Signal Of Need Research Articles

Vocal communication regulates sibling competition over food stock

Animals resolve conflicts over the share of resources by competing physically or signalling motivation with honest signals of need. In some species, young siblings vocally signal to each other their hunger level and the most vocal individual deters its siblings from competing for the non-divisible food item delivered at the next parental visit. This so-called sibling negotiation for forthcoming food has been studied only in this context. It therefore remains unclear whether siblings could also negotiate access to a pool of divisible resources, a situation that is similar to a group of individuals competing for an accessible food resource. To tackle this issue, we placed barn owl (Tyto alba) nestlings singly in artificial nests containing several mice, and we simulated the presence of a sibling calling at low or high rate using playback experiments. If nestling barn owls vocally negotiate over a divisible food stock, we propose the following two predictions. First, nestlings would vocally signal before eating from this stock of food, and second, numerous playback vocalizations would inhibit feeding. Accordingly, singleton nestlings vocalized just before consuming food stored in their artificial nest and they delayed the consumption of the food stock if hearing many playback calls. The production of such food-associated vocalizations has been observed in foraging adults in various birds and mammals, but never in young animals and when resource is divisible and easily accessible. Our study raises the possibility that vocal communication could evolve in a variety of competitive contexts. We present here the first experimental evidence that sibling barn owls use food-associated vocalizations to compete over the preys stored in the nest. Owlets emit calls just before consuming an available food item and broadcasting calls induces nestlings to temporarily refrain from eating from the food stock. This raises the possibility that vocal communication can mediate the share of a food stock accessible to all competitors.

Effects of variation in nestling hunger levels and begging on the provisioning behavior of male and female Eastern Phoebes (Sayornis phoebe)

ABSTRACT Nestling birds solicit food from their parents using conspicuous vocalizations and visual begging displays, and evidence suggests begging represents an honest signal of need that adults use to determine provisioning rates. Less is known about how adult males and females may differ in response to changes in nestling begging behavior as a result of variation in hunger levels or how nestling begging and adult provisioning may vary seasonally in multi-brooded species. To examine these parent-offspring interactions, we manipulated hunger levels of nestling Eastern Phoebes (Sayornis phoebe) during the 2011 breeding season in central Kentucky. Both first and second broods were divided into three treatments: whole brood fed (n = 12), whole brood deprived (n = 16), and some fed/some deprived (split; n = 14). Food-deprived nestlings begged with increased intensity, and fed broods begged with less intensity. Adult Eastern Phoebes adjusted their provisioning rates accordingly, provisioning food-deprive...

Begging and ectoparasite attraction

Honesty of offspring begging behaviours is the keystone to understanding the evolution of parent–offspring communication. Three main begging costs have been traditionally advocated that ensure the reliability of offspring signalling: energy expenditure, loss of inclusive fitness and attraction of predators. Here, we propose that ectoparasites may eavesdrop on begging signals, especially acoustic signals, for host detection, a never considered but potentially generalized cost of begging that will constrain the evolution of exaggerated begging displays. Ectoparasitic insects possess a diversity of auditory systems for intraspecific communication that may be used to detect begging calls of host offspring. The use of auditory cues for host detection offers some advantages to ectoparasites, particularly in environments in which long-distance detection of hosts is necessary. There are well-known examples of interspecific eavesdropping on host auditory signals by parasites that include parasitoid flies attracted to calling crickets and cicadas, and frog-biting midges and mosquitoes attracted to frog calls. Eavesdropping on begging signals may have evolved in those parasites searching for hosts that display begging behaviours, which include not only birds but also mammals and some reptiles and insects with parental care of juveniles. Considering begging costs due to detection by ectoparasites may help us understand the reliability, and therefore the evolution, of signals of need and parent–offspring communication.

