Parents and offspring in an evolutionary game: the effect of supply on demand when costs of care vary

Abstract

Current models of parent-offspring communication do not explicitly predict the effect of parental food supply on offspring demand (ESD). However, existing theory is frequently interpreted as predicting a negative ESD, such that offspring beg less when parental supply is high. While empirical evidence largely supports this interpretation, several studies have identified the opposite case, with well-fed offspring begging more than those in poorer condition. Here, we show that signalling theory can give rise to either a negative or a positive ESD depending on the precise form of costs and benefits. Introducing variation among parents in the cost of care, we show that the ESD may change sign depending upon the quantitative relation between two effects: (i) decreased supply leads to increased begging because of an increase in marginal fitness benefit of additional resources to offspring, (ii) decreased supply leads to reduced begging because it is associated with a decrease in parental responsiveness, rendering begging less effective. To illustrate the interplay between these two effects, we show that Godfray's seminal model of begging yields a negative ESD when care is generally cheap, because the impact of supply on the marginal benefits of additional resources then outweighs the associated changes in parental responsiveness to begging. By contrast, the same model predicts a positive ESD when care is generally costly, because the impact of care costs on parental responsiveness then outweighs the change in marginal benefits.

Sibling conflict and dishonest signaling in birds

“…These results suggest that siblings that are not even born yet, and, indeed, may never be born, cast a competitive shadow back in time, which selects for exaggerating need to parents. The logical next step would be to explore how parents' response to begging is affected by the same life history factors (6,49,50,58). Longevity and lifetime fecundity have already been shown to influence other aspects of parental care, such as how parents respond to nest predators, with species that have less potential for future reproduction engaging in riskier defense behavior (59).…”

“…Therefore, unborn siblings, which may potentially exist in the future, could potentially impact the honesty of signaling between current offspring and their parents. If parents are saving resources for future breeding attempts, then this could make them less responsive to their current brood's begging, and hence select for their offspring to exaggerate their signals (3,6,46,47). We estimated conflict with future siblings from the relative number of potential future breeding attempts: adult life expectancy multiplied by the maximum number of successful broods parents can raise each year.…”

“…We next looked at whether variation in within-brood relatedness due to promiscuity also impacts offspring honesty (3)(4)(5)(6)(18)(19)(20). We used the percentage of broods with at least one extrapair offspring as our measure of promiscuity.…”

“…Offspring can solicit food using either signals of need or quality, and the same life history traits may influence the evolution of both kinds of signals (1,2). Our predictions were derived from signal-of-need models, where parents are assumed to have enough food to feed all their offspring and are selected to feed the offspring with the greatest need (1)(2)(3)(4)(5)(6)(7)(8). In contrast, when parents do not have enough food to feed all their offspring, they are expected to pay attention to signals of quality and to feed the highest quality offspring (7)(8)(9)(10)(11)(12)(13)(14).…”

“…Evolutionary theory suggests that signaling between offspring and their parents will evolve in response to the environment. At one extreme, if parents have enough resources to rear all their offspring, then offspring can be selected to honestly signal their need for food, so the offspring in the worst condition is fed (1)(2)(3)(4)(5)(6)(7)(8).…”