Heat-induced maternal effects shape avian eggshell traits and embryo development and phenotype at high incubation temperatures

Phenotypic plasticity is an important avenue by which organisms may
persist in the face of rapid environmental change. Environmental cues
experienced by the mother can also influence the phenotype of offspring, a
form of plasticity called maternal effects. Maternal effects can
adaptively prepare offspring for the environmental conditions they will
experience, however, their ability to buffer offspring against
environmental stressors as embryos is understudied. Using captive zebra
finches, we performed a maternal-offspring environmental match-mismatch
experiment utilizing a 2x2x2 factorial design. Mothers were exposed to a
mild heat conditioning (38 °C) or control (22 °C) treatment as juveniles,
an acute high heat (42 °C) or control (22 °C) treatment as adults, then
paired for breeding. The eggs produced by those females were incubated at
a hyperthermic (38.5 °C) or optimal temperature (37.2 °C). We found that
when mothers were exposed to a mild heat conditioning as juveniles, their
embryos exhibited reduced water loss, longer development times, and
produced hatchlings with heavier pectoralis muscles when incubated at high
incubation temperatures, compared to embryos from control mothers. Mothers
exposed to both the mild heat conditioning as juveniles and a high heat
stressor as adults produced eggs with a higher density of shell pores, and
embryos with lower heart rates during development. However, there was a
cost when there was a mismatch between maternal and embryo environment.
Embryos from these conditioned and heat-stressed mothers had reduced
survival at control incubation temperatures, indicating the importance of
offspring environment when interpreting potential adaptive effects.