Models of parent-offspring conflict. V. Effects of the behaviour of the two parents

In the absence of any parent-offspring conflict, the total parental investment per offspring should be less when two parents collaborate in caring for the offspring than when only one parent invests. This does not necessarily mean that offspring fare less well when both parents invest. The ‘ideal’ amount of parental investment for an offspring to take is always greater than is ‘ideal’ for the parent to allocate (Trivers 1974). The offspring's optimum is higher if the offspring's action affects the reproductive success of only one parent and lower if both parents are affected (e.g. two-parent investment, or lifelong monogamy). The difference between the parental optimum and the offspring optimum depends on the mating system and on the form of conflict (between successive broods, or within broods), and prescribes a ‘conflict range’. The extent of conflict cannot be deduced solely from a knowledge of the average relatedness between siblings. The conflict is likely to be resolved by an ESS in which intermediate (compromise) levels of investment are paid out to offspring, which nevertheless continue to make costly demands for yet more investment. The degree of conflict can be measured by the extent to which offspring subject their parents to aggressive demands for extra investment, and is likely to be greater when two parents collaborate equally over investment than when only one parent invests. When only one parent invests, conflict is higher if sibling-competition is between siblings in the same broods (intra-brood) than when it is between progeny in successive broods (inter-brood). However, the reverse will tend to be the case when both parents invest equally.     

Models of parent-offspring conflict. I. Monogamy

Theoretical models for Trivers (1974) concept of parent-offspring conflict are examined for species in which the effects of the conflict are felt by full sibs. A rare conflictor gene will spread if (f(m) greater than 1/2(m + 1), where f(m) is the fitness gained by a conflictor relative to a non-conflictor offspring (f(m) greater than 1), and m is the amount of parental investment taken by a conflictor relative to m = 1 for a non-conflictor. The range of m alleles which can spread against the parent optimum decreases as the cost to the parent increases until a point is reached where there is no conflict of evolutionary interests. There would be no polymorphism for conflictor: non-conflictor alleles unless special conditions prevail. The conflictor allele which spreads most rapidly as a rare mutant against the parental optimum is not an evolutionarily stable strategy (ESS). The ESS for parent-offspring conflict in monogamous species has mo = f(mo)/2[df(mo)/dmo]. The analytical solutions are confirmed throughout by simulations.     

Antagonistic Parent-Offspring Co-Adaptation

Background

In species across taxa, offspring have means to influence parental investment (PI). PI thus evolves as an interacting phenotype and indirect genetic effects may strongly affect the co-evolutionary dynamics of offspring and parental behaviors. Evolutionary theory focused on explaining how exaggerated offspring solicitation can be understood as resolution of parent-offspring conflict, but the evolutionary origin and diversification of different forms of family interactions remains unclear.

Methodology/Principal Findings

In contrast to previous theory that largely uses a static approach to predict how “offspring individuals” and “parental individuals” should interact given conflict over PI, we present a dynamic theoretical framework of antagonistic selection on the PI individuals obtain/take as offspring and the PI they provide as parents to maximize individual lifetime reproductive success; we analyze a deterministic and a stochastic version of this dynamic framework. We show that a zone for equivalent co-adaptation outcomes exists in which stable levels of PI can evolve and be maintained despite fast strategy transitions and ongoing co-evolutionary dynamics. Under antagonistic co-adaptation, cost-free solicitation can evolve as an adaptation to emerging preferences in parents.

Conclusions/Significance

We show that antagonistic selection across the offspring and parental life-stage of individuals favors co-adapted offspring and parental behavior within a zone of equivalent outcomes. This antagonistic parent-offspring co-adaptation does not require solicitation to be costly, allows for rapid divergence and evolutionary novelty and potentially explains the origin and diversification of the observed provisioning forms in family life.     

