Home Bowhead Whale Population StudiesAge Estimation

Age Estimation

Age Estimation

Principal Investigators J. Craig George, Ph.D.
Collaborators Judy Zeh (University of Washington), Jeff Bada (Scripps Institution of Oceanography), Erich Follmann (UAF), Ray Tarpley (St. George’s University), Cheryl Rosa, John Reynolds and Dana Wetzel (Mote Marine Laboratory)
Funding NSB, NOAA

How Long Do Bowhead Whales Live?

The studies below investigated this question, which started with the local TEK on bowhead lifespan, “bowheads live two human lifetimes.” The discovery of old stone and metal harpoon tips in whales harvested in the 1980’s and 1990’s led to more speculation on the age of bowheads being much greater than the 50 or 60 years estimated by scientists. A lifespan estimate, using baleen analysis, corpora counts in ovaries of mature females and aspartic acid racemization in eye lenses, was obtained. While the maximum lifespan is unknown, these multiple lines of evidence point to extended longevity in bowheads, at least up to 150 years of age and possibly as much as 200 years.

Why Do Bowheads Live So Long?

  • Slow metabolism and growth rates
  • Delayed maturity
  • Highly labile low density prey field (George et al. 1999)
  • Demands of high lipid storage in thick blubber (thermoregulation and “life-insurance”)
  • Adjustment of longevity to ensure reproduction: “Survive the lean years and reproduce in the good ones.” (Kraus, S.D. and R.M. Rolland. 2007. Right whales in the urban ocean. In: Kraus, SD and Rolland, RM (eds.). The Urban Whale: North Atlantic Right Whales at the Crossroads. Harvard University Press, Cambridge, Massachusetts.)

Body Length versus Age Estimates for Bowhead Whales:

This graph shows the body length versus age estimates using four different lines of evidence: AAR, baleen isotope cycles, and corpora counts. Each dot represents one whale aged with one of the three methods. The baleen method is only useful for whales 20 years of age or younger. The corpora counting method is useful only for females that have reached sexual maturity. The AAR method is useful for all ages. This data provides good evidence that bowhead whales live to at least 150 years of age or longer. (Source: Lubetkin 2008)

Aging Methods:

  • Aspartic Acid Racemization
  • Isotope Analysis of Baleen
  • Ovarian and Pregnancy Data
  • Tympanic Bone

Aspartic Acid Racemization (AAR)

What is Aspartic Acid?

Left: These are models of isomers of aspartic acid, the left-handed version (L) and the mirror-imaged, right-hand version (R). Right: This image shows the layers of an eye lens, with the oldest layer, or nucleus, on the inside and newest layers on the outside. The inner nucleus is used in the AAR analysis. (Image source: Brignole, E. and J. McDowell. 2001. Amino Acid Racemization. Today’s Chemist at Work.)

Aspartic acid is an amino acid found in proteins in all living organisms. It comes in two structural forms, or isomers. These two forms have the same molecular formula but the molecular arrangements are mirror-images of each other. The two different isomeric forms are called D and L enantiomers, and they rotate in polarized light in different directions, one form to the left (the L form) and one form to the right (the D form). In new tissue, all newly-formed aspartic acid molecules are of the L isomer.


Lens from a bowhead whale eye. Photo by Cheryl Rosa

What is Racemization?

Once the L isomer is formed, it naturally undergoes a racemization reaction, or a reversible conversion to the D isomer. Living organisms maintain the amino acids in the L form, reversing the reaction from the D form back to the L form; thus, a living organism maintains a state of disequilibrium or a ratio less than one (D/L < 1). In a dead organism, this racemization reaction continues naturally in both directions (L to D, and D back to L) until an equilibrium is formed. At equilibrium there are equal amounts of each form, 50% L form and 50% D form, giving a ratio of one (D/L = 1). The rate of this reaction is calculated for each organism based on the temperature of the animal, with higher temperatures resulting in higher rates of racemization.

For the aging of eyes, the inner nucleus of the lens is used for the AAR analysis. This lens nucleus is not active, or rather, it is functionally dead. The racemization towards equilibrium began in the nucleus at birth or even during the fetal stage. In the laboratory, the nucleus is removed from the eye lens, the amino acids are extracted, and the amino acid forms are analyzed to obtain a D/L ratio. Comparing the D/L ratio to the rate of racemization curve allows the scientists to come up with an age estimate. See George et al. (1999) for more details on the methods and results.

