Reduced Egg Investment Can Conceal Helper Effects in Cooperatively Breeding Birds

Strategic female reproductive investment in response to male attractiveness in birds

Life-history theory predicts that individuals should adjust their reproductive effort according to the expected fitness returns on investment. Because sexually selected male traits should provide honest information about male genetic or phenotypic quality, females may invest more when paired with attractive males. However, there is substantial disagreement in the literature whether such differential allocation is a general pattern. Using a comparative meta-regression approach, we show that female birds generally invest more into reproduction when paired with attractive males, both in terms of egg size and number as well as food provisioning. However, whereas females of species with bi-parental care tend to primarily increase the number of eggs when paired with attractive males, females of species with female-only care produce larger, but not more, eggs. These patterns may reflect adaptive differences in female allocation strategies arising from variation in the signal content of sexually selected male traits between systems of parental care. In contrast to reproductive effort, female allocation of immune-stimulants, anti-oxidants and androgens to the egg yolk was not consistently increased when mated to attractive males, which probably reflects the context-dependent costs and benefits of those yolk compounds to females and offspring.

Helpers at the nest compensate for reduced maternal investment in egg size in carrion crows

Life history theory predicts that mothers should trade off current and future reproductive attempts to maximize lifetime fitness. When breeding conditions are favourable, mothers may either increase investment in the eggs to improve the quality of the offspring or save resources for future reproduction as the good raising environment is likely to compensate for a 'bad start'. In cooperatively breeding birds, the presence of helpers improves breeding conditions so that mothers may vary the number, size and quality of the eggs in response to the composition of the group. Here, we show that in cooperatively breeding carrion crows Corvus corone corone, where nonbreeding males are more philopatric and more helpful at the nest than females, breeding females decreased egg size as the number of subordinate males in the group increased. However, despite the smaller investment in egg size, fledglings' weight increased in groups with more male subordinates, improving post-fledging survival and indicating that helpers fully compensated for the initial 'bad start'. These results highlight a 'hidden effect' of helpers that bears profound implications for understanding the ultimate function of helping.

The relationship between maternal phenotype and offspring quality: do older mothers really produce the best offspring?

Maternal effects are increasingly recognized as important drivers of population dynamics and determinants of evolutionary trajectories. Recently, there has been a proliferation of studies finding or citing a positive relationship between maternal size/age and offspring size or offspring quality. The relationship between maternal phenotype and offspring size is intriguing in that it is unclear why young mothers should produce offspring of inferior quality or fitness. Here we evaluate the underlying evolutionary pressures that may lead to a maternal size/age-offspring size correlation and consider the likelihood that such a correlation results in a positive relationship between the age or size of mothers and the fitness of their offspring. We find that, while there are a number of reasons why selection may favor the production of larger offspring by larger mothers, this change in size is more likely due to associated changes in the maternal phenotype that affect the offspring size-performance relationship. We did not find evidence that the offspring of older females should have intrinsically higher fitness. When we explored this issue theoretically, the only instance in which smaller mothers produce suboptimal offspring sizes is when a (largely unsupported) constraint on maximum offspring size is introduced into the model. It is clear that larger offspring fare better than smaller offspring when reared in the same environment, but this misses a critical point: different environments elicit selection for different optimal sizes of young. We suggest that caution should be exercised when interpreting the outcome of offspring-size experiments when offspring from different mothers are reared in a common environment, because this approach may remove the source of selection (e.g., reproducing in different context) that induced a shift in offspring size in the first place. It has been suggested that fish stocks should be managed to preserve these older age classes because larger mothers produce offspring with a greater chance of survival and subsequent recruitment. Overall, we suggest that, while there are clear and compelling reasons for preserving older females in exploited populations, there is little theoretical justification or evidence that older mothers produce offspring with higher per capita fitness than do younger mothers.