The coadaptation of parental supply and offspring demand

The evolution of parent-offspring interactions for the provisioning of care is usually explained as the phenotypic outcome of resolved conflicting selection pressures. However, parental care and offspring solicitation are expected to have complex patterns of inheritance. Here we present a quantitative genetic model of parent-offspring interactions that allows us to investigate the evolutionary maintenance of a state of resolved conflict. We show that offspring solicitation and parental provisioning are expected to become genetically correlated through coadaptation and that their genetic architecture is dictated by an interaction between patterns of selection and the proximate mechanisms regulating supply and demand. When selection is predominately on offspring solicitation, our model suggests that the genetic correlations between provisioning and solicitation are usually positive if provisioning reduces solicitation. Conversely, when selection is predominately on parental provisioning, the correlations are mostly negative as long as parents show a positive response to offspring demand. Empirical estimates of the genetic architecture of traits involved in family interactions fit these predictions. Our model demonstrates how the evolutionary maintenance of parent-offspring interactions can result in variable patterns of coadaptation, and it provides an explanation for the diversity of family interactions within and among species.     

The quantitative genetic basis of offspring solicitation and parental response in a passerine bird with biparental care.

The coevolution of parental investment and offspring solicitation is driven by partly different evolutionary interests of genes expressed in parents and their offspring. In species with biparental care, the outcome of this conflict may be influenced by the sexual conflict over parental investment. Models for the resolution of such family conflicts have made so far untested assumptions about genetic variation and covariation in the parental resource provisioning response and the level of offspring solicitation. Using a combination of cross-fostering and begging playback experiments, we show that, in the great tit (Parus major), (i) the begging call intensity of nestlings depends on their common origin, suggesting genetic variation for this begging display, (ii) only mothers respond to begging calls by increased food provisioning, and (iii) the size of the parental response is positively related to the begging call intensity of nestlings in the maternal but not paternal line. This study indicates that genetic covariation, its differential expression in the maternal and paternal lines and/or early environmental and parental effects need to be taken into account when predicting the phenotypic outcome of the conflict over investment between genes expressed in each parent and the offspring.     

The influence of mating system on the intensity of parent-offspring conflict in primates

An evolutionary conflict of interest exists between parents and their offspring over the partitioning of parental investment (PI) among siblings. When the direct fitness benefits to offspring of increased PI, outweigh the inclusive fitness costs from lost future sibling fitness, selection should favour the evolution of offspring selfishness over altruism. In theory, this conflict is heightened when females are not strictly monogamous, as current offspring should be less altruistic towards future half-siblings than they would be towards full-siblings. Using data collected on foetal growth rate (representing prenatal PI) in primates, I test the prediction from theory that the resolution of the parent-offspring conflict will be closer to the offspring's evolutionary optima in polyandrous species than in more monandrous species. Using phylogenetic comparative analysis, and controlling for allometry, I show that offspring are able to obtain more PI when the probability of future full-siblings decreases, and that this is most pronounced in taxa where there is the opportunity for direct foetal access to the maternal bloodstream. These results support the hypothesis that the resolution of prenatal PI conflict is influenced by both a species' mating system and by its placental structure.     

Shared and unshared parental investment,parent-offspring conflict and brood size

Taking as our starting point Trivers' (1974) account of parent-offspring conflict, we develop models of the influence of brood size on the optimal level of parental investment (PI) in the whole brood for parent and offspring, and on the magnitude of conflict between them. A modification of Trivers' model is proposed. In general, the benefit of an act of PI to an offspring in a brood of size N is (N+1)/N times the benefit to its parent. Therefore as brood size increases, offspring benefit approaches parental benefit, and this is because an increasing proportion of the offspring's benefit is being gained through siblings, to which offspring and parent are equally related. A distinction is drawn between ‘shared’ and ‘unshared’ types of PI. When PI is shared the total benefit accruing is not directly gained by all offspring but is shared amongst them (e.g. food brought to the young). In contrast, unshared PI can simultaneously benefit some or all of the brood (e.g. types of anti-predator defence). For shared investment, PI and conflict are predicted to increase with brood size. Two models of unshared anti-predator defence are described. If the predator characteristically takes the whole brood when it strikes (e.g. altricial nestlings) PI is predicted to increase and conflict decline with brood size, although this effect is inhibited or even reversed for high risk defence tactics because of the higher cost to larger broods if the parent dies. When the predator takes a single offspring (e.g. precocial birds) the parent's optimum PI is independent of brood size, the offspring's optimum PI declines in larger broods and conflict again declines with brood size. The parent is commonly expected to win the conflict over anti-predator care. Predictions concerning PI levels gain support from existing data, largely for birds, but evaluation of those for conflict must await the collection of new data. The distinction between shared and unshared investment is applicable to altruistic behaviour in general.     

