Populations with different densities often show genetically-based differences in life histories. The divergent life histories could be driven by several agents of selection, one of which is variation in per-capita food levels. Its relationship with population density is complex, as it depends on overall food availability, individual metabolic demand, and food-independent factors potentially affecting density, such as predation intensity. Here we present a case study of two populations of a small live-bearing freshwater fish, one characterised by high density, low predation risk, low overall food availability, and presumably low per-capita food levels, and the other by low density, high predation risk, high overall food availability, and presumably high per-capita food levels. Using a laboratory experiment we examined whether fish from these populations respond differently to food limitation, and whether size at birth, a key trait with respect to density variation in this species, is associated with any such differential responses. While at the lower food level growth was slower, body size smaller, maturation delayed and survival reduced in both populations, these fitness costs were smaller in fish from the high-density population. At low food, only 15% of high-density fish died, compared to 75% of low-density fish. This difference was much smaller at high food (0% vs. 15% mortality). The increased survival of high-density fish may, at least partly, be due to their larger size at birth. Moreover, being larger at birth enabled fish to mature relatively early even at the lower food level. We demonstrate that sensitivities to food limitation differ between study populations, consistent with selection for a greater ability to tolerate low per-capita food availability in the high-density population. While we cannot preclude other agents of selection from operating in these populations simultaneously, our results suggest that variation in per-capita food levels is one of those agents.

Please find this information in the manuscript and supporting information.

General note: Please also read the ReadMe files that accompany each dataset; they explain the column headers.

Fish data

This file contains the experimental treatment, ancestral population, measured life-history traits, maternal identity and maternal size of 80 second-generation laboratory-reared individuals. Note that the body sizes and age at maturity of one individual (H7) may contain measurement errors. We thus excluded this individual from all analyses except that of survival to maturity, where the individual was retained but its size at birth was not used as a covariate in the analysis. The data in this file were used to produce Figs. 3-7.

Water data

This file contains measurements of water quality from both study locations across three years. Note that concentrations of nitrate were below the detection limit in Trout Pond, and that concentrations of nitrite and total phosphorus were undetectable in both water bodies. The data in this file were used to produce Fig. 2.

Measuring trial

This file provides the results of a smaller laboratory experiment aimed at estimating the correlation between standard length and dry mass in newborn fry. The fish used for this experiment were separate from those used in the main study. The results and further details of the experiment can be found in the Methods section.