Maternally-transferred thyroid hormones and life-history variation in birds

1. In vertebrates, thyroid hormones (THs) play an important role in the
regulation of growth, development, metabolism, photoperiodic responses and
migration. Maternally transferred THs are important for normal early-phase
embryonic development when embryos are not able to produce endogenous THs.
Previous studies have shown that variation in maternal THs within the
physiological range can influence offspring phenotype. 2. Given the
essential functions of maternal THs in development and metabolism, THs may
be a mediator of life-history variation across species. 3. We tested the
hypothesis that differences in life histories are associated with
differences in maternal TH transfer across species. Using birds as a
model, we specifically tested whether maternally transferred yolk THs
co-vary with migratory status, developmental mode, and traits related to
pace-of-life (e.g. basal metabolic rate, maximum lifespan). 4. We
collected un-incubated eggs (n = 1-21 eggs per species, median = 7) from
34 wild and captive bird species across 17 families and 6 orders to
measure yolk THs (both triiodothyronine, T3 and thyroxine, T4), compiled
life-history trait data from the literature, and used Bayesian
phylogenetic mixed models to test our hypotheses. 5. Our models indicated
that both concentrations and total amounts of the two main forms of THs
(T3 and T4) were higher in the eggs of migratory species compared to
resident species, and total amounts were higher in the eggs of precocial
species, which have longer prenatal developmental periods, than in those
of altricial species. However, maternal yolk THs did not show clear
associations with pace-of-life related traits, such as fecundity, basal
metabolic rate, or maximum lifespan. 6. We quantified interspecific
variation in maternal yolk THs in birds and our findings suggest higher
maternal TH transfer is associated with the precocial mode of development
and migratory status. Whether maternal THs represent a part of the
mechanism underlying the evolution of precocial development and migration
or a consequence of such life histories is currently unclear. We therefore
encourage further studies to explore the physiological mechanisms and
evolutionary processes underlying these patterns.