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.