Table 1.

Summary of the general linear mixed model fitting birth weight of Soay sheep twin lambs in relation to their own sex, the sex of their co-twin and the interaction of their own sex and co-twin sex, as well as other variables known to explain variation in birth weight (see §2). (Significance of model terms was assessed by adding them sequentially to produce the final model. Lamb sex gives weight of male versus female lambs; co-twin sex gives weight of lambs with a male versus female co-twin; nt is the population density in the year of the lamb's birth; nt−1 is the density in the previous year (n = 354 lambs of 184 litters of 119 mothers).)

fixed effectsparameter estimate (s.e.)F-valued.f.p-value
 constant1.826 (0.146)
 capture age (in days)0.105 (0.014)43.021,227.4<0.001
 birth date0.0156 (0.0032)26.671,155.3<0.001
nt0.0011 (0.0002)12.291,18.50.002
nt−1−0.0017 (0.0002)56.521,20.5<0.001
 mother's age0.0129 (0.010)5.021,158.90.026
 mother's age2−0.0094 (0.0033)14.421,148.9<0.001
 mother's weight (kg)0.0378 (0.0087)17.851,172.1<0.001
 lamb sex0.0231 (0.051)26.341,271.5<0.001
 co-twin sex−0.180 (0.050)5.191,268.00.024
 lamb sex × co-twin sex0.206 (0.074)7.661,152.90.006
 mother's weight20.181,175.50.67

variance componentsΩ (s.e.)

 birth year0.0047 (0.0041)
 mother ID0.028 (0.0095)
 litter ID0.010 (0.0082)
 residual0.061 (0.0066)