volume ratio. Consider the tremendous body bulk of a sperm whale compared to its surface area. Its surface to volume ratio is small. In contrast, consider a mouse. The “bulk” of its tiny body is more comparable to its surface area. Its surface to volume ratio is big.
So for the same reason a tortilla cools faster than a casserole, the bigger the body, the smaller the ratio and the more heat that is retained. Does that mean the dolphin calves are colder than their mothers? One thing is for sure. Calves quickly develop surprising bulk. So among our little calves, Vice and Dose are probably getting the worst of it. Vice is really small for his age, pouring more resources into healing egregious injuries than growth. Dose has been under stress since getting snared by fishing line.
In the bigger picture, large species also have lower weight-specific metabolic rates than small species. So although a sperm whale eats more than a mouse, each “unit” of its body weight takes less energy to run. One ramification is that large animals can fast longer than small animals. A sperm whale can go without food far longer than a mouse. Some whales go half a year without eating!
Streamlining also means dolphins and whales store more energy in the form of blubber than smaller terrestrial animals. Blubber isn’t as good as insulating as thick luxurious fur unless that fur gets wet. To prevent excessive heat loss and hypothermia, blubber varies in thickness and composition across species. A dolphin of warm tropical waters has thinner blubber with less lipid than a comparably-sized porpoise of cold temperate waters. As a result, the tropical dolphin loses heat four times faster than the porpoise.
Bottlenose dolphin blubber thickness varies seasonally. Since our waters are relatively warm for most of the year, we can only hope that the dolphins put on sufficient weight this fall to increase their blubber.
But blubber also varies across individuals, depending on their general size, reproductive state, and nutritional status. Some of our dolphins had bad skin conditions this fall. Bet and Valiant seemed to be healing by Christmas but not Slight or LA Stick. Split, the dolphin who kept vigil over her dead baby for a week this summer, had a very bad case of skin mottling and is probably under particularly intense stress in this cold.
In addition, an ecological principle has been prodding me since it got cold. Across a species’ geographical range, animals adapted to the warmer regions are smaller than their cold temperature counterparts. For example, the largest bears (polar and Kodiak brown bears) live in the coldest places. The smallest bears (Malaysian sun bears) live in the warmest places.
The same holds true for bottlenose dolphins. The largest bottlenose dolphins live in the coldest waters. The smallest live in the warmest waters.
Our waters get pretty warm here in the summer. I remember being struck by “how small’ our local dolphins are compared to the dolphins I studied along the Pacific coast.
Now, about those little tiny snowy egrets and monk parrots….
Dr. Weaver studies wild dolphins under federal permit GA1088-1815, National Oceanic and Atmospheric Administration. Send her an e-mail at dazzled@tampabay.rr.com or visit www.dazzlingdolphins.com.
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Article published on Thursday, Jan. 22, 2009
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