The young of birds and mammals frequently solicit food and other resources from their parents in ways that appear to be costly either in terms of energetic expenditure or because they may attract predators. Costly solicitation has been explained as a means by which the young manipulate their parents into providing more resources than the parental optimum. Alternatively, communication between parents and young can be interpreted as an evolutionarily stable signaling system. A model is developed of two young in a brood who compete for a fixed amount of resources distributed by their parents. It is shown that an evolutionarily stable signaling system can exist in which the parent obtains accurate information about the resource needs of its young and on which it bases its resource distribution decisions. Young with greater resource requirements solicit at higher levels, but the system is stable because any misrepresentation is selected against. Essential for stability is that signaling must be costly; it is the costs that penalize misrepresentation and allow stability. The necessity of costs for a stable signaling system is a direct result of potential parent-offspring conflict over resource share. The model is used to investigate the influence on expected levels of solicitation of within-brood relatedness, the condition of the offspring and the offspring's brood mate, and the total amount of resources available for the brood. The cases of identical and nonidentical young are considered. The basic model is extended to study situations in which the costs of solicitation are experienced by both brood members and to offer a preliminary analysis of the joint effects of within- and between-brood competition for resources.
The motor training hypothesis, proposed in its first form nearly half a century ago and broadened subsequently, states that the function of play is adaptive modification of the developing neuromuscular system. Evidence from many mammalian species indirectly supports the motor training hypothesis, but the exact nature of developmental change prompted by play remains unknown. We reviewed literature on the anatomical and physiological effects of exercise in mammals and categorized these as effects available to individuals at any age, versus effects available only during a discrete period of postnatal development, and transitory effects, which decay soon after exercise ends, versus permanent effects. We found that most effects are available at any age and are transitory; we argue that they are not likely primary benefits of play. However, two effects that influence motor performance-modification of cerebellar synaptogenesis and modification of skeletal muscle fiber type differentiation-are available only during a short period of postnatal development and appear to be permanent. In three species for which both kinds of data were available, the age distribution of play closely matched the age distribution of these two types of experience-modifiable development. We propose that play may not be motor training in the broad sense, but rather it may be behavior designed to influence specific types of development.
Judging from studies of homing and territorial behavior, many animals value familiar home ranges or territories This article discusses a new proximate explanation for this phenomenon: individuals may learn site-specific serial motor programs that enhance their ability to move rapidly, safely, and efficiently around obstacles and barriers in familiar areas. The literature on motor learning in humans and on hurdle race training in humans and horses yields a number of specific predictions on how animals should behave, if they practice and learn serial motor programs that facilitate high-speed locomotion along complicated routes or pathways. Support for some of the predictions of the motor learning hypothesis is already available in the literature on animal play, exploration, maze learning, and spatial orientation, and other predictions of this hypothesis should be readily testable using small mammals.
We model the relationship between mating probability and risk taking for lekking animals disturbed by predators. Our dynamic model is based on different mating probabilities among the lekking males and a decline in both predation risk and mating probability with hiding time after a predator attack at the lek. The model predicts that a lekking male with low expected mating probability should hide for a longer period after a predator attack than a male with high mating probability. It also predicts that males should hide for a longer period when predation risk is high and that a high mating advantage of a rapid return after an attack reduces the differences in optimal hiding time among males with different mating probabilities. To test the first prediction from the model, we have flushed great snipe (Gallinago media) males from leks and compared their hiding times to their temporary expected mating probabilities. As predicted by our model, males with the highest expected probabilities of mating had the shortest hiding times. Empirical data also showed that individuals adjusted their hiding time to temporary changes in their probability of mating. Such plasticity in mating behavior may reduce differences among males in lifetime reproductive success and thus also reduce the intensity of sexual selection.
Demographic models of tree populations assume that seed availability does not depend on the populations themselves. We develop models to assess the consequences of fecundity and dispersal for population structure and diversity. Results show that population structure and reproductive success are importantly affected by seed production and dispersal for realistic parameterization of time scales describing thinning, disturbance, maturation, and longevity. Maturation age affects mean and variance in seed rain. Populations with well-dispersed seed have a structure that is most sensitive to maturation age when disturbance is frequent. With restricted dispersal, delayed maturation means increased variability in seed rain, maximized when half of all patches support reproductive individuals. Density-dependent thinning compensates for the initial variability conferred by limited dispersal but not enough to permit the neglect of fecundity and dispersal at the disturbance frequencies and thinning rates typical in many forests. Longevity matters most when it is short and disturbance rare. To assess the effects of dispersal on reproductive success, we partition the contributions of seed-rain mean and variance. Fecundity and population structure affect both the mean and the variance in seed rain, albeit in different ways. Dispersal affects only the variance. The partitioned contribution of mean and variance are used to consider two cases: how dispersal consequences for reproductive success depend on life-history schedules and disturbance regime, and boundary growth rates of a globally dispersed population invading a resident population with restricted dispersal. In both cases, restricted dispersal has important consequences on the scales observed in many real forests. Most models of forest tree dynamics assume a globally dispersed seed pool that is disconnected from the populations that should produce that seed. This assumption leads to two opposing (offsetting?) consequences for species diversity: artificially high diversity due to continuous seed supply and artificially low diversity due to lack of sites where good competitors with restricted dispersal should be absent.
This article discusses how the presence of dynamic change in traits affecting inter-specific interactions changes the ways in which interactions between species are distinguished, classified, and measured. The prevalence of models lacking any trait dynamics has led to methods of identifying and measuring indirect effects that are not valid when phenotypically or evolutionarily plastic traits affect interspecific interactions. When traits are dynamic, the number of links in a dynamic model can no longer be used to distinguish direct and indirect effects, and classifying interactions by a single sign denoting effect on equilibrium density becomes problematic. The presence of trait dynamics also changes the interpretation of manipulative experiments that have been used to measure indirect effects. Because both traits and population densities can transmit indirect effects, a given ordered series of species will often transmit several indirect effects, which may have opposite signs. Because some traits can change very rapidly, dynamic equations describing population growth rates may often include the densities of species that interact indirectly with the given species. Some of the conceptual problems in comparing magnitudes of direct and indirect effects are illustrated by simple models of a three-species food chain. Different methods of measuring effect magnitudes can give different conditions for when direct effects are larger than indirect effects. Other terminology related to indirect effects, including interaction modification and higher-order interaction, is critically discussed. Given the present paucity of information about trait dynamics, it may be premature to attempt to compare magnitudes of direct and indirect effects.
Both resource control of heterotrophic biomass and heterotrophic regulation of plant populations imply that heterotrophic biomass should constitute an increasingly smaller proportion of total system biomass as the turnover time of autotrophs increases. Although this trend is widely accepted, it has seldom been tested, perhaps because comparable data for many ecosystems are hard to collect. The plankton are an exception to this difficulty. Because the biomasses of both autotrophs and heterotrophs can be estimated with relative ease and because workers use similar techniques, data are available from a wide range of lakes. Both literature data, representing a wide geographical range, and data from a localized set of lakes show that the ratio of heterotrophic to autotrophic biomass (H/A ratio) is well above unity where autotrophic biomass is low and declines where autotrophic biomass is high. A similar pattern is found in marine and terrestrial systems In terrestrial systems, this pattern has been explained by changes in the turnover time of the autotrophic biomass, but in lakes energetic subsidies from the littoral and the watershed are likely needed to support the relatively high heterotrophic biomass of many oligotrophic systems.