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Published on TBNWeekly.com - Jan. 25, 2007
Dolphin Watch
Breakfast at the bottom
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Photo by ANN WEAVER
Bottlenose dolphins often toss their fish food over the water, which sometimes functions to reorient the fish for easier swallowing. Other times, it seems they are just playing with their food
 
The morning seas were softly green and still. Front Slash, an adult bottlenose dolphin named for the diagonal slit assaulting the forward edge of her dorsal fin, swam methodically across a quiet cove.

The water was shallow. We tracked her progress by the trail of footprints she left on the water surface from beats of her powerful peduncle.

Then she dove. Moments later, a large mud plume ascended, a temporary stain of tan sand presumably punched from its rest on the sea floor. It dispersed slowly like a glass of chocolate milk released at the bottom of the bathtub. The fish finder showed nothing.

What was Front Slash doing? She was having breakfast at the bottom, extricating food hidden in the silty mud of the sea floor.

Dolphins hunt for hidden fish with sight and sound. In murky water, they hunt with sounds, even penetrating the sea floor with beams of sound shot from their amazing foreheads. These sounds change energy when they hit something and bounce back in its image like some kind of acoustic hologram that dolphins are uniquely designed to read.

Who was she eating? Candidates include eels and wrasses who shelter in the sand but we don’t know.

Perhaps understandably, dolphin behaviorists know little about dolphin feeding, a most important behavior. Study area waters are seldom clear enough for scientists to peer into their depths. Feeding dolphins are fleet.

Boat-based scientists see little beyond the occasional glimpse of a preoccupied dolphin gulping air before descending again. Plus, local bottlenose dolphins may share maritime feasts with other large predators, which discourages direct observation of feeding by scientists swimming in murky waters.

Regardless, since it is illegal for people to swim with free-ranging dolphins in the United States, the swimming option is out for dolphin behaviorists working here. That’s OK. It takes tremendous work to simultaneously swim and collect sound dolphin data.

Elsewhere in the world, remarkable research by gutsy swimming scientists Kelly Rossbach and Denise Herzing revealed that bottlenose dolphins sometimes feed by plunging their jaws, called the rostrum, deep into the sand for prey. Some of them penetrate the sand to their pectoral (arm) fins. They called the behavior crater feeding because of the ‘moonscape’ of pockmarks left behind on the sandy ocean floor.

If Front Slash created her mud plume to extricate a bit of breakfast by shoving her face into the sand, it’d be like you eating popcorn by shoving your face into the bowl instead of using your hands. Who would’ve thought dolphins feed like this?

Yet, Front Slash’s rostrum is beautifully designed for the job. I’m intrigued by mud pluming because we should’ve predicted it from the design of the bottlenose dolphin’s face.

All animals specialize on particular foods. What‘s amazing is that these specializations are reflected in the physical design of animal faces (domestic breeds excepted).

Functional face designs are easiest to see in birds. Chickens pluck tiny seeds off the ground; their beaks are perfect little tweezers for the job. Pelican beaks are long to maximize their chances of nabbing fish. Heron beaks are razor-sharp daggers that reflect their habit of stabbing prey, a good reason not to feed herons. Flamingo and spoonbill beaks are dramatically designed for shushing through water to imprison mouthfuls of small prey.

Mammals have functional faces too. Consider deer and rhinos. Deer have long delicate muzzles. Like needle-nose pliers, these delicate jaws are beautifully designed for the cervid habit of carefully plucking only sweet tender leaves as they browse the branches. Their delicate muzzles show that deer are selective feeders, picky about what they eat.

On the other hand, white rhinos have broad blunt faces. This design fits beautifully with their habit of mowing groundcover. With a mouth like this, rhinos are justifiably less selective about what they eat than deer.

The next time you’re at the zoo, study animal faces for clues about their feeding specializations. Then read about their ecology. It’s remarkable how the two go together.

Who would’ve thought that bottlenose dolphins pluck food from under the sand like deer feeding on foliage?

Much of the sea floor is in our area is silty mud, easily disturbed. It is home to lots of food as long as the water is undisturbed by pollution. In many areas, though, its covered with a plethora of sharp empty bivalve shells. Whose leftovers are these?

Dr. Weaver studies wild dolphins under federal permit GA1088-1815, National Oceanic and Atmospheric Administration. Contact her at acweaver@tampabay.rr.com.
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