Flight performance
When testing monarchs on a tethered flight mill (Fig. 2A), eastern
monarchs flew longer (F1,54=8.81, P=0.004) and thereby
realized greater flight distances than western monarchs (Fig. 2B, C;
F1,54=4.56, P=0.037). In contrast, western monarchs flew
with greater power than eastern monarchs (Fig. 2D;
F1,54=6.00, P=0.018), which is expected in butterfly
populations adapted to shorter flight distances
(McKay, Ezenwa & Altizer, 2016). Female
and male butterflies did not differ in flight duration
(F1,51=0.052, P=0.82), flight distance
(F1,51=0.10, P=0.75), or power
(F1,51=0.007, P=0.93). These differences in flight
performance between eastern and western migratory monarchs correspond
with their drastically different migration distances.
We also found significant differences in wing morphology of eastern and
western monarchs. Wing morphology measurements showed that eastern
monarchs had larger wings than western monarchs (PC1:
F1,54=11.0, P=0.002), confirming previous studies
(Altizer & Davis, 2010;
Freedman & Dingle, 2018). Wing
morphology is an important determinant of migration in many winged
animals (Altizer & Davis, 2010) and
migratory monarchs have larger forewings than non-migratory monarchs
(Dockx, 2007;
Altizer & Davis, 2010;
Dockx, 2012;
Li et al. , 2016;
Yang et al. , 2016;
Flockhart et al. , 2017). In
contrast, eastern and western monarchs did not differ in wing shape
(PC2: F1,54=2.05, P=0.16). Additionally, wing size and
shape did not significantly affect flight duration (PC1:
F1,53=1.40, P=0.24; PC2: F1,52=0.34,
P=0.56), flight distance (PC1: F1,53=1.73, P=0.19; PC2:
F1,52=0.82, P=0.36) or flight power (PC1:
F1,53=0.47, P=0.50; PC2: F1,52=1.68,
P=0.20). We also found no differences in wing size (PC1:
F1,53=0.042, P=0.84) and shape (PC2:
F1,53=1.42, P=0.24) between females and males (within
location).
Average speed did not vary significantly between eastern and western
monarchs (F1,54=0.001, P=0.98) or between male and
female butterflies (F1,51=0.20, P=0.66); moreover, wing
size and wing shape did not affect average speed (PC1:
F1,53=1.05, P=0.31; PC2: F1,52=1.81,
P=0.19). Similarly, average weight loss during flight trials did not
vary between eastern and western monarchs (F1,54=0.0007,
P=0.98), and also was not affected by sex, wing size, and wing shape
(Sex: F1,51=0.84, P=0.36; PC1:
F1,53=0.002, P=0.97; PC2: F1,52=1.42,
P=0.24).