TROC in the Procellariiformes and the pace-of-life
In mammals, shorter telomeres prevailed in large-bodied and long-lived species (Gomes et al., 2011; Risques & Promislov, 2018), and short telomeres are associated with repressed telomerase activity in somatic cells at adulthood (Tian et al., 2018). In contrast, we found that bird Adult TL was not related to the body size PC 1 axis. The variance in body sizes within birds is less extensive than for mammals, which might partially explain the difference observed between these two major clades of vertebrates; that is, too low a range in body size of birds to reveal strong negative selection on telomere length. Adjustment for the phylogenetic tree reduced the association to a negligible value (Table 2). Still, our within-orders regression data suggested negative relationships between Adult TLand body size, and TROC and both body size and the slow-fast continuum in Procellariiformes, an order with an extensive range of body sizes of long-lived seabirds. The former relationship suggests that, as previously found in rodents, the small and long-lived species in theHydrobatidae may have evolved specific mechanisms promoting long-lifespan while preserving telomerase activity and long telomeres over life (Tian et al., 2018). The latter link reveals that the relatively long-living and small-bodied Procellariiformes exhibit long telomeres in adulthood (Oceanodroma leucorhoa and Hydrobates pelagicus (both Hydrobatidae ), and to a lesser extend Fulmarus glacialis )) along with particularly slow embryonic growth rates (0.104, 0.166 and 1.181vs. a mean procellariform value of 1.938 g/day; note that variation in clutch size along the slow-fast continuum is negligible for procellariiform species because they lay only a single egg each breeding season; ESM1). Thus, long telomeres in this order were not significantly related to body size (PC1), but were related to the slow end of the slow-fast continuum (PC2) and thus to species longevity. This relationship is reversed in the Passeriformes, where long telomeres were related to the fast (short lifespan) end of the slow-fast continuum of life history.
One of the pathways that could decelerate the rate of cell division, and hence the rate of telomere erosion (Monaghan & Ozanne, 2018), is a prolonged incubation duration (Vedder et al., 2018). Prolonged incubation might also be associated with the maintenance of high telomerase activity throughout life in procellariiform species (Haussmann et al., 2004). An alternative explanation could be that theHydrobatidae , which are cavity nesters where chicks grow in a very contained and stable environment, even after hatching, may exhibit metabolism that favours the maintenance of the lengths of the telomeres (Stier, Metcalfe, & Monaghan, 2019). This pattern supports the antagonistic pleiotropy of telomere length processes, the main hypothesis proposed to explain why species in different phylogenetic clades have evolved large differences in telomere lengths (Hemann & Greider, 2000). This perspective deserves future exploration, notably on telomerase activity in the soma among the species of Procellariformes.