Results of the AAR study:

Using this process, it was determined that bowheads may reach ages greater than 100 years of age (see graph below). At least one of the bowhead eye lens samples was found to be over 200 years of age (George et al. 1999).

Bowhead whale body length versus racemization age from eye lens:


The top line are female bowhead whales and the bottom line are males. Estimated age-at-length for bowhead whales using the aspartic acid racemization technique. Female bowhead whales tend to be larger than males. (Source: Rosa et al. 2004)

Isotope Analysis of Baleen

Most BCB bowhead whales make annual migrations from the Bering Sea in the winter to the eastern Beaufort Sea in the summer. Thus, they are feeding in different regions in the spring and the fall. These different regions are isotopically different; that is, they have different carbon isotopes (or different forms of the carbon atom) in the environment and in the prey species. These different forms, or isotopes, will be incorporated into the bowhead tissues formed after those feedings. Therefore, the carbon isotope found in the layers of baleen plates should show a “signature” of the two different feeding areas, the Bering Sea and the Beaufort Sea, which provided the carbon for that growth. Oscillations between the two different feeding areas, or isotope signatures, in the baleen match up with their annual migration pattern. Counting the number or oscillations, or cycles, gives a measurement which correlates to the age in years of the animal

This baleen aging method can only be used for whales up to about 20 years of age. After that age, their baleen plates have grown to full-size, and reach a steady state between growth and wear; older years will have worn off making accurate aging after that time impossible. The baleen aging method can be used to help verify other methods (for instance, amino acid racemization or corpora aging in females).


Baleen plates on a harvested bowhead whale. There are about 300 plates on each side of the bowhead’s mouth. (Photo credit: NSB-DWM)


A piece of bowhead baleen about 13 feet long, held by Craig George, Todd O’Hara and Lara Dehn. (Photo credit: DWM)


This graph shows the amounts of carbon-13 isotope found in the baleen layers. The tip of the baleen is the oldest part and the thick base is the newest part of the plate. The amount of carbon-13 isotope found in the baleen is determined by how much of the isotope is in their food source. Throughout their migratory route, the isotopes amount vary which is seen as an annual variation. The peaks for each year show the spring growth and the troughs show the fall growth. (Source: Lubetkin 2008)

Ovarian and Pregnancy Data – Corpora Age Method

The use of this method of aging requires:

  • An estimate of the age of bowhead whales at maturity, which is estimated to between the late teens to late twenties. The most reliable trait for estimating maturity at this time is body length, with the majority of females reaching maturity at 13.5 meters in length. (George et al. 2004).
  • The total number of corpora albicantia (or corpus albicans, singularly) and corpora lutea (or corpus luteum, singularly) are counted in the sectioned ovary. After an egg (or ova) is released from a follicle in the ovary during ovulation in mammals, the follicle becomes a corpora luteum (or CL). The CL swells and releases hormones to help maintain the pregnancy in the case of the egg being fertilized. After the pregnancy, whether the pregnancy was successful or not, the CL shrinks into a corpus albicans (or CA). The CA is made of tough, fibrous scar tissue which remains visible as a record of ovulation. (Tarpley and Hillmann, 1999).


Bowhead whale ovaries with a CL on the lower ovary, from spring whale harvested in 2004. (Photo credit: Craig George)

  • The ovulation and pregnancy rate for the bowhead whale. The calculation of ovulation and pregnancy rate of every three to four (3-4) years is estimated and the cycles shown below.


This diagram shows the reproduction cycle of the female bowhead for two different scenarios. The top timeline would be the cycle for three years between pregnancies. The bottom timeline shows the cycle for four years between pregnancies. (Source: George et al. 2004)

Note:

Some of the drawbacks for using this method of aging is that it cannot be used for males or for immature females. Also, it is not possible to determine whether a corpus is the result of a successful pregnancy or if the mother was not successful at carrying her fetus to term. However, for mature females, it can be combined with other methods for aging for verification. It has also been a very useful tool in the study of the female reproductive cycle which is important for understanding population growth rates.

Publications:

Other Eye Related Research:

Journal articles on Baleen Aging:

Journal articles on Ovarian Corpora Age Method:

More journal articles on Aging of Bowhead Whales:

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