Fine whines improve with age

We sought to reopen and broaden discussions of offspring solicitation, offering 2 alternatives to the reigning Signal of Need (SoN) paradigm. Hinde and Godfray’s view that ‘‘most available evidence’’ appears to support SoN is beyond doubt but whether that indicates a healthy science or a worrisome narrowing of active-choice possibilities is less clear. After all, landslide elections occur whenever just 1 party is on the ballot. In this case, however, considerable data already support Signal of Quality (SoQ), so that idea has a well-deserved place at the table. Parental favoritism toward stronger offspring is a perfectly reasonable complement to the widespread life-history strategy of initial overproduction. Lab conditions with ad libitum food and zero predation may essentially eliminate foraging costs and lead parents to ‘‘invest’’ differently, but taxa that evolved outdoors routinely face tougher choices. In solving their real-life problems, parents are likely to use any reliable information they can find, with signals being only one source category. We should remain open to factors overlooked in early models, freely adjusting theory to nature, not vice versa. Of the active-choice hypotheses considered in our review, one was cast in elegant equations; the others, to date only as verbal arguments. (Actually, Grafen’s SoQ—the simplest handicap application—needs only a kinship constraint added to the costs to move from sexual signaling over to begging.) And we agree with several commentary authors that begging models should be dynamic, so all current models fall short. To be very clear, we can easily imagine how resource-rich parents and cooperative sibships ‘‘might’’ profit within a SoN system, but we see little in nature to suggest that this heart-warming possibility occurs regularly. We certainly appreciate what Wright called the ‘‘central role’’ of kinship in limiting selfishness (e.g., in SoQ, see our figure 2a), but that limitation is finite as specified explicitly by Hamilton’s rule. We are glad to see Grodzinski et al. distinguish their Fuel Gauge Model from our Signal of Hunger (SoH). The latter was derived independently, but we chose to err on the side of giving fuel gauge due precedence for any perceived overlap. Their point on efficient functioning of the ‘‘digestive engine’’ clarifies things nicely. Less happily, several commentators misread our call for a moratorium on overly simplistic hunger experiments as a universal ban on hunger, period. We made it clear (APPENDIX) that hunger can factor somehow into Godfray’s ultimate version of need. But how? Far too many workers have used only short-term measures (food deprivation) and then plugged that in as the SoN model’s ultimate variable. Showing repeatedly that hungrier babies escalate is of no further value ‘‘as an end unto itself’’, but Kolliker may be right that we should have elaborated further. If assessing how hunger affects signaling is merely a convenient first step toward subsequent linkage with compelling fitness measures (presumably what Wright dubbed its ‘‘potential utility’’), we have no problem, but that key second step is often missing. Meanwhile, Wright’s assertion that satiety ‘‘almost never’’ occurs must remain an open issue. We have frequently observed senior egret and sparrow sibs ingesting so much that they cease begging and even nap through subsequent meals. Kilner and Johnstone seem intent on framing our distinction between Godfray’s ‘‘need’’ and empiricists’ ‘‘hunger’’ in starkly polarized terms (‘‘wholly distinct’’), but we were not so extreme (see ‘‘limited equivalency’’; box 1). Their more interesting question (why should adults actively promote brood reduction?) is easily answered: Parents sometimes gain from favoring their strongest offspring via various measures (many well shy of infanticide). The active-choice phase of parental care begins well before physical sibling competition becomes effective, so parents stand to conserve valuable investment (Grafen’s image of not wasting food on a lost cause). We agree enthusiastically with Kilner and Johnstone’s closing point that Godfray’s model has served its purpose admirably. Good models are tools for promoting clear thinking, but the actual thinking that ensues is not the modeler’s job. They can only lead a horse to water. In our opinion, much of the empirical work ‘‘supporting’’ SoN was incautious and uncritical about various key assumptions, especially parental life-history objectives and whether sibling interests are ‘‘sufficiently’’ aligned to drive the evolution of an SoN system. Science proceeds best when trying to falsify predictions from an array of alternative hypotheses, rather than enshrining just one. The points we raised in our review may prove incorrect, as we shall see when they are tested. Who knows at this point whether a proximate and ephemeral phenomenon like hunger will turn out to track fitness consequences adequately? Or whether a given reproductive effort will require parental sacrifice of some offspring to enhance sibling prospects? We do not. But these are important questions, answers to which cannot be provided by assumptions. The empiricist’s job is to consider alternatives without bias before seeking answers. For family dynamics, the inequality at the heart of Hamilton’s rule shows that outcomes may tip either way. Our review proposes that behavioral ecologists interested in offspring solicitations must consider diverse functional explanations. Sometimes—within or between taxa—the signals may truly be begging (SoN); other times, boasting (SoQ). And sometimes they may be just a bit of bookkeeping (SoH). We hope readers will pursue falsification criteria for all 3 hypotheses and devise additional ones as well.