Evidence for begging as an honest signal of offspring need in the biparental mimic poison frog

Whether begging behaviour mediates food provisioning has been extensively studied in birds. However, begging behaviour occurs without direct offspring competition in some species and thus may be driven exclusively by intrabrood dynamics. We studied begging behaviour of individually housed offspring of the biparental mimic poison frog Ranitomeya imitator. We tested whether (1) begging is an honest signal of need or hunger, (2) begging is costly and (3) parents allocate food according to offspring need. Under manipulation of long-term diet, food-limited tadpoles increased begging effort over the course of development. Tadpoles that were induced to beg suffered a cost of taking longer to reach developmental stages and showed a marginal cost on growth rate. Finally, parents were more likely to feed the tadpole that was exposed to a nonsupplemented diet over its food-supplemented sibling. In R. imitator, begging behaviour appears to signal offspring need honestly and thus may predict differential food provisioning among offspring.

Parental influence on begging call structure in zebra finches (Taeniopygia guttata): evidence of early vocal plasticity

Begging calls are signals of need used by young birds to elicit care from adults. Different theoretical frameworks have been proposed to understand this parent–offspring communication. But relationships between parental response and begging intensity, or between begging characteristics and proxies of a young’s need remain puzzling. Few studies have considered the adjustment of nestling begging features to previous experience as a possible explanation of these discrepancies. In this study, we tested the effect of a heterospecific rearing environment on individual developmental trajectories of the acoustic structure of nestling begging calls. Fifty-two zebra finch chicks were fostered either to Bengalese finch or to zebra finch parents, and begging calls were recorded at several stages of nestling development. Acoustic analyses revealed that the development of the spectral features of the begging calls differed between experimental conditions: chicks reared by Bengalese finches produced higher pitched and less broadband begging calls than chicks reared by conspecific parents. Differences were stronger in males than females and were not explained by differences in growth rate. We conclude that nestling begging calls can be plastic in response to social interactions with parents.

Testing the bargaining vs. inclusive fitness models of suicidal behavior against the ethnographic record

Suicide is responsible for more deaths than all wars and homicides combined. Despite over a century of research on this puzzling and tragic behavior, and a recent increase in the number of treatments and intervention programs, it remains a global scourge. There is abundant research on suicidality in Western populations, but research on suicide among non-Western peoples is limited. Most notably, few studies analyze suicidality within small scale, non-industrial societies. Using ethnographic data from 53 cultures, this study tests two evolutionary theories of suicidal behavior: (1) deCatanzaro’s inclusive fitness model, which proposes that successful suicide would increase the inclusive fitness of individuals with low reproductive potential who are a burden on kin, and (2) the bargaining model, which proposes that suicide attempts are a costly signal of need, with completed suicides an unfortunate byproduct. These models were operationalized into two sets of variables, which were used to code 474 textual accounts of suicide extracted from the Probability Sample of the Human Relations Area Files. Results indicate limited support for the inclusive fitness model, which might apply primarily to older adults in harsh environments, and widespread support for most elements of the bargaining model, especially among younger healthy adolescents and adults.

Males respond to female begging signals of need: a handicapping experiment in the pied flycatcher, Ficedula hypoleuca

The ‘female nutrition’ hypothesis proposes that food provided by males during incubation is an important energy source for females in bird species in which females alone incubate. Females should be able to communicate their needs through begging signals to mates and males may compensate for the energetic limitations of females through their feeding visits, owing to their overlapping reproductive interests. To test whether female begging during incubation is an honest signal of energetic need and whether mates respond to it we experimentally handicapped female pied flycatchers at the beginning of incubation by clipping two primary flight feathers on each wing. Experimental manipulation led females to intensify begging displays arising from condition impairment and males accordingly increased their incubation feeding rates. Female begging intensity explained more than half of the variation in male incubation feeding rate, thereby showing that female nutrition is the main factor explaining male incubation feeding. Moreover, handicapped females consumed a higher proportion of male food deliveries during the first few days after hatching and weighed less at the end of the nestling period than control females. Handicapping had no influence on female incubation behaviour, hatching and breeding success, nestling and male condition or female nestling provisioning. The provisioning rates of males in the late nestling stage were higher in experimental nests. This is the first experimental study showing that males adjust incubation feeding rates to behavioural displays of need by their mates. The ability of females to modify their begging displays according to need may be an important adaptation that allows females to maintain a good energetic condition during incubation.

