Introduction
Telomeres are repetitive nucleotide sequences at the terminal regions of eukaryote chromosomes. They serve to protect healthy chromosomes from DNA repair mechanisms that otherwise act on the terminal ends of chromosomes, and organize the replication of DNA during cell division (de Lange 2009). During replication, telomere sequences may be lost, thus shortening the telomere end (Olonikov 1973). Telomerase may replace the lost sequences, thus lengthening it (Chan and Blackburn 2004). Nonetheless, cells with over-shortened telomeres become “senescent” or “self-destruct” (termed apoptosis). When apoptosis occurs, the DNA-encoded information of the cell is removed from the organism (Blackburn 2000). Thus, telomeres may play a role in both cell senescence (or alternatively in cell immortalization due to the activity of telomerase; Tian et al. 2018), accumulation of senescent cells in organs (Campisi 2005), and organismal senescence (Blasco 2007; Young 2018). Further, accelerated telomere loss at a given chronological age may indicate decreasing organismal condition, especially during early development, and may underline both physiological stress and a shorter life (viz., advanced senescence; Haussmann et al. 2003; Bize et al. 2009; Boonekamp et al. 2014; Sudyka et al. 2016; Pepper et al. 2018; Whittemore et al. 2019; Sheldon et al. 2021).
Previous studies have suggested that due to the association of rate of telomere loss from chromosomes and organismal senescence, telomere dynamics during life are closely and functionally associated with lifespan among species of different body sizes and pace of life (e.g. , Dantzer and Fletcher 2015; Tricola et al. 2018). These studies have primarily focused on birds, and recognized the possible importance of body size and historical patterns (viz., the influence of phylogeny) in explaining a general pattern of relatively slowed loss of telomeres during life in larger species with syndromes of slower paces of life. These studies applied phylogenetic comparisons to 14 and 19 species of birds (TRF data), respectively, relatively small samples for robust phylogenetic analyses (though Dantzer and Fletcher 2016 presented an analysis that assumed no significant phylogenetic associations). Tricola et al. (2018) suggested that the rate of telomere shortening exhibited a strong historical pattern that may have coevolved with lifespan. Both studies concluded that larger species with relatively slower lifecycles exhibited reduced rates of telomere shortening.
These key previous studies raised a series of questions that might be examined with a larger sample of species of birds. First, how flexible are telomere traits over phylogenetic history? Tricola et al. (2018) found that telomere length was not strongly influenced by the phylogenetic pattern, but telomere rate of change (TROC) was. Alternatively, Criscuolo et al. (2021) found that neither adult telomere length nor TROC showed a strong phylogenetic pattern in a sample of, a corrected sample size of 52 bird species, a different result that needs to be explained.
Second, how do longevity, body size, and the pace of life interact with telomere dynamics? Dantzer and Fletcher (2015) found that all three variables covaried strongly with TROC, and TROC was lower for the longest-lived bird species. The latter result was confirmed by Tricola et al. (2018) with or without body mass used as a covariate. In a phylogenetic comparative analysis of 9 species of birds and mammals, Le Pepke and Eisenberg (2020) reported a low rate of telomere loss in long-lived species, but a trivial effect of body mass. These studies took a phylogenetic comparative approach where regressions were used to “control” for the phylogenetic patterns underlying variables. Still, none of them quantified how those phylogenetically-controlled patterns of telomere dynamics change with lifespan at a given body size; that is, when influences of body size are statistically controlled (Udroiu 2020).
Third, does telomere length per se change with species lifespan, with or without the influences of body size taken into account? Gomes et al. (2011) studied 61 species of mammals, and found that adult telomeres were shorter in the longest-lived species. Both body mass and longevity showed significant negative associations with telomerase and telomere length, respectively, independent of the phylogenetic pattern among the species (Gorbunova and Seluanov 2009). The conclusion of Gomes et al. (2011) was that, for large species to evolve long lifespans, replicative aging occurred (i.e., short telomeres combined with repressed telomerase activity). A result confirmed by a re-analysis of the same dataset recently (Pepke and Eisenberg 2021), with emphasis on an inverse association of telomere length with body mass. These results suggest that short telomeres may have co-evolved in long-lived species as a consequence of body size, underlying the necessity for large species to control for higher risks of cell immortalization by a widespread cellular mechanism (Tian et al. 2018). Interestingly, the phylogeny-adjusted analyses of Tricola et al. (2018) conducted in birds found no significant association of telomere length and longevity, whether or not influences of body mass were included. However, longevity and body mass are known to show strong covariance (Dantzer and Fletcher 2015), and thus may be collinear. The analyses of more bird species by Criscuolo et al. (2021), however, found no significant association of adult telomere length and either body size or the pace of life, with or without inclusion of the phylogenetic pattern in the analyses, though they did not specifically examine longevity and its association with telomere length.
The purpose of our present study was to examine associations of telomere length and TROC on the one hand, and body size and longevity on the other hand in birds. We did this using a restricted sample size of 30 of the bird species (see below for justification) reviewed in the meta-analysis of Criscuolo et al. (2021). We first asked whether any of the variables showed evidence of strong phylogenetic pattern, using the Bayesian meta-analysis approach of Hadfield and Nakagawa (2010). Longevity and body size are closely associated in birds (e.g. , Bennett and Owens 2002; Dantzer and Fletcher 2015; Criscuolo et al. 2021). Larger species reflect many aspects of life histories that covary over evolutionary time, in part because it takes longer to grow and survive to a large body size (Dobson 2007). Further, bird species vary along a “slow-fast continuum” that reflects alternative paces of life that are independent of body size (Gaillard et al. 1989). Thus, we examined variation in longevity that was strongly associated with body size (“mass-predicted lifespan”), and longevity that was statistically independent of body size (“mass-independent lifespan”). The latter reflects changes in lifespan that can be described as varying with the pace of life (Dobson and Oli 2007; Criscuolo et al. 2021). We compared both of these aspects of lifespan to telomere length and TROC, with and without statistical adjustment for influences of phylogeny.
We also addressed a further issue with respect to how TROC is measured. First, our sample included estimates of TROC that used mean differences in telomere lengths between chicks and older birds (after Criscuolo et al. 2021). Such estimates have the advantage of including the chick period, when the greatest rates of telomere loss occur as birds age (Sidorov et al. 2009; Monaghan and Ozanne 2018), but they have the disadvantage of including individuals that do not survive to adulthood (Fletcher and Dantzer 2015). A bias may thus occur between the samples of younger and older birds. Thus, we estimated TROC only from samples of adult birds. A further problem is that some estimates of telomere length measure only DNA sequences of the terminal telomere repeats, whereas other methods include DNA sequences from the body of the chromosome (Remot et al. 2021). We restricted our analyses to those studies that used the former methods and thus produced the best estimates of telomere lengths.