Senescent birds redouble reproductive effort when ill: confirmation of the terminal investment hypothesis

Abstract

This study reports an experimental confirmation of the terminal investment hypothesis, a longstanding theoretical idea that animals should increase their reproductive effort as they age and their prospects for survival and reproduction decline. Previous correlational and experimental attempts to test this hypothesis have yielded contradictory results. In the blue-footed booby, Sula nebouxii, a long-lived bird, after initial increase, male reproductive success declines progressively with age. Before laying, males of two age classes were challenged with lipopolysaccharide to elicit an immune response, which induced symptoms of declining survival prospects. Reproductive success of immune-challenged mature males fell, while that of immune-challenged old males showed a 98% increase. These results demonstrate that senescent males with poor reproductive prospects increase their effort when those prospects are threatened, whereas younger males with good reproductive prospects do not.

Figure 1

Age-dependent reproductive success of blue-footed booby males from two cohorts. Reproductive success (fledglings produced per pair) was estimated as the standardized residuals from annual reproductive success of the colony. Showing longitudinal data on 13 males from cohort 1988 and 63 males from cohort 1989, which were ringed as chicks and survived 13 years. Individual reproductive performance varied with age (F_1,179=21.69, p<0.0001) and age² (F_1,174=18.57, p<0.0001) and this effect was more pronounced in males hatched in 1988, although the interaction was no significant (cohort×age², F_1,174=2.67, p=0.10; cohort×age, F_1,179=2.13, p=0.14; cohort, F_1,195=0.49, p=0.48; male identity, p<0.01; study area, p>0.5).

Figure 2

Reproductive success (mean+s.e.) of control (PBS) and experimental (LPS) mature and old males. Experimental treatment had a different effect according to male age on (a) the number of hatchlings (treatment, F_1,44=2.25, p=0.14; age, F_1,44=0.90, p=0.34; treatment×age, F_1,44=4.28, p=0.044) and (b) the number of fledglings (treatment, F_1,44=2.95, p=0.09; age, F_1,44=0.25, p=0.62; treatment×age, F_1,44=5.35, p=0.025). Male breeding experience, included in the analyses, had a strong quadratic effect on the number of hatchlings (breeding experience, F_1,44=14.49, p=0.0004; breeding experience², F_1,44=17.44, p=0.0001) and on the number of fledglings (breeding experience, F_1,44=9.31, p=0.004; breeding experience², F_1,44=13.05, p=0.0008). Previous reproductive output did not influence the number of hatchlings (F_1,43=1.34, p=0.25) or fledglings (F_1,43=0.81, p=0.37). Sample size of each group is indicated.