Optimal allocation of resources to growth and reproduction: Implications for age and size at maturity

Does primiparity affect the efficiency of converting energy to offspring production in the guinea-pig?

In small mammals the reproductive performance of first-time breeders is of great relevance to fitness. However, young primiparous mothers reproduce generally less successfully than multiparous ones. One constraining factor may be inefficiency of novice breeders in converting energy to offspring production. To test this hypothesis I investigated the energy cost of gestation and lactation in the guinea-pig (Cavia porcellus). I compared mothers' offspring production (growth of pups), body mass, and energy intake, as well as the solid-food intake of their pups (during lactation) and energy expenditure (resting oxygen consumption) between young primiparous, old primiparous, second-litter multiparous, and third-litter multiparous females. Young primiparous females showed significantly lower offspring production than multiparous females. For older primiparous females the level was intermediate. Both young and older primiparous females showed a lower efficiency of converting energy to offspring production than their multiparous conspecifics. Parity affected lactation most strongly. Part of the inefficiency of primiparous guinea-pig mothers can be explained by the lower contribution to the energy budget made by their offspring from independent consumption of solid food.

Stochastic variation in food availability influences weight and age at maturity

Variation in mean food availability, and in the variance around the mean, affects the growth rate during development. Previous theoretical work on the influence of environmental quality or growth rates on the phenotypic traits age and size at maturation assumed that there is no variation in growth rate or food availability within a generation. We develop a stochastic dynamic programming (SDP) model of the foraging behaviour of aphidophagous syrphids, and use this model to predict when syrphids should pupate (mature) when average food availability changes, or varies stochastically, during development. The optimal strategy takes into account not only the availability of food, but also the timing of its availability. Food availability, when small, influences developmental time, but not weight at pupation. Food availability, when large, influences weight at pupation, but not developmental time. When the food supply is low, the optimal strategy adjusts the size at pupation downwards for stochastic as opposed to deterministic availability of food. The conclusions reinforce the need for life-history studies to consider state dependence and short-term variability in growth rates.

To Grow or to Reproduce? The Role of Life-History Plasticity in Food Web Dynamics

The size of an individual is a key feature influencing and determined by a species' life history and ecology. Here, I consider how life-history plasticity within a single species can influence the outcome of food web interactions along a productivity gradient. An individual can either reproduce early but remain susceptible to predators throughout its life (strategy 1) or delay reproduction and grow to a predator-invulnerable size refuge (strategy 2). At low productivity, strategy 1 is favored because the probability of growing to a size refuge is low compared to the probability of being eaten. Here, the system is consumer controlled, and predators have large effects on the food web. At high productivity, strategy 2 is favored because high food availability increases the probability of prey attaining size refuge before being eaten. Consequently, the system becomes less consumer controlled, and predators have weaker effects on food web dynamics. At intermediate productivity, either strategy 1 or strategy 2 can be favored, depending on the initial conditions of the system. Field and laboratory experiments with a common freshwater snail Helisoma trivolis and its insect predator Belostoma flumineum support both the key assumptions and predictions of the models.

Effects of temperature on individual growth rate and body size of a freshwater amphipod

Individual growth rates of the freshwater amphipod Hyalella azteca (Saussure) were measured in the littoral zone oftwo small oligotrophic Ontario lakes and in growth experiments over a natural temperature gradient (10, 15, 20, and 238C).Field observations showed that a temperature of 208C is important for both the induction and termination of reproductiveresting stages in H. azteca. Growth rates were more affected by temperature in small than in large individuals. Growthparameters are related to rearing temperature by linear regressions, which can be used as a simple model for bioenergeticscalculations in crustaceans. A negative relationship between water temperature and maximum size attained by the amphipodswas found. The largest adults were absent in studied populations when summer temperatures were high, and this phenomenon,which has also been observed in other aquatic invertebrates, was bioenergetically determined. Energy-budget estimationsshowed negative net growth efficiency (K2) in the largest adults at temperatures above 208C. The relationship between K2andtemperature showed a dome-shaped pattern, K2values for larger amphipods being maximal at lower temperatures. Seasonalmigrations of adult H. azteca from shallow littoral to deeper cold habitats, observed in lakes during the warmest periods, appearto be temperature-induced and bioenergetically advantageous, despite probable increases in predation risk experienced inspatially simple deep-water habitats.