Factors affecting vocalization in Tengmalm's owl (Aegolius funereus) fledglings during post-fledging dependence period: scramble competition or honest signalling of need?

Begging behaviour of nestlings has been intensively studied for several decades as a key component of parent-offspring conflict. There are essentially two main theories to account for intensity of food solicitation among offspring: that intensity of begging is related to some form of scramble competition between nest mates or that it offers honest signalling of need to parents. The vast majority of studies which have addressed begging behaviour have been based on observations of, and experiments on, nestlings and have not considered begging behaviour, during the post-fledging period. Begging vocalizations in this post-fledging phase of dependence have rarely been studied, despite the importance of vocalizations as a communication method between offspring and parents, particularly for nocturnal species. We radiotracked 39 fledglings of the Tengmalm’s owl (Aegolius funereus) in two years with different availability of prey: 2010 (n = 29 fledglings) and 2011 (n = 10 fledglings) and made 1320 nightly localizations in which we recorded presence or absence of begging calls. Within years, the most important measures related to the probability of vocalization were body condition at fledging, time of night, number of surviving siblings, age and weather conditions. Begging intensity increased with age in both years; however, in the year with low prey availability fledglings vocalized significantly more often. The main factor causing these differences between years was probably the different availability of prey, affecting breeding success, post-fledging behaviour, and thus also both short- and long-term needs of offspring. We believe that our results suggest honest signalling of their fledgling’s need.

Signals of need and quality: Atlantic puffin chicks can beg and boast

Need and hunger models of honest begging predict that lower-quality offspring should call more, or beg, to signal their poor body condition or hunger. In contrast, quality models of begging predict that offspring of higher fitness should call more, or boast, to signal their viability to parents. We observed 2 types of calls in Atlantic puffin (Fratercula arctica) chicks: a shorter peep call and a longer screech call. Poorly fed chicks screeched during a higher proportion of parental visits than well-fed chicks. Food-supplemented chicks showed a decrease in the proportion of food visits with screech calls, whereas control chicks did not. Chicks in good body condition peeped more than chicks in poor body condition and these chicks showed a greater increase in the peep call rate after supplemental feeding than chicks that started off in poorer condition. Screech calls may signal need and/or hunger to parents, whereas peep calls may signal chick quality. This combination of signals should allow parents to make strategic resource-based decisions, allocating more food to hungry or lower-quality chicks when resources are abundant and preferentially feeding high-quality chicks when resources are scarce.

Calling by Domestic Piglets during Simulated Crushing and Isolation: A Signal of Need?

This study examined whether piglet distress vocalizations vary with age, body weight and health status, according to the predictions of the honest signalling of need evolutionary model. Vocalizations were recorded during manual squeezing (a simulation of being crushed by mother sow) and during isolation on Days 1 and 7 after birth in piglets from 15 litters. We predicted that during squeezing, younger, lighter and sick piglets would call more intensely because they are in higher risk of dying during crushing and therefore they benefit more from the sow’s reaction to intensive vocalization. For isolation, we predicted that lighter and younger piglets would call more because they are more vulnerable to adverse effects of the separation. Calls were analyzed in their time and frequency domain. The rate of calling, call duration, proportion of high-pitched calls and eight acoustic parameters characterizing frequency distribution and tonality were used as indicators of acoustic signalling intensity. Piglets that experienced “squeezing” on Day 1 produced more intense acoustic distress signalling than on Day 7. Lighter piglets called more during squeezing than heavier piglets. Health status did not significantly affect any of the indicators of intensity of vocalization during squeezing. In isolation, none of the parameters of vocalization intensity were affected either by the age or by the weight of the piglets. In summary, the model of honest signalling of need was confirmed in the squeezed situation, but not in the isolation situation.

