
A migratory lifestyle is associated with shorter telomeres in a songbird - Mz
http://www.aoucospubs.org/doi/full/10.1642/AUK-16-56.1
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reasonattlm
Telomere dynamics are sufficiently different between mice and humans to be
cautious about extracting meaning from mouse studies.

Birds have very much more different telomere dynamics.

So aside from that, average telomere length is typically measured in immune
cells from a blood sample. Telomere length shortens with each cell division,
and stem cells delivery new cells with long telomeres. So in immune cells you
are going to get a fuzzy metric that depends on everything known to influence
immune function filtered through a combination of immune cell replication and
replacement rates. So stress, injury, exertion, diet, infection, age, etc,
etc.

What you end up with is a statistical measure that can support any number of
theories, being fairly distant from underlying processes, and influenced by
many processes. There's a lot of variation in the literature, and many
contradictory datasets and interpretations.

That again is entirely distinct from the evolutionary background that leads to
a species having telomeres that start at a given length, and there are wide
variations in that metric across species - just look at mice and humans.