Costly Solicitation, Timing of Offspring Conflict, and Resource Allocation in Plants

The theory of parent–offspring conflict is extended toplants that produce many offspring in one reproductive event.The energetic cost of begging signals and the timing of offspringconflict are explicitly taken into account. We find that ifthe indirect costs of increased provisioning of selfish offspringare borne by their brood mates, then offspring are selectedto solicit in so costly a way that a substantial part of parentalinvestment in a brood goes to solicitation rather than offspring'sgrowth and survival. Consequently, offspring conflict oftenresults in smaller seed size than the parental optimum in theabsence of conflict, although each offspring still consumesmore resources than the amount its mother is willing to give.While the optimal sex allocation can be shown to be independentof solicitation and sibling conflict, the overall reproductiveeffort is always lowered by parent–offspring conflict.The timing of offspring conflict during the period of parentalinvestment is demonstrated to be an important factor that influencesthe outcome of parent–offspring conflict. The more resourcesare allocated to individual offspring before the occurrenceof offspring solicitation, the less offspring should solicit,and hence the closer the offspring size to the parental optimum.Copyright 2000 Annals of Botany Company Evolutionarily stable strategy, parent–offspring conflict, parental investment, reproductive resource allocation, seed size, solicitation, timing of offspring conflict     

Intrafamilial conflict and parental investment: a synthesis

We outline and develop current theory on how inherent genetic conflicts of interest between the various family members can affect the flow of parental investment from parents to offspring, and discuss the problems for empirical testing that this generates. The parental investment pattern realized in nature reflects the simultaneous resolution of all the conflicts between the family players. This depends on the genetic mechanism, the mating system and reproductive constraints, on whether extra demand by progeny affects current or future sibs, and particularly on the behavioural mechanisms underlying demand (begging or solicitation) and supply (provision of parental investment by parents). The direction of deviation from the optimal parental investment for the parent(s) depends on the slope of what we term the 'effect of supply on demand', the mechanism that determines how changes in food supply affect begging levels. If increasing food increases begging (positive slope), less parental investment is supplied than the parental optimum and if increasing food decreases begging (negative slope), more parental investment is supplied. The magnitude of deviation depends on both the 'effect of supply on demand' and on the 'effect of demand on supply' (the mechanism determining how changes in begging affect food supply, which always has a positive slope). We conclude that it will often be impossible to deduce the extent of underlying conflict by establishing the amount of parental investment given relative to the ideal optimum for the parent. Some possible directions for future research are discussed.     

Parental investment and family dynamics: interactions between theory and empirical tests

The pattern of parental investment (PI) seen in nature is a product of the simultaneous resolution of conflicts of interest between the members of a family. How these conflicts are resolved depends upon the mating system, the genetic mechanism, on whether extra PI affects current or future offspring, and the behavioural mechanisms underlying supply and demand of PI. Until recently very little empirical work has been done to underpin these key determinants of conflict resolution. This review examines recent empirical progress in understanding both (1) how conflict is resolved and (2) its evolutionary consequences. How offspring demand interacts with parental supply of resources determines how conflict is resolved. Two extremes are: passive parental choice of competing offspring, relating to offspring control of resource allocation, and active parental choice relating to parental control. Although most previous empirical work has tended to conclude or assume that parents primarily control resource allocation decisions, recent studies explicitly examining predictions from theoretical analyses have shown that offspring control of resource allocation is more important than previously realised. The amount of PI supplied at resolution depends not on who controls food allocation, however, but on the nature of the supply and demand mechanisms. These have yet to be established experimentally, but a recent regression model illustrates how this could be achieved in the field. Determination of the effect of supply on demand (ESD) and the effect of demand on supply (EDS) mechanisms is critical to parent–offspring conflict theory, which has not been adequately tested empirically. There is an underlying, and until recently untested, assumption of models of intrafamilial conflict that there is genetic variation for both offspring demand and parental supply behaviours, so that the behaviours can coevolve. Recent studies on great tits, burrower bugs and mice all found evidence for genetic variation in supply and demand behaviours, but the predicted negative correlation between genes expressed in mothers and their offspring (i.e. parent–offspring coevolution), was found only for burrower bugs. The lack of a negative relationship for great tits and mice may have been a consequence of antagonistic coevolution between the sexes (sexual conflict). These studies illustrate the importance of the underlying genetics and mating system in determining conflict resolution, and point to the need for new models (especially of interbrood competition) taking differences in the genetics and the co-evolution of the ESD and EDS mechanisms into account. We also discuss the importance of the comparative approach in determining evolutionary consequences of conflicts, and use the recent work on growth costs of begging to illustrate the difficulties of measuring costs of conflict in an evolutionary currency. The recent growth in empirical work on conflicts in families illustrates an increasing, and increasingly productive, integration between theoreticians and empiricists.     

