Buzz
Trees and whales can’t reveal their age, and most weren’t documented from birth. So, how do you determine their age? What methods can you use to find out the age of objects without relying on a calendar?
1. Dendrochronology
Dendrochronology, literally meaning “the study of tree time,” is commonly referred to as tree-ring dating. Trees in temperate climates develop new rings annually, with summer rings being less dense due to rapid growth and winter rings being denser and darker due to slower growth. By extracting a core sample from a living tree, scientists can count these rings to determine its age. Additionally, the width of the rings provides insights into the environmental conditions of a region during specific years.
2. Otoliths
Extracting an otolith from a red snapper, courtesy of Wikimedia Commons, fair use
All vertebrates possess otoliths, or “ear stones,” which aid in balance and interpreting gravity and movement. These structures remain roughly the same size throughout life. In fish, however, otoliths grow alongside their bodies. Similar to tree rings, seasonal dietary changes create visible rings in fish otoliths, revealing their age. Since most fish never stop growing, their otoliths continue to expand, even if only slightly each year.
3. Epiphyseal fusing
Tibia and fibula of a 12-year-old, courtesy of Gilo1969, under Creative Commons license
The epiphysis is a rapidly growing cell plate located at the ends of long bones. From birth to early adulthood, these plates change in size and shape until they vanish once growth stops. Before disappearing, their size and level of closure can provide an approximate age at death for humans or great apes. However, since epiphyseal plates are rarely useful for aging individuals past adolescence, they are primarily used to estimate the age of children and young teens in forensic or anthropological contexts.
4. Tooth formation
Image courtesy of Dozentist, under Creative Commons license
Babies are typically born without visible teeth, but their teeth are already forming inside their skulls. By the ninth week of gestation, tooth “buds” for primary (baby or milk) teeth begin to develop. Even before primary teeth emerge, permanent teeth start forming above them. From birth until the full set of permanent teeth appears (usually around ages 14 or 15), forensic experts can assess tooth development stages to estimate age at death. Wisdom teeth, which often emerge in late teens or early 20s, are rarely used for aging due to their inconsistent development and variability in modern humans.
5. Cementum annuli
What if a person’s teeth are already fully grown and firmly set? Interestingly, the cementum, which secures tooth roots, forms microscopic rings of alternating collagen and mineralization patterns. These rings allow age at death to be determined, provided the teeth are intact and not burned. The term “cementum annuli” translates to “yearly cementum,” and this method was first discovered in deer. While it makes sense for deer and other animals in environments with seasonal food changes, it’s unclear why humans exhibit the same pattern. Despite the lack of a clear mechanism, the correlation between cementum rings and known ages is strong enough to make this method widely accepted.
6. Tooth wear
Image courtesy of Ernst Vikne, under Creative Commons license
You might know the phrase, but have you ever wondered about the origin of “don’t look a gift horse in the mouth”? In the past, when livestock and work animals were common gifts, horses were often exchanged. Inspecting a horse’s mouth to determine its age was considered impolite, as it implied ingratitude. The eruption and wear patterns on the teeth of many grazing animals are reliable indicators of age. If you were checking a horse’s teeth, you were essentially questioning the value of the gift. So, don’t question the cost of a gift, don’t inspect a gift horse’s mouth, and be thankful for what you receive—even if it’s an old mare only fit for grazing!
7. Amino acid racemization
Antarctic Glaciers
Living organisms contain numerous proteins composed of amino acids. In almost all cases, these amino acids are initially formed in a “left-facing” orientation. However, after death or when tissue becomes biologically inactive, amino acids naturally transition into a racemic state, where equal amounts of left- and right-facing forms exist. Over time, the ratio approaches 50:50. Although various factors influence the rate of this process, once the racemization rate is known, the age at death or inactivation can be estimated.
Consider the inner eye of baleen whales as an example. The whale’s eye develops in the womb, with new tissue forming around the existing tissue, making the innermost layer akin to a tree core. This inner eye can reveal the whale’s age. For instance, a recently killed fin whale was found with a 19th-century harpoon embedded in its blubber. By analyzing the racemization levels in the inner eye, researchers concluded it was highly likely the whale was an adult when first harpooned, confirming the artifact’s authenticity.
8. Carbon-14 decay
Today's Chemist
While carbon-14 dating is most famous for estimating the time since an organism’s death and is widely used in paleoanthropology, it also applies to biologically inactive tissues in animals. These tissues contain a measurable level of the radioactive isotope carbon-14, which decays into stable carbon-12 at a predictable rate. By analyzing the ratio of carbon-14 to carbon-12, scientists can establish a timeline for the tissue’s age.
If the time since an organism’s death is known (determined through tissues that remain biologically active until death), the carbon-14 ratio in tissues that become inactive after a certain age (like adult teeth post-eruption) can be compared to the death timeline. This helps estimate the organism’s probable age. Often, amino acid dating and carbon-14 dating are used together to achieve a more precise estimate of age at death or tissue inactivation.
9. Earwax plugs
Thinkstock
If you’ve ever struggled with hearing due to earwax buildup, be thankful you’re not a blue whale! A recently developed method for determining age involves extracting the earwax “plug” from deceased baleen whales. Over their adult lives, these whales accumulate alternating layers of light and dark earwax, reflecting their migration patterns and diet. Similar to tree rings, each layer can be analyzed to detect exposure to toxins, physical stress, or environmental contaminants. This method also reveals how long pollutants, like banned pesticides, persist in ocean ecosystems.