The Evolutionary Puzzle of Suicide

Mechanisms of self-destruction are difficult to reconcile with evolution’s first rule of thumb: survive and reproduce. However, evolutionary success ultimately depends on inclusive fitness. The altruistic suicide hypothesis posits that the presence of low reproductive potential and burdensomeness toward kin can increase the inclusive fitness payoff of self-removal. The bargaining hypothesis assumes that suicide attempts could function as an honest signal of need. The payoff may be positive if the suicidal person has a low reproductive potential. The parasite manipulation hypothesis is founded on the rodent—Toxoplasma gondii host-parasite model, in which the parasite induces a “suicidal” feline attraction that allows the parasite to complete its life cycle. Interestingly, latent infection by T. gondii has been shown to cause behavioral alterations in humans, including increased suicide attempts. Finally, we discuss how suicide risk factors can be understood as nonadaptive byproducts of evolved mechanisms that malfunction. Although most of the mechanisms proposed in this article are largely speculative, the hypotheses that we raise accept self-destructive behavior within the framework of evolutionary theory.

Individual vocal signatures in barn owl nestlings: does individual recognition have an adaptive role in sibling vocal competition?

To compete over limited parental resources, young animals communicate with their parents and siblings by producing honest vocal signals of need. Components of begging calls that are sensitive to food deprivation may honestly signal need, whereas other components may be associated with individual-specific attributes that do not change with time such as identity, sex, absolute age and hierarchy. In a sib-sib communication system where barn owl (Tyto alba) nestlings vocally negotiate priority access to food resources, we show that calls have individual signatures that are used by nestlings to recognize which siblings are motivated to compete, even if most vocalization features vary with hunger level. Nestlings were more identifiable when food-deprived than food-satiated, suggesting that vocal identity is emphasized when the benefit of winning a vocal contest is higher. In broods where siblings interact iteratively, we speculate that individual-specific signature permits siblings to verify that the most vocal individual in the absence of parents is the one that indeed perceived the food brought by parents. Individual recognition may also allow nestlings to associate identity with individual-specific characteristics such as position in the within-brood dominance hierarchy. Calls indeed revealed age hierarchy and to a lower extent sex and absolute age. Using a cross-fostering experimental design, we show that most acoustic features were related to the nest of origin (but not the nest of rearing), suggesting a genetic or an early developmental effect on the ontogeny of vocal signatures. To conclude, our study suggests that sibling competition has promoted the evolution of vocal behaviours that signal not only hunger level but also intrinsic individual characteristics such as identity, family, sex and age.

Evolution of learning and levels of selection: A lesson from avian parent–offspring communication

In recent years, it has become increasingly clear that the evolution of behavior may be better understood as the evolution of the learning mechanisms that produce it, and that such mechanisms should be modeled and tested explicitly. However, this approach, which has recently been applied to animal foraging and decision-making, has rarely been applied to the social and communicative behaviors that are likely to operate in complex social environments and be subject to multi-level selection. Here we use genetic, agent-based evolutionary simulations to explore how learning mechanisms may evolve to adjust the level of nestling begging (offspring signaling of need), and to examine the possible consequences of this process for parent–offspring conflict and communication. In doing so, we also provide the first step-by-step dynamic model of parent–offspring communication. The results confirm several previous theoretical predictions and demonstrate three novel phenomena. First, negatively frequency-dependent group-level selection can generate a stable polymorphism of learning strategies and parental responses. Second, while conventional reinforcement learning models fail to cope successfully with family dynamics at the nest, a newly developed learning model (incorporating behaviors that are consistent with recent experimental results on learning in nestling begging) produced effective learning, which evolved successfully. Third, while kin-selection affects the frequency of the different learning genes, its impact on begging slope and intensity was unexpectedly negligible, demonstrating that evolution is a complex process, and showing that the effect of kin-selection on behaviors that are shaped by learning may not be predicted by simple application of Hamilton’s rule.