How costly is the honest signaling of need?

ESS models of biological signaling have shown that costly signals can provide honest information. In the context of parent-offspring conflict over the allocation of resources by parents to their young, the theory explains costly offspring solicitation behavior as an accurate signal of offspring need to parents who cannot assess offspring condition directly. In this paper, we provide a simple but general characterization of the honest signaling of need in models of parent-offspring conflict: the offspring's signaling cost is proportional to the parent's fitness loss from satisfying the offspring's resource requirement. The factor of proportionality is given by a measure of the extent of parent-offspring conflict that depends only on coefficients of relatedness. These results hold for interbrood conflict with uniparental investment even if the relationship between offspring condition and resource requirement is not monotonic, and extend to cases of biparental care, uncertainty concerning the parent's condition, and intra-brood conflict. Copyright 1999 Academic Press.     

Do honest signalling models of offspring solicitation apply to insects?

Honest signalling models predict that the intensity of solicitation by offspring influences the level of provisioning provided by parents and reflects offspring need. The empirical evidence supporting these predictions primarily comes from studies of birds or mammals. Thus, although parental care of altricial offspring is taxonomically widespread, the generality of these models is not well known. To investigate whether honest signalling models apply to insects, we manipulated parent and offspring behaviour in the burying beetle Nicrophorus orbicollis, a species with advanced parental care. First, within biparental care, we manipulated the brood size to alter the parents'' perception of offspring need. We measured the care giving behaviour of male and female parents to examine whether either adjusts its level of care according to offspring need. In the second experiment, because two parents together provision the brood more often than single parents, we manipulated the number of care givers (uniparental and biparental care) and measured offspring solicitation to assess whether offspring change their behaviour in response to need. Our results show that parent behaviour is broadly consistent with the first prediction of the models; both sexes provisioned larger broods more often than smaller broods. Larval solicitation was also consistent with the second prediction; larvae that were provisioned less often begged more. Our results provide evidence that honest signalling models can be applied to insects as well as vertebrates, although there are also subtle differences in care giving behaviour that may be important.     

Family dynamics through time: brood reduction followed by parental compensation with aggression and favouritism

Parental food allocation in birds has long been a focal point for life history and parent–offspring conflict theories. In asynchronously hatching species, parents are thought to either adjust brood size through death of marginal offspring (brood reduction), or feed the disadvantaged chicks to reduce the competitive hierarchy (parental compensation). Here, we show that parent American coots (Fulica americana) practice both strategies by switching from brood reduction to compensation across time. Late‐hatching chicks suffer higher mortality only for the first few days after hatching. Later, parents begin to exhibit parental aggression towards older chicks and each parent favours a single chick, both of which are typically the youngest of the surviving offspring. The late‐hatched survivors can equal or exceed their older siblings in size prior to independence. A mixed allocation strategy allows parents to compensate for the costs of competitive hierarchies while gaining the benefits of hatching asynchrony.     

Maternal effects and parent–offspring conflict

Maternal effects can provide offspring with reliable information about the environment they are likely to experience, but also offer scope for maternal manipulation of young when interests diverge between parents and offspring. To predict the impact of parent–offspring conflict, we model the evolution of maternal effects on local adaptation of young. We find that parent–offspring conflict strongly influences the stability of maternal effects; moreover, the nature of the disagreement between parents and young predicts how conflict is resolved: when mothers favor less extreme mixtures of phenotypes relative to offspring (i.e., when mothers stand to gain by hedging their bets), mothers win the conflict by providing offspring with limited amounts of information. When offspring favor overproduction of one and the same phenotype across all environments compared to mothers (e.g., when offspring favor a larger body size), neither side wins the conflict and signaling breaks down. Only when offspring favor less extreme mixtures relative to their mothers (something no current model predicts), offspring win the conflict and obtain full information about the environment. We conclude that a partial or complete breakdown of informative maternal effects will be the norm rather than the exception in the presence of parent–offspring conflict.     