Female Cape sugarbirds (Promerops cafer) modify egg investment both for extra-pair mates and for male tail length

The differential allocation hypothesis predicts that females should invest more in reproduction when paired with attractive males. We measured egg volume in Cape sugarbirds (Promerops cafer), a sexually dimorphic passerine, in relation to paternity of the offspring and in response to an experimental tail length treatment. We manipulated tail length, after pair formation, but before egg laying: males had their tails either shortened or left unmanipulated. Our manipulation was designed to affect female allocation in a particular breeding attempt rather than long-term mate choice: males with shortened tails would appear to be signalling at a lower level than they should given their quality. We found that egg volume was smaller in the nests of males with experimentally shortened tails but larger when the offspring were the result of extra-pair matings. Both these findings are consistent with the differential allocation hypothesis. We suggest that tail length may be used by females as a cue for mate quality, eliciting reduced female investment when breeding with social mates; and with males with shortened tails.

Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird

Wherever individuals perform cooperative behaviours, each should be selected to adjust their own current contributions in relation to the likely future contributions of their collaborators. Here, we use the sentinel system of pied babblers (Turdoides bicolor) to show that individuals anticipate contributions by group mates, adjusting their own contribution in response to information about internal state broadcast by others. Specifically, we show that (i) short-term changes in state influence contributions to a cooperative behaviour, (ii) individuals communicate short-term changes in state, and (iii) individuals use information about the state of group mates to adjust their own investment in sentinel behaviour. Our results demonstrate that individual decisions about contributions to a cooperative effort can be influenced by information about the likely future contribution of others. We suggest that similar pre-emptive adjustments based on information obtained from collaborators will be a common feature of cooperative behaviour, and may play an important role in the development of complex communication in social species.

Multiple benefits of cooperative breeding in purple-crowned fairy-wrens: a consequence of fidelity?

1. Kin selection is one of the mechanisms that can explain apparent altruism by subordinate individuals in cooperatively breeding species, if subordinates boost the production of kin. We compared productivity and breeder survival in pairs with and without subordinates in a genetically monogamous cooperatively breeding bird, the purple-crowned fairy-wren Malurus coronatus. 2. Additive effects of subordinate help increased productivity. Total feeding rates to the nest were increased by two or more subordinates, and fledgling production was greater in larger groups. Not all subordinates contributed to nestling feeding, and the effect of group size was greater when non-contributors were excluded from analyses, suggesting that increased fledgling production was a direct result of help. 3. Compensatory effects of subordinate help improved breeder survival. Assisted breeders reduced their workload by 20-30%, irrespective of the number of helpers. Although re-nesting intervals were not affected by group size, reduced breeder feeding rates resulted in improved survival and breeders in larger groups survived better. 4. Subordinates and nestlings are usually progeny of the breeding pair in this species, and benefits of cooperative breeding are very different from three congeners with extremely high levels of extra-group paternity (EGP). In these Malurus, fledgling production and survival of male breeders are not enhanced in larger groups. This is consistent with the expectation that kin-selected benefits vary with relatedness, and thus levels of EGP. 5. We tested whether benefits of cooperative breeding in 37 avian species varied with levels of extra-group mating. Both direct and phylogenetically controlled comparisons showed that improvement of (male) breeder survival and enhanced productivity are more likely when fidelity is higher, as predicted when investment of subordinates correlates with relatedness to offspring. This pattern highlights the importance of considering the genetic mating system for understanding the evolution of cooperative breeding.

Life history, habitat saturation and the evolution of fecundity and survival altruism

Hamilton's rule provides a general description of the conditions for the evolution of altruism. But altruism can take different forms depending on which life-history trait is affected by the helping behavior (fecundity vs. survival helping). In particular, these different forms of helping may have very different demographic consequences, which may feed back on evolution. We examine the interplay between various forms of helping and demography in viscous populations with empty sites. A key component of our analysis is the local density of empty sites experienced by a focal individual, which provides a measure of habitat saturation. Habitat saturation is shown to have contrasting effects depending on (1) whether the physiological costs and benefits of helping affect fecundity, survival or both; and (2) whether the costs of helping are paid in a density-dependent or density-independent manner. For a given level of habitat saturation and with density-dependent reproduction, we find that the conditions for the evolution of helping should be more favorable in the survival altruism life cycle with a cost on fecundity, and more stringent in the fecundity altruism life cycle with a cost on survival. More generally, our analysis stresses the importance of taking into account the feedback between population demography, life history, and kin selection when investigating the selective pressures on altruism.

