Age-specific reproduction in a long-lived species: the combined effects of senescence and individual quality

Abstract

Apparent changes in breeding performance with age measured at the population level can be due to changes in individual capacity at different ages, or to the differential survival of individuals with different capabilities. Estimating the relative importance of the two is important for understanding ageing patterns in natural populations, but there are few studies of such populations in which these effects have been disentangled. We analysed laying date and clutch size as measures of individual performance in a population of mute swans (Cygnus olor) studied over 25 years at Abbotsbury, UK. On both measures of breeding performance, individuals tended to improve up to the age of 6 or 7, and to decline after about the age of 12. Individuals with longer lifespans performed better at all ages (earlier laying, larger clutches) than animals that ceased breeding earlier. We conclude that the apparent mean increase in performance with age in mute swans is due to both individual improvement and differential survival of individuals who perform well, while the decline in older age groups is due to individual loss of function. Our results underline the need to take individual differences into account when testing hypotheses about life histories in wild populations.

Figure 1

(a) Fitted values for the LMM exploring age effects on laying date (model 1), illustrated with three ALR classes; 3–6 years, grey diamonds; 7–11, black squares; 12 or more, open triangles. Note that for illustrative purposes the fitted values for the continuous variable ALR are averaged across the three classes to give three curves representing ALR. Owing to different numbers of individuals in each age/ALR class, the effects of different years and the effects of individuals, the averaged points do not appear as continuous curves. (b) Mean and Bonferroni CIs for the residualized laying date for six age/ALR classes. Since there are 15 possible comparisons, the Bonferroni α-level is set to 0.05/15=0.003 for each test, and the CIs are 99.7% CIs. Standard errors are pooled standard errors, and the d.f. for the tests are 2008. In fact, only seven meaningful comparisons are considered (age classes across ALR groups (four) and age differences within age classes (three)). Horizontal bars show significant differences with an indication of the Bonferroni adjusted significance of the difference (^*p<0.05, ^**p<0.005). Symbols are the same as given in (a). (c) Fitted values for the LMM exploring effects of RRL on laying date (model 2), illustrated with three ALR classes. Note that RRL is plotted from right to left, so that last reproduction occurs at the r.h.s. Symbols are the same as given in a.

Figure 2

(a) Fitted values for the LMM exploring age effects on clutch size illustrated with three ALR classes. Symbols are the same as given in figure legend 1. (b) Mean and Bonferroni CI for the residualized clutch size for six ALR classes. See legend of figure 1b. The difference between age classes 7–11 and age classes 3–6 for ALR group, 7–11 is not significant (t=2.68, adjusted p=0.11) and is considerably less than the equivalent difference in the ALR=12+ group (t=3.67, adjusted p=0.004). Symbols are the same as given in figure legend 1. (c) Fitted values for the LMM exploring effects of RRL on clutch size illustrated with three ALR classes. Symbols are the same as given in figure 1a.