Results
Heart rate showed a significant decrease with time after arrival (LMM,
estimate = -0.020, df = 208, t = -10.530, p < 0.001), but
variation in heart rate was not explained by the treatment date (LMM,
estimate = 0.002, df = 11, t = 0.485, p=0.637) or treatment order (LMM,
estimate = 0.051, df = 11, t = 0.682, p =0.509). Model egg color type
had no statistical effect on incubating females’ (n=14) heart rate
immediately after return to the nest (LMM, estimate = 0.009, df = 208, t
= 0.824, p = 0.411, Fig. 2). Furthermore, initial heart rate did not
differ between rejecter and acceptor females during the non-mimetic egg
trials (LMM, estimate = -0.053, df = 9, t = -0.875, p = 0.404). Eight
out of 14 females rejected the non-mimetic egg within 2 hrs.
Subjects exposed to mimetic or non-mimetic model eggs did not show
significant differences in circulating corticosterone concentrations 2
hours after experimental parasitism (LM, estimate: -0.118, t = -0.913,
df = 40, p = 0.367; Fig. 2). Plasma corticosterone concentrations
decreased across the season (LM, estimate = -0.012, t = -4.301, df = 40,
p < 0.001). None of the mimetic blue eggs were rejected (n=22)
whereas 7 out of 21 (33%) of robins rejected the non-mimetic egg within
2 hours of its addition (Fisher’s exact test, p=0.0036). However, plasma
corticosterone did not differ between females who rejected or accepted
the non-mimetic egg (LM, estimate: -0.008, df = 18, t = -0.044, p =
0.966; Fig. 3).
Prior to false discovery rate (FDR) correction, 312 pituitary-expressed
genes were statistically significantly differentially expressed between
birds exposed to the mimetic and non-mimetic egg treatments
(Supplementary Table 1) two hours following the experimental parasitism.
Among these, POMC showed higher expression in pituitary in birds exposed
to non-mimetic model eggs (n=6) compared to birds exposed to mimetic
eggs (n=7, fold-change = 0.719, p = 0.042, Fig. 4). One of the most
significantly differentially expressed genes prior to FDR correction was
ATF3, a transcription factor specifically associated with the stress
response (Hai et al. 1999) , which was upregulated in the birds
exposed to non-mimetic eggs (fold change=0.323, p<0.001). No
genes were significantly differentially expressed between the model egg
treatments following FDR correction. POMC expression levels and
corticosterone concentrations were not correlated with each other
(Spearman’s rho = 0.055, df = 11, p = 0.863, Fig. 5).