Reliable Information Content and Ontogenetic Shift in Begging Calls of Grey Warbler Nestlings

The most critical assumption of communication models regarding parent–offspring conflict is that food solicitation displays of genetic offspring are honest signals to elicit beneficial parental care. A critical requirement of honesty is the reliable change of perceivable aspects of begging calls with physiological needs. We experimentally tested whether and how the acoustic structure and begging call rate of individual Grey Warbler Gerygone igata nestlings change with hunger level and age. We also examined a rarely documented component of chick begging calls, namely the temporal dynamics of acoustic modulation after nestlings heard parental feeding calls. Begging call structure narrowed in frequency range and, surprisingly, decreased in amplitude as chick hunger levels increased. We also found that begging calls changed with chick age, with the frequency increasing and the duration decreasing for older chicks. These results indicate that the acoustic properties of nestling Grey Warbler begging calls are complex and may be used to signal several aspects of nestling traits, including hunger level and age (or size, a correlate of age). Overall, begging calls of Grey Warbler chicks appear to be honest, implying that parents are likely to benefit from relying on the acoustic features of their progeny’s calls which predict chick need. Our results have important implications regarding the reliability and information content of nestling solicitation signals for the brood parasite shining cuckoo Chrysococcyx lucidus exploiting Grey Warbler parental care, in that these begging‐call mimetic specialist cuckoos might also need to match closely the dynamics of acoustic features of their host chicks’ calls.     

Role assessment,reserve strategy,and acquisition of information in asymmetric animal conflicts

It was formerly argued that alternative evolutionarily stable strategies (ESSs) are possible for animal contests characterized by some asymmetry that can be perceived with perfect accuracy. Where roles A and B refer to the asymmetry between opponents, ESSs are: ‘fight when A, retreat when B’, and vice versa. Either can be an ESS, but only if the ‘reserve strategy’ (=what an animal does when it fights) is sufficiently damaging. We examine the ‘war of attrition’ (winner = opponent that persists longer). In a population at either ESS, reserve strategy is never normally shown; it is therefore subject to drift unless the selective action of rare individuals which break the convention is considered. These could arise either by mutation or by mistakes in role assessment. When mutations and mistakes simply specify that occasionally an animal fights when it ‘should’ retreat, selection adjusts reserve strategy to a level where only one ESS (the ‘commonsense’ ESS) is possible, if the asymmetry is relevant to payoff. Thus for asymmetries in fighting ability or resource value, the individual with the lower score will retreat. However, we are particularly concerned with cases where both payoff-relevant aspects (fighting ability and resource value) are asymmetric. If opponents sustain contest costs at rates K_A and K_B, and their resource values are V_A and V_B, an ‘optimal assessor’ strategy defined by the interaction between the two asymmetries, is a unique ESS. It obeys the rule ‘fight on estimating role A, where V_A/K_A>V_B/K_B; retreat in B’. If mistakes can occur in both roles, but are very rate, the ESS is not fundamentally altered though there will be infinitesimal tendencies for persisting in role B. Selection to improve assessment abilities intensifies as abilities improve, but is weak if roles A and B are rather similar. Over a range of similarity between roles, an ‘owner wins’ convention may be adopted if ownership correlates positively with role A and an individual cannot tell when it would otherwise pay him to break the convention. We also examine a contest in which information about roles can be acquired only during a contest itself, and at a cost. Much depends on the rate at which information is acquired relative to the rate at which costs are expended, and on whether contests normally escalate in intensity, remain at the same level, or de-escalate. Selection favours short contests when costs are high relative to resource value, where the outcome of a round contains much information about fighting ability, and where the actual disparity in fighting ability is large.     

Begging and bleating: the evolution of parent-offspring signalling

The evolution of biological signalling in the face of evolutionary conflicts of interest is an active area of evolutionary ecology, and one to which Maynard Smith has made important contributions. We explore the major theoretical challenges in the field, concentrating largely on how offspring signal to their parents when there is the potential for parent-offspring conflict. Costly offspring solicitation (begging etc.) has been interpreted in terms of a Zahavi Grafen honest handicap signal, but this has been challenged on the grounds of' the costs of signalling. We review this controversy and also explore the issue of pooling versus separating signalling equilibrium. An alternative explanation for costly begging is that it is due to sibling competition, and we discuss the relationship between these ideas and signalling models in families with more than one offspring. Finally we consider signal uncertainty, how signalling models can be made dynamic, and briefly how they may be tested experimentally.     

