Sex differences in biparental care as offspring develop: a field study of convict cichlids (<Emphasis Type="Italic">Amatitlania siquia</Emphasis>)

Parental investment theory states that parents should contribute more to older offspring. Differences between the sexes also influence how each parent contributes to offspring in biparental species. Here, we examined a naturally occurring population of biparental convict cichlids in Costa Rica to determine how each parent cared for offspring during two distinct offspring development stages. Consistent with the predictions of the reproductive value hypothesis, we hypothesized that the levels of parental contribution would be relative to the value that each parent places on a brood. We predicted that female parents would contribute more than male parents because female convict cichlids have lower future reproductive success than males. Additionally, we predicted that both parents should contribute more to older offspring, either due to the young’s increased susceptibility to predation (i.e., the vulnerability hypothesis) or because of the longer period of time parents have been interacting with older offspring (i.e., feedback hypotheses). This increase in investment by males should coincide with a change in the coordination of care between parents. Detailed observations of parental pairs in their natural habitat supported these predictions. Females contributed more to broods than males and were relatively unaffected by offspring age while males spent significantly more time with older, free-swimming fry. Additionally, males tended to leave younger offspring more than females did, and were more likely to do so consecutively with younger offspring. This suggests that the coordination of duties between parents changes as parental investment changes. Overall, these data support both the reproductive value and the vulnerability hypotheses, but not necessarily the feedback hypothesis.     

The Dilution Effect and Differential Predation Following Brood Adoption in Free-ranging Convict Cichlids (Cichlasoma nigrofasciatum)

Parental investment in unrelated offspring is potentially maladaptive but may be promoted by natural selection if the presence of foreign young enhances the survival of the parents' own young. We experimentally augmented broods of free-ranging convict cichlids (Cichlasoma nigrofasciatum) to test whether survival of the adopting parents' young (fry) increases, in relation to that of control broods, after the addition of smaller foreign fry, and whether such an increase can be attributed to the effect of brood dilution acting alone or to a combination of brood dilution and the effect of differential predation on adopted young. Total fry survival did not differ between experimental (E) broods and control (C) broods, but E broods had significantly more large (host) fry after 5 days and 10 days than C broods did. In E broods, small (foreign) fry suffered higher rates of predation than large fry, indicating differential predation. In E broods starting at 7.0 and 7.5 mm standard length (SL), observed fry mortalities did not differ significantly from mortalities expected from the effect of brood dilution. However, E broods starting at 8.0 mm SL had significantly lower mortalities than expected, indicating that parents that adopt smaller foreign fry can increase the survival of their own fry by the combined effects of brood dilution and differential predation. Within E broods, growth of smaller foreign fry was significantly slower than that of larger host fry, