Artificial selection reveals the energetic expense of producing larger eggs

Table 1 Body composition, metabolic rate (RMR) and oxidative damage (ROMs) of breeding and non-breeding females

Trait	Breeding	Non Breeding	t	df	P
Generation 4
Body Mass
Total Body Mass (g)	288 ± 35	258 ± 28	2.84	10.90	0.016
Non-Repro. Mass (g)	262 ± 33	258 ± 28	0.36	11.07	0.724
Body Size
Tarsus Length (mm)	40.0 ± 1.4	39.8 ± 1.1	0.38	12.09	0.712
Reproductive and Associated Organs
Repro. Organs (g)	15.18 ± 1.73	0.31 ± 0.12	39.30	8.36	<0.001
Liver (g)	7.98 ± 1.06	4.66 ± 0.80	9.78	9.60	<0.001
Protein and Lipid Reserves
Pectoral Muscles (g)	51.85 ± 7.53	51.38 ± 4.67	0.08	14.43	0.939
Body Fat (g)	4.96 ± 3.00	8.59 ± 3.94	−3.29	21.45	0.003
Generation 5
Body Mass (g)¹	254 ± 20	240 ± 17	4.20	28	<0.001
RMR (mL O₂ min⁻¹)	6.01 ± 0.67	3.52 ± 0.37	22.73	28	<0.001
ROMs (mM H₂O₂)	0.882 ± 0.263	0.801 ± 0.228	1.22	27	0.232

Means ± SD are shown. In generation 4, females were measured once, either in breeding (N = 65) or in non-breeding (N = 10) condition. In generation 5, females (N =29) were measured twice, once in breeding and once in non-breeding condition. Repro. is abbreviation for Reproductive. Significant results are displayed in bold
¹The difference in body mass between the two states is less than in generation 4 due to measuring the birds at different times of day (here the majority of females had already laid an egg)

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