Evolution of longevity through optimal resource allocation

Models of life history evolution predict optimal traits of a simplified organism under various environmental conditions, but they at most acknowledge the existence of ageing. On the other hand, genetic models of ageing do not consider the effects of ageing on life histroy traits other than fecundity and longevity. This paper reports the results of a dynamic programming model which optimizes resource allocation to growth, reproduction and somatic repair. A low extrinsic (environmentally caused) mortality rate and high repair efficiency promote allocation to repair, especially early in life, resulting in delayed ageing and low growth rates, delayed maturity, large body size and dramatic enhancement of survival and maximum lifespan. The results are generally consistent with field, comprative and experimental data. They also suggest that the relationships between maximum lifespan and age at maturity and body size observed in nature may be by-products of optimal allocation strategies.

Optimal allocation of resources explains interspecific life-history patterns in animals with indeterminate growth

In a previous paper it was suggested that interspecific allometries of physiological (respiration, assimilation and production rates) as well as life-history (age and size at maturity, life expectancy) parameters are by-products of body-size optimization in animals with determinate growth. Here the analysis is extended to animals with indeterminate growth, such as fish or reptiles. In a seasonal environment it is optimal to grow after maturation, and increasingly more resources should be devoted to reproduction year after year. This leads to growth curves that closely resemble Bertalanffy’s curves characterized by growth constant k and asymptotic length l ∞ . Sets of parameters describing mortality and productivity were generated with a random number generator. Then the schedule of growth was calculated for each ‘species’ obtained, under the assumption of optimal allocation of resources. Interspecific comparisons in a set of such species resemble the empirical patterns discovered by Beverton & Holt and discussed extensively by Charnov: mortality rate and growth constant k are positively related, length at maturity and asymptotic length l ∞ are positively correlated, and k and l ∞ are negatively correlated. Data for fish and reptiles match these predictions.

Resource dependent growth and body condition dynamics in juvenile snakes: an experiment

We examined the manner in which animals adjust the proportion of energy allocated to growth and storage in response to food availability. We compared univariate growth and length-mass relationships between juvenile adders (Vipera berus) reared under two different feeding regimes. Animals in the low- and high-food experimental groups were fed suckling mice once and twice weekly, respectively. Snout-vent length, body mass, and body condition (residual scores from log-log regression of body mass on snout-vent length) were measured shortly after birth, and at 4, 9, and 14 weeks. We found that growth in length and mass, as well as changes in length-mass relationships, differed between treatments; snakes with access to more food not only increased faster in length but were also heavier at the completion of the experiment than were similar sized less frequently fed snakes. There was no association between body condition of individuals measured at birth and at the end of the experiment, whereas size at birth was a good predictor of final size. Our results provide evidence for resource-dependent allocation strategies in V. berus, and suggest that somatic growth is less sensitive to environmental fluctuations than body condition, presumably because body size is of greater importance for fitness.

Size- and age-related variation in the seasonal timing of nesting activity, nest characteristics, and female choice of parental male pumpkinseed sunfish (Lepomis gibbosus)

Parental males from a population of pumpkinseed sunfish (Lepomis gibbosus) were captured throughout the spawning season to examine age- and size-related seasonal trends in nesting activity. Females of spawning pairs were also captured to determine if male body size or nest characteristics influenced their selection of a mate. Large/old (≥ 80 mm; age 4–6 years) parental males commenced nesting earlier in the spring–summer spawning season than small/young (< 80 mm; age 2 and 3 years) parental males. As the spawning season progressed, the proportion of large/old males in the nesting population decreased until more than 70% of the males nesting late in the season were small/young individuals. Small individuals nesting late in the season were inferior in body condition to those that nested earlier in the season. These findings support the hypothesis that size-related differences in energy reserves and metabolism in centrarchids result in size-dependent variation in the timing of nesting activity. The length of females captured spawning at a nest site was not significantly correlated with the length of the parental male, water depth, or distance from shore; however, females captured spawning in firm (sand and gravel) substrates were significantly larger than those spawning in soft (silt or woody debris) substrates. Although all observed nests in the various substrates contained eggs, the positive relationship between body size and fecundity in female pumpkinseeds suggests that a selective advantage for males nesting in firm substrate may accrue through the attraction of larger females.