Parent-absent signalling of need and its consequences for sibling competition in the barn swallow

In species with parental care, competition among siblings for access to limited parental resources is common. Sibling competition can be mediated by begging behaviour, a suite of different visual and acoustic displays by which offspring solicit parental care. These are mostly addressed to the parents upon food provisioning, but can also be performed in the absence of the attending parents. This so-called parent-absent begging (PAB) may function as an intrabrood communication signal and potentially affect intrabrood competition dynamics for access to food. We investigated the role of PAB in moulding sibling interactions and its effect on food intake among altricial barn swallow (Hirundo rustica) nestlings, both under normal and experimentally reduced food intake. Frequency of PAB increased after food deprivation. Nestlings that had performed PAB increased their begging intensity upon the subsequent parental feeding visit, while siblings reduced their own begging level, but only when they had not been food-deprived. As a consequence, nestlings which had performed PAB before parental arrival had larger chances of receiving food. However, nestlings did not benefit from displaying PAB when competing with food-deprived siblings. Our findings show that PAB reliably reflects need of food, indicating that a nestling will vigorously compete for the subsequent food item. By eavesdropping siblings' PAB displays, nestlings may optimally balance the costs of scrambling competition, the direct fitness gains of being fed and the indirect fitness costs of subtracting food to needy kin. However, large asymmetries in satiation between competitors may lead individual offspring to monopolize parental resources, irrespective of PAB displays.

How Do Female Red-winged Blackbirds Allocate Food within Broods?

Abstract. Nestlings communicate with parents via begging, but what does begging signal and how do parents allocate food to their nestlings? We tested the signal-of-need (SoN) and signal-of-quality (SoQ) hypotheses for nestling begging in the Red-winged Blackbird (Agelaius phoeniceus) by attempting to determine whether begging is negatively (SoN) or positively (SoQ) condition dependent, and by attempting to identify the attributes of nestlings that parents use to allocate food within broods. We quantified begging by its mean intensity (scale 0–7) and mean duration and parental allocation by the number of times each nestling was fed. We found that the intensity and duration of begging were not correlated with nestlings' size (estimated by body mass and tarsus length), condition (estimated from the residual of mass regressed on tarsus length), age, sex, or testosterone concentration, so begging did not appear to be negatively or positively condition dependent. A generalized linear model showed that mean inte...

Mouth colour components of begging are dynamic signals of quality in European starling nestlings

Offspring solicit food from their parents through begging signals. Nestling skin and flange coloration are begging signals that appear to convey information about nestling need or condition, and several experiments have shown that modifications of nestling coloration affect parental allocation decisions. However, it is important to examine the short‐term changes in these signalling components in response to food constraints since such dynamic changes are required for signals to indicate condition or need. Using a food deprivation experiment, we tested whether flange and skin reflectance in European starling Sturnus vulgaris nestlings change after a three‐hour interval. We investigated whether flange and skin reflectance changed according to the predictions arising from the ‘signal of quality’ or ‘signal of need’ hypotheses on the function of begging signals. We found that flange carotenoid and UV reflectance changed according to the signal of quality hypothesis with nestlings in good condition increasing their signal expression in response to the food deprivation, whereas those in poor condition decreased their signal expression. With the use of vision modelling, we show that changes in flange reflectance are detectable by starling parents. In contrast, we found a correlation going in the opposite direction for changes in skin UV reflectance. Nestlings with low lipid reserves increased their reflectance compared to nestlings with high reserves. However, vision modelling showed that short‐term changes in skin UV reflectance are not large enough to be detectable by the parents. Our study shows that flange carotenoid and UV reflectance are dynamic components of begging with short‐term variations that can be used by parents as signals of nestling quality.

Commentary

Commentary

Begging for a better future: how far can behavioral ecologists go without specifying mechanisms?

