Artificial size selection experiment reveals telomere length dynamics and fitness consequences in a wild passerine

Telomere dynamics could underlie life-history trade-offs among growth,
size, and longevity, but our ability to quantify such processes in
natural, unmanipulated populations is limited. We investigated how 4 years
of artificial selection for either larger or smaller tarsus length, a
proxy for body size, affected early-life telomere length (TL) and several
components of fitness in two insular populations of wild house sparrows
over a study period of 11 years. The artificial selection was expected to
shift the populations away from their optimal body size and increase the
phenotypic variance in body size. Artificial selection for larger
individuals caused TL to decrease, but there was little evidence that TL
increased when selecting for smaller individuals. There was a negative
correlation between nestling TL and tarsus length under both selection
regimes. Males had longer telomeres than females and there was a negative
effect of harsh weather on TL. We then investigated whether changes in TL
might underpin fitness effects due to the deviation from the optimal body
size. Mortality analyses indicated disruptive selection on TL because both
short and long early-life telomeres tended to be associated with the
lowest mortality rates. In addition, there was a tendency for a negative
association between TL and annual reproductive success, but only in the
population where body size was increased experimentally. Our results
suggest that natural selection for optimal body size in the wild may be
associated with changes in TL during growth, which is known to be linked
to longevity in some bird species.