Unlike humans, humpback whales have no laugh lines, grey hairs or sagging skin to give away their age.
Once the whales are about one year old, their size and external features do not change reliably as they age. As they can live for up to 95 years, accurate age estimates are important to scientists studying the animals’ biology and population dynamics.
Now scientists have developed the first DNA-based test for estimating the age of humpback whales, based on recent advances in human medicine.
The non-lethal test looks at age-related changes in the DNA of skin samples that can be collected from live whales using biopsy darts.
The research, published in Molecular Ecology Resources in March, was conducted by the Australian Antarctic Division, in collaboration with the Center for Coastal Studies in the United States, and the Australian Genome Research Facility.
Australian Antarctic Division molecular biologist Dr Simon Jarman, said the new test relied on changes in the ‘DNA methylation’ of genes involved in the ageing process. DNA methylation is a biochemical process where a methyl group (CH3) is added to specific DNA building-blocks. This process alters the expression of genes.
‘We’ve long known that DNA methylation is involved in processes like sex determination and the development of many cancers, but it has only recently been shown to be involved in the ageing process,’ Dr Jarman said.
‘We used information on age-associated DNA methylation in human and mouse genes, to identify similar gene regions in humpback whales. We then developed an assay using three of the most informative methylated genes, which estimates age with a high level of precision.’
Estimating age is important for monitoring the recovery of whale populations following commercial whaling. When combined with genetic information about the relatedness of individuals in a population (parents, siblings, offspring), age data improves methods for estimating the size of whale populations. It is also critical to understanding how biological characteristics of whales change with age.
Age estimates in live whales have been made by analysing lipid (fat) profiles from blubber samples. However, lipid profiles vary depending on the whales’ diet, which varies depending on their foraging range. Scientists can also identify whales through unique markings on their tail flukes. If a whale’s fluke was photographed when it was a calf, its age can be easily calculated on future sightings.
Scientists have tried to estimate the age of dead humpback whales using growth layers in a waxy structure within the animals’ ears. However the method is unreliable and can’t be used on live whales.
‘Our new DNA methylation-based test provides a minimally invasive and more reliable test that does not kill the whale, that is less labour intensive than photographic identification, and that can be used on any humpback whale found in any ocean,’ Dr Jarman said.
‘Almost everyone who studies humpback whales collects biopsy sample for population genetic analysis, so this method also fits into established sampling programs.’
The test does need to be ‘calibrated’ for the species being studied. For example, for this study the research team used a population of 40 humpback whales of known age, ranging from a few weeks to 30 years, living in the Gulf of Maine in the United States. The ‘humpback epigenetic age assay’ was conducted on skin biopsies from these whales to establish its accuracy.
‘We found this test to be more accurate than other non-lethal methods of ageing whales and it was also very good at determining the age order of a parent and its offspring,’ Dr Jarman said.
‘We can now use this test on any humpback whale anywhere in the world. We’ll have to calibrate it again for other whale species, but we already have a lot of individual pilot whales and sperm whales of known age, from strandings, and there are monitored populations of southern right whales in South America and killer whales in the north Atlantic.’
Dr Jarman hopes to extend the test to seabirds, in particular penguins, to study age-specific mortality and fertility.
For wildlife biologists, the test is an exciting application of technology that has been more recently associated with human forensic settings — such as age estimation from corpses or blood stains at crime scenes.
No whales were harmed in the course of this research.
Polanowski AM, Robbins J, Chandler D, Jarman SN (2014). Epigenetic estimation of age in humpback whales. Molecular Ecology Resources doi: 10.1111/1755–0998.12247
Corporate Communications, Australian Antarctic Division