Food-supplementing parents reduces their sons' song repertoire size

Food-supplemented parents typically produce more offspring, as numerous experiments on vertebrate populations have shown. 'Propagule' (egg or neonate) size and parental care may also be affected, with implications concerning the adult quality of offspring, although few experiments have addressed whether food-supplementing one generation affects adult quality in the next. We conducted a food supplementation experiment on song sparrows (Melospiza melodia) and tested whether song repertoire size, a demonstrated indicator of male quality, differed between the adult sons of fed (food-supplemented) and unfed (non-food-supplemented) parents. Counterintuitively, fed parents produced sons with smaller adult song repertoires, who may thus be expected to contribute fewer offspring, and fewer grand-offspring, to the population. Fed and unfed parents invested equally in the total biomass of their clutches and broods, and average nestling condition was comparable, but because fed parents produced more offspring, average egg and nestling sizes were reduced. Fed and unfed parents apportioned care differently within their broods, and we suggest compensatory growth of offspring emerging from light eggs, or egg size itself, may have affected adult repertoire size. Conceivably, the conservation benefits of food-supplementing populations could attenuate over time if fed parents produce offspring of poorer quality than themselves.

Maternal effects in cooperative breeders: from hymenopterans to humans

The environment that an offspring experiences during its development can have lifelong consequences for its morphology, anatomy, physiology and behaviour that are strong enough to span generations. One aspect of an offspring's environment that can have particularly pronounced and long-lasting effects is that provided by its parent(s) (maternal effects). Some disciplines in biology have been quicker to appreciate maternal effects than others, and some organisms provide better model systems for understanding the causes and consequences of the maternal environment for ecology and evolution than others. One field in which maternal effects has been poorly represented, and yet is likely to represent a particularly fruitful area for research, is the field of cooperative breeding (i.e. systems where offspring are reared by carers in addition to parent(s)). Here, we attempt to illustrate the scope of cooperative breeding systems for maternal effects research and, conversely, highlight the importance of maternal effects research for understanding cooperative breeding systems. To this end, we first outline why mothers will commonly benefit from affecting the phenotype of their offspring in cooperative breeding systems, present potential strategies that mothers could employ in order to do so and offer predictions regarding the circumstances under which different types of maternal effects might be expected. Second, we highlight why a neglect of maternal strategies and the effects that they have on their offspring could lead to miscalculations of helper/worker fitness gains and a misunderstanding of the factors selecting for the evolution and maintenance of cooperative breeding. Finally, we introduce the possibility that maternal effects could have significant consequences for our understanding of both the evolutionary origins of cooperative breeding and the rise of social complexity in cooperative systems.

Helpers and egg investment in the cooperatively breeding acorn woodpecker: testing the concealed helper effects hypothesis

In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the “concealed helper effects” hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall’s acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.

Does interspecific competition affect offspring provisioning?

Offspring size is one of the most well-studied life-history traits, yet it is remarkable that few field studies have examined the manner in which the relationship between offspring size and performance (and thus, optimal offspring size) is affected by the local environment. Furthermore, while offspring size appears to be plastic in a range of organisms, few studies have linked changes in offspring size to changes in the relationship between offspring size and performance in the field. Interspecific competition is a major ecological force in both terrestrial and marine environments, but we have little understanding of its role in shaping selection on offspring size. Here we examine the effect of interspecific competition on the relationship between offspring size and performance in the field for the marine bryozoan Watersipora subtorquata along the south coast of Australia. Both interspecific competition and offspring size had strong effects on the post-metamorphic performance of offspring in the field, but importantly, they acted independently. While interspecific competition did not affect the offspring size-performance relationship, mothers experiencing competition still produced larger offspring than mothers that did not experience competition. Because larger offspring are more dispersive in this species, increasing offspring size may represent a maternal strategy whereby mothers produce more dispersive offspring when they experience high competition themselves. This study shows that, while offspring size is plastic in this species, post-metamorphic factors alone may not determine the size of offspring that mothers produce.

