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.

To examine the assumption, underlying much ecological theory, that a "Jack-of-all-trades is master of none", a comparison was made of the aquatic predation of four colubrid snakes, two aquatic specialists (Thamnophis couchi, T. melanogaster) and two terrestrial-aquatic generalists (T. sirtalis, and T. elegans). Observations were made in the field, then juvenile snakes were compared under controlled laboratory conditions. The specialists and generalists had qualitatively different foraging repertoires. The specialists made lengthy dives, crawled slowly on the underwater substrate and made long-distance underwater responses to prey: orientation, approach, pursuit, and frontal attack. The generalists dived briefly, with rapid serpentine locomotion, or searched "terrestrially" by wandering along the shoreline and snatching prey from the water surface. Underwater, the generalists' only prey-directed response was sudden short-distance attack, and they appeared to rely on chance encounters with prey. There were significant differences between the two pairs. In comparison with the generalists, the specialists 1) captured more fish, 2) spent more time on aquatic search, 3) spent more aquatic search time diving, 4) spent more diving time on the underwater substrate, 5) made more attacks in every category of search in open water, and 6) attacked underwater from a greater distance. Significant differences were not found in attack frequencies in those search categories where underwater vision is probably unimportant (aerial attacks and attacks in underwater crevices), nor in aerial attack distances. The specialists' behavioral superiority in respect of their specialization supports the "Jack-of-all-trades" assumption. The observations suggest that this superiority is associated with superior underwater visual acuity (which may be due to superior visual accommodation). The generalists' principal underwater searching technique (serpentine diving) appears to be energetically costly, and may only be profitable when aquatic prey are especially vulnerable. Nerodia sipedon, member of a genus that is closely related to Thamnophis but more aquatic, had a foraging repertoire qualitatively similar to that of the generalists. The similarity may be associated with N. sipedon's nocturnal habits and presumed reliance on non-visual prey cues.

Abstract. In marine ecosystems climatic fluctuation and other physical variables greatly influence population dynamics, but differential effects of physical variables on the demographic parameters of the two sexes and different age classes are largely unexplored. We analyzed the effects of climate on the survival and recruitment of both sexes and several age classes of a long-lived tropical seabird, the Blue-footed Booby (Sula nebouxii ), using longterm observations on marked individuals. Results demonstrated a complex interaction between yearly fluctuations in climate (both local and global indexes, during both winter and breeding season) and the sex and age of individuals. Youngest birds' survival and recruitment were commonly affected by local climate, whereas oldest birds' parameters tended to be constant and less influenced by environmental variables. These results confirm the theoretical prediction that sex-and age-related variation in life-history demographic traits is greater under poor environmental conditions, and they highlight the importance of including variability in fitness components in demographic and evolutionary models. Males and females showed similar variation in survival but different recruitment patterns, in relation to both age and the spatial scale of climatic influence (local or global). Results indicate different lifehistory tactics for each sex and different ages, with birds likely trying to maximize their fitness by responding to the environmental contingencies of each year.