USING TOOTH WEAR TO ESTIMATE AGES OF SEA OTTERS
(THIS PAGE IS UNDER CONSTRUCTION.
WE WILL POST THE WORK AS IT PROGRESSES!)
Sea otter biologists who capture live sea otters and wildlife veterinarians that do sea otter necropsies like to estimate the ages of the animals by examining the amount of tooth wear that has occurred. Other morphological parameters such as body size and pelage color are also used to arrive at an age estimate. A small tooth, a premolar, is extracted from each otter and is sent off to a laboratory that sections and stains them. An observer uses a microscope to count growth rings in the tooth section to arrive at the "official" age. We like to think that we're pretty good at estimating the age of sea otters by judging tooth wear, etc., but, since it can take several months to get the "official" age back from the laboratory, we don't get good feedback in order to improve our estimates.
Through the kindness of Jim Bodkin, our sea otter biologist, I obtained skulls from dead sea otters (most were picked up as part of annual beach walks in Prince William Sound done to estimate winter kill) for which an "official" age, obtained from a sectioned tooth, was available. My eventual goal is to develop a display and training tool by scanning in the dentition of sea otters of various ages. For now, I am just posting some examples of sea otter tooth wear.
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Here's a graph showing the "official" ages of the population of skulls from which I am drawing my subsample. I can't include all of the skulls because some are incomplete or are missing a lot of teeth. Note that the very young and the old sea otters are the ones that die at higher rates than the prime age animals. |
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The lower half of this figure is an example of a sectioned and stained premolar tooth, showing the growth rings. Normally, the observer just examines the section under a microscope and counts the rings by eye. Here I used an image analysis program to scan the growth rings and quantify intensity (darkness). Arrows have been drawn between the low intensities on the graph to the dark rings.
So, how old do you think this otter was? |
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Okay, so finally I'm getting to the pictures of the skulls. This is an example of the scans I'm getting by just placing the skull and disarticulated mandible on the flatbed scanner. The lighting is a bit uneven, due to the elevation away from the plane of scan of the anterior skull by the maxillary canines and of the posterior mandible by the ramus of the mandible. Still, this may work well enough that we can eliminate an intermediate step of photographing each skull. |
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This is a composite of the dentition of three skulls showing three levels of tooth wear. I chose these three skulls to illustrate the range of wear patterns without knowing the "official" ages obtained from tooth sectioning. ç Slight wear.
ç Medium wear.
ç Heavy wear. It is not uncommon for beach-cast skulls to be missing teeth.
Given that our population of skulls has "official" ages of from 0 to 22 years old, how old do you think these three otters were when they died? |
AHA! Just by chance, the selection of the three skulls shows why estimating ages by tooth wear alone can be misleading. The "official" ages of the three otters whose skulls are shown above are: Slight wear (top) 9 years old; Medium wear (middle) 9 years old, Heavy wear (bottom) 20 years old. So how close were your estimates?
This example may illustrate the effects that food preferences have on tooth wear. Individual sea otters tend to have favorite foods that they learn from their mothers. An otter that specializes in sea urchins may have less tooth wear than another otter of the same age that prefers clams with heavy shells that must be crushed.
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Occasionally we find a sea otter whose bones are stained a light orchid color (top). These animals specialize in eating purple sea urchins and absorb the pigment from the urchins.
The lower skull is from a sea otter that ate predominately other foods. This is the typical color of skulls we see. |
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