Differential reproductive success favours strong host preference in a highly specialized brood parasite

Obligate avian brood parasites show dramatic variation in the degree to which they are host specialists or host generalists. The screaming cowbird Molothrus rufoaxillaris is one of the most specialized brood parasites, using a single host, the bay-winged cowbird (Agelaioides badius) over most of its range. Coevolutionary theory predicts increasing host specificity the longer the parasite interacts with a particular avian community, as hosts evolve defences that the parasite cannot counteract. According to this view, host specificity can be maintained if screaming cowbirds avoid parasitizing potentially suitable hosts that have developed effective defences against parasitic females or eggs. Specialization may also be favoured, even in the absence of host defences, if the parasite's reproductive success in alternative hosts is lower than that in the main host. We experimentally tested these hypotheses using as alternative hosts two suitable but unparasitized species: house wrens (Troglodytes aedon) and chalk-browed mockingbirds (Mimus saturninus). We assessed host defences against parasitic females and eggs, and reproductive success of the parasite in current and alternative hosts. Alternative hosts did not discriminate against screaming cowbird females or eggs. Egg survival and hatching success were similarly high in current and alternative hosts, but the survival of parasitic chicks was significantly lower in alternative hosts. Our results indicate that screaming cowbirds have the potential to colonize novel hosts, but higher reproductive success in the current host may favour host fidelity.

Maternal effects as generators of evolutionary change: a reassessment

Despite being a center of debate in biology for centuries, the connection between the generation of novel adaptive variation and its inheritance remains a contentious issue. In evolutionary and behavioral ecology, assigning natural and sexual selection a creative and anticipatory role unmasks the need to explicitly consider the link between a trait's functional importance and its inheritance and results in confusion about selection as an adaptive modifier of development versus selection as a passive filter of already produced forms. In developmental genetics, an emphasis on regulatory versus coding aspects of molecular evolutionary change overlooks the fundamental question of the origination of an inherited developmental toolkit and assignment of its regulatory functions. Because maternal effects, by definition, combine developmental induction of functionally important changes and their inheritance, they bridge the origin and evolution of organismal adaptability, at least on short time scales. The explosion of empirical studies of maternal effects raises a question--are maternal effects ubiquitous but short-term adjusters and fine-tuners of an evolved form with only secondary importance for evolutionary change? Or are they a particularly clear example of a stage in a continuum of inheritance systems that accumulates, internalizes, and passes on the most consistent and adaptive organism-environment interactions? Here I place recent empirical studies of avian maternal effects into the evolutionary framework of variation, selection, and inheritance to examine whether maternal effects provide a window into evolutionary processes.

How helpers help: disentangling ecological confounds from the benefits of cooperative breeding

Evolutionary explanations for helping in cooperative breeding systems usually require a positive effect of helping on the fitness of the breeders being assisted. However, such helper effects have proven surprisingly difficult to quantify. Cockburn et al. (this issue) apply detailed statistical analyses to long-term field data on the enigmatic superb fairy-wren. They show that it is possible to disentangle the complex web of ecological and evolutionary interactions that confound so many studies. Whilst fairy-wren helpers may not increase nest productivity, they do increase future survival of breeding females. This study points the way for future statistical explorations of long-term data in other cooperative birds and mammals.

Maternal investment tactics in superb fairy-wrens

In cooperatively breeding species, parents often use helper contributions to offspring care to cut their own costs of investment (i.e. load-lightening). Understanding the process of load-lightening is essential to understanding both the rules governing parental investment and the adaptive value of helping behaviour, but little experimental work has been conducted. Here we report the results of field experiments to determine maternal provisioning rules in cooperatively breeding superb fairy-wrens (Malurus cyaneus). By manipulating carer: offspring ratios, we demonstrate that helpers allow females to reduce the rate at which they provision their brood. Female reductions, however, were less than that provided by helpers, so that chicks still received food at a faster rate in the presence of helpers. Despite this, chicks fed by parents and helpers were not heavier than those provisioned by parents alone. This is because maternal load-lightening not only occurs during the chick provisioning stage, but also at the egg investment stage. Theoretically, complete load-lightening is predicted when parents value themselves more highly than their offspring. We tested this idea by 'presenting' mothers with a 'choice' between reducing their own levels of care and increasing investment in their offspring. We found that mothers preferred to cut their contributions to brood care, just as predicted. Our experiments help to explain why helper effects on offspring success have been difficult to detect in superb fairy-wrens, and suggest that the accuracy with which theoretical predictions of parental provisioning rules are matched in cooperative birds depends on measuring maternal responses to helper presence at both the egg and chick stages.