Parent-offspring conflict over clutch size

Summary We present a model for sexually-reproducing diploids in which a female can produce a variable (generally large) clutch size, where the sibs then compete over some fixed resource, and where certain offspring use siblicide to reduce the primary clutch/brood size created by the mother. Where siblicide involves neither direct energy loss (e.g. fighting cost) nor gain (e.g. cannibalism) to an offspring, the optimal clutch size for an offspring can differ from the optimum for the mother, i.e. there can be parent-offspring conflict over clutch size. The magnitude of this evolutionary conflict (measured in terms of difference between clutch size optima) increases with multipaternity of the brood and with the steepness of the initial decline in offspring survivorship (through sib-competition as further offspring are added to the brood). However, the disparity in clutch size optima may not be great. Where the integer clutch size optima are the same, there will clearly be no conflict. Where this differs, resolution of the evolutionary conflict could involve much apparent behavioral conflict, commonly manifest as siblicidal aggression.The ESS (evolutionarily stable strategy) for such a game will depend upon the direct costs and benefits of siblicide, as well as on the indirect costs to sibs via relatedness. If the only costs of siblicide arise through relatedness, then offspring will win in the sense that the eventual clutch size will match the offspring optimum. Whether or not the mother will produce this clutch size depends on the mechanism controlling siblicide. A siblicidal ESS will occur when offspring are programmed to kill a fixed number/proportion of a brood (victim-based siblicide), but not if programmed to reduce the sibship to the offspring optimum (survivor-based siblicide). With survivor-based siblicide, the mother can do no better than to lay the offsprings' optimal clutch size.     

Compromised strategy resolves intersexual conflict over pre-copulatory guarding duration

Summary An evolutionarily stable strategy (ESS) on pre-copulatory mate-guarding duration is separately obtained for males and females, by assuming that either the male or female can control perfectly the timing of guarding. A difference between sexes in an ESS brings on an intersexual conflict, in particular when the ESS of the actively searching sex (usually male) is longer than that of the other. We analyse two extreme situations, in which the female mating stages are either perfectly synchronized or uniformly distributed. The analysis reveals that (1) the male ESS for guarding duration is longer than the female ESS in the synchronized case if the sex ratio is male-biased, (2) the difference in ESSs is higher for a more male-biased sex ratio, less guarding costs or a higher encounter rate, and (3) an asynchronous female mating cycle extends the conflict region towards female-biased sex ratios. We show by including conflict costs in fitnesses of both sexes that intersexual conflict may be resolved by a compromised solution, where the starting time of mate guarding is an intermediate value between the ESSs of the two sexes. This compromised strategy depends on both fitness increments of winning the conflict and physical power in controlling the opponent and tends to approach the ESS of the commoner sex in highly biased sex ratios. If both actors engaged in a conflict have enough information on each other, a compromise without an overt struggle may be reached.     

Asynchronous hatching in the burying beetle,Nicrophorus quadripunctatus,maxmizes parental fitness

Life history theory predicts that natural selection favours parents who balance investment across offspring to maximize fitness. Theoretical studies have shown that the optimal level of parental investment from the offspring's perspective exceeds that of its parents, and the disparity between the two generates evolutionary conflict for the allocation of parental investment. In various species, the offspring hatch asynchronously. The age hierarchy of the offspring usually establishes competitive asymmetries within the brood and determines the allocation of parental investment among offspring. However, it is not clear whether the allocation of parental investment determined by hatching pattern is optimal for parent or offspring. Here, we manipulated the hatching pattern of the burying beetle Nicrophorus quadripunctatus to demonstrate the influence of hatching pattern on the allocation of parental investment. We found that the total weight of a brood was largest in the group that mimicked the natural hatching pattern, with the offspring skewed towards early hatchers. This increases parental fitness. However, hatching patterns with more later hatchers had heavier individual offspring weights, which increases offspring fitness, but this hatching pattern is not observed in the wild. Thus, our study suggests that the natural hatching pattern optimizes parental fitness, rather than offspring fitness.