Mock et al.’s critical review of the ‘‘honest begging’’ literature is certainly welcome and constructive. We agree that there are some problems with Signal of Need (SoN) including its interactions with Signal of Quality (SoQ) (Lotem 1998), and the fact that it makes opposing predictions for different fitness functions (Cotton et al. 1999) and ‘‘assumes away’’ the dynamic nature of parent–offspring interactions (Godfray and Johnstone 2000). We are obviously pleased with the endorsement of the fuel gauge model (FGM; Grodzinski and Lotem, 2007) but feel that its presentation as a version of the Signaling of Hunger idea (SoH), that is, as an alternative to SoN, gives it a different interpretation than originally intended. The distinction may look subtle but it is critical: In our view, the field does not require another normative model but crucially requires mechanistic models connecting the theoretical terms ‘‘need’’ and ‘‘quality’’ with biological reality. The FGM is therefore not an alternative to SoN, but a mechanistic model explaining how food deprivation can cause a gradual increase in need (i.e., in the marginal fitness value of receiving extra resources). The first component of the FGM is that sated (full) offspring have a need level of zero simply because they are not receptive to being fed, so food cannot be converted into offspring fitness. However, this food-receptivity component, termed ‘‘being ready for additional food’’ by Mock et al., is not sufficient to explain further increases in begging with food deprivation (receptivity alone could be indicated by an allor-none signal or cue). This is where the second component of the FGM becomes essential. The reason that an ‘‘almost empty’’ offspring is needier than a ‘‘half empty’’ offspring is that although they may be equally ready to receive a meal, the almost empty one is at greater risk of becoming completely empty, which might temporarily stop its ‘‘digestive engine’’ converting food into growth (Grodzinski and Lotem 2007). Alternatively, ‘‘efficiency-based’’ models suggest that fullness reduces digestive efficiency, providing a different functional reason for why food-deprived offspring will gain more fitness from being fed (Grodzinski and Lotem 2007; Wright et al. 2010). Importantly, specifying the alternative mechanisms through which SoN may work enables us to return to ultimate reasoning with relevant experimental results (Grodzinski et al. 2009). If the justified criticism of being ‘‘content with SoN’’ leads to this integrating approach, as we suggest, we are not really ‘‘expanding from signal of need,’’ but breaking it down to concrete examples that can be studied and understood at the mechanistic level. In contrast, using ‘‘hunger’’ or ‘‘fullness’’ without clarifying their relation to fitness might not take us much further than need or quality. We greatly sympathize with Mock et al.’s call to move beyond the ‘‘hunger experiment’’ and the honest signaling preoccupation but feel that expanding to SoH or SoQ may not be the best way. In theory, begging should be adjusted in relation to both differential costs (quality) and benefits (need), but offspring must assess those through proximate factors, such as digestive physiology, hormonal levels, competitive state, and past experience. Understanding these mechanisms is necessary for decoding the message conveyed by begging and assessing the extent to which begging is honest and parental response to it is adaptive. On the bright side, it seems that the field is already moving in this direction with an increasing number of studies into the genetic and phenotypic mechanisms of parent–offspring communication (e.g., Bell 2008; Grodzinski et al. 2008; Dor and Lotem 2009, 2010; Hinde et al. 2009; Wright et al. 2010). We should perhaps recall that the honest-signaling revolution was mainly a conceptual one. It provided a stable solution for the problem of cheating, facilitating the use of normative models like those used for animal foraging to study animal communication. Now that behavioral ecologists gradually stopped asking whether foraging is optimal and are instead using optimality as a working hypothesis for studying the evolution of behavioral mechanisms (recently reviewed by McNamara and Houston 2009), we can stop asking whether begging is honest as a goal in itself. Instead, we can use working hypotheses about costly signaling and evolutionary stability (implying some general concept of honesty) to study the mechanisms that make offspring begging and parental response to it adaptive.

