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Biology

a Whale’s tale

Hamilton, V., Evans, K., & Hindell, M. A. (2017). From the forests to teeth: Visual cross-dating to refine age estimates in marine mammals. Marine Mammal Science.

Have you ever wondered how marine biologists know the age of the mammals they study? Of course, some may be fortunate enough to witness the birth of a juvenile and then track their growth through time. Most often though, birth in the wild is unseen by interested scientists and estimates of approximate age may be assigned based on developmental stages.

Exact age can be trickier to determine. A solution to the problem is to apply the method dendrologists (tree scientists) use to date trees—they count the rings. Similar to trees, the teeth of marine mammals grow incrementally in layers (Figure 1). These layers can be counted to determine the age of an individual- just like tree rings! For example, if a tooth has 20 layers, and the layering rate is 1 layer per year, than the estimated age would be 20 years.

Sperm Whale tooth, scale is faint. Source: https://commons.wikimedia.org/wiki/File:Sperm-whale-tooth2_hg.jpg

Figure 1: Sperm Whale tooth, scale is faint. Source: https://commons.wikimedia.org/wiki/File:Sperm-whale-tooth2_hg.jpg

Traditionally, marine mammal scientists have based age estimates on a single tooth. To accommodate the finite, sometimes difficult to distinguish layers, multiple ‘readers’- one who counts the layers-submit counted ages, and the mode (or most common value) is used.

Hoping to improve age estimates, a group of scientists working in Australia sought to move beyond traditional methods and tried an alternate approach. They proposed that rather than using a single tooth with multiple readers, that instead they could count the layers of multiple teeth from a single individual, and then cross-date the teeth with each other.

The proposed method is dependent on groups of layers defined as a set of alternating ridges and grooves called Growth Layer Groups (GLGs). Some may have unique morphology or a marker line from mineralization interference.  GLG’s are a function of environmental variation being recorded in teeth. Layers that are not included in GLGs are called accessory. GLGs are special because in a single mammal, they are recognizable on multiple teeth. This is important because it allows scientists to cross-date the teeth.

FIgure 2: Beached sperm whale

FIgure 2: Beached sperm whale

Cross-dating is identifying like GLGs in a pair of teeth. It is helpful because it can alleviate discrepancies in age. For example, if two teeth both have the same GLG but one was counted at 22 years and the other counted at 20 years, the researchers know the age is between 20-22 years. After their initial estimation, they can go back and look carefully to recount layers and try to find the layers they missed in the younger tooth; perhaps a few accessory layers were not visible in one tooth- that would be enough to cause a difference in the age estimations.  When only a single tooth is evaluated, missing layers may go unnoticed, offsetting the age.

Beached sperm whale. Source: https://commons.wikimedia.org/wiki/File:Sperm_Whale_-_Alturlie_Point_-_geograph.org.uk_-_1241380.jpg

Figure 3: Beached sperm whale. Source: https://commons.wikimedia.org/wiki/File:Sperm_Whale_-_Alturlie_Point_-_geograph.org.uk_-_1241380.jpg

Before implementing the new approach, the scientists had to first collect teeth. Unfortunately tooth collection means that teeth were accessible, and in this case, it was because pods of sperm whales had beached themselves (figure 2).   Beaching events occurred three times in two locations in Tasmania, Australia: two at Ocean beach in 2003 and 2007, and a third on Flinders Island in 2004 (figure 3).   In total, between 2 and 4 mandibular (lower jaw) (figure 4) teeth were collected from 15 male sperm whales (feature image).

Figure 3: Sperm whale head anatomy. Source: https://commons.wikimedia.org/wiki/File:Sperm_whale_head_anatomy.svg

Figure 4: Sperm whale head anatomy. Source: https://commons.wikimedia.org/wiki/File:Sperm_whale_head_anatomy.svg

After the teeth were collected, they were etched (coated with a protective layer) and photographed before the GLGs were counted (see figure 1 in the original article). Notable GLGs were described based on their appearance and the year of deposition was estimated. The notable GLGs were compared to other GLGs on the individuals other teeth.  Offsets between the teeth were the investigated and adjustments were made to tighten the age constraint.

Some difficultly in this newer method is introduced because some teeth preserve layers better than other. This can be a natural causation related to the shape, and wear and tear of the teeth. It can also be a function of etching techniques.

All in all, scientists were satisfied with the adjustments that the new method enabled them to make to the ages. For each individual whale, the GLGs did help reduce the error associated with estimated age. The researchers acknowledge that the best method to use will be dependent on the goals of the study.

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