New labels for old whines

With characteristically colorful prose, Mock et al. (2011) critically review empirical work on nestling begging displays inspired by Godfray’s (1991) ‘‘signal of need’’ model. They contrast Signals of Need (SoN) with 2 ‘‘parsimonious alternative types of honest begging,’’ Signals of Hunger (SoH) and Signals of Quality (SoQ). Starting with SoH, these are supposed to provide a proximate indication of offspring fullness. This, Mock et al. assure us, makes them distinct from SoN, which on their interpretation can only provide information about offspring reproductive value. Indeed, they strongly criticize existing studies for interpreting evidence that begging reflects hunger as support for SoN; as Mock et al. define them, hunger and need are wholly distinct. This criticism seems to us to be misplaced and based on a false dichotomy. In models of begging, ‘‘need’’ simply refers to the benefits that offspring stand to gain from eliciting additional investment (e.g., Godfray 1991; Godfray and Johnstone 2000). These benefits may vary in relation to short-term need, that is, offspring hunger in Mock et al.’s terminology or in relation to long-term need, that is, offspring condition or reproductive value (e.g., Price et al. 1996; Lotem 1998). The logic of the handicap principle is applicable in the same way to both cases, and both have thus been interpreted as SoN; this is why so many authors have advanced evidence for the impact of short-term food deprivation on begging as support for SoN. It is of course interesting to explore how shortand long-term need respectively contribute to variation in begging intensity—see, for instance, Price et al. (1996), Lotem (1998), and Kilner et al. (1999). Nevertheless, in our view, it is confusing to treat signals of short-term need as an entirely separate category of display from signals of long-term need, and it is misleading to suggest that existing theory focuses only the latter and ignores the former. Turning to SoQ, Mock et al. suggest that begging may advertise merit rather than need and may serve to minimize parents’ wasting investment on dependents with dim prospects. In our view, there is indeed a real difference between SoQ and SoN, and, because most discussion of begging has indeed focused on SoN, we see no harm in Mock et al.’s effort to revive interest in SoQ. Nevertheless, Mock et al.’s suggestion is not entirely novel—Royle et al. 2002, for instance, have previously shown that models of begging can generate precisely the pattern Mock et al. predict in which parents provide more resources to chicks in better condition. Furthermore, we see 2 underlying problems with SoQ as a general hypothesis for the evolution of offspring begging. First, it is puzzling why parents would need to intervene actively in the process of food allocation in order to favor offspring in better condition, as SoQ suggests, when these offspring are superior competitors in any case. If the parents’ goal is to sacrifice weaker offspring with poorer prospects, they could simply allow sibling competition to take its course (e.g., Royle et al. 2002; Kolliker and Richner 2004). By contrast, SoN provides a stronger reason why parents should attempt to seize control of food allocation because this allows them to override differences in competitive ability and instead feed according to need. Second, although there are indeed phases of development during which infanticide is common, and offspring might benefit by dissuading parents from targeting them for ‘‘disinvestment,’’ this risk does not persist throughout chick growth (Mock and Parker 1997). SoQ might thus play a role at certain key points in development, when the risk of infanticide looms large, but looks unconvincing as a general explanation for begging that persists throughout the rearing of the young. As a final thought, we wonder how much is to be gained by setting up alternatives to Godfray’s (1991) original model of begging, which Mock et al. treat as the definitive statement of the SoN hypothesis. Godfray’s analysis has served its purpose admirably by forcing empiricists to think more clearly about the details of interactions within the family. As a result, we now know that real animal families are far more complicated and interesting than his model assumed, with provisioning responses determined by parental life history, prenatal signals of parental quality, and partner behavior as well as by begging intensity. Equally, Godfray’s model has stimulated many refinements of the theory, with more recent models attempting to incorporate some of the complexities revealed by empirical studies of parent–offspring interaction, from Godfray’s own 1995 model of competitive begging (Godfray 1995), to analyses of sibling negotiation, prenatal influences, and partial begging. In our view, future work should focus on incorporating these complexities into Godfray’s (1991) framework for thinking about family life rather than on replacing SoN with artificial alternatives.

Fine whines improve with age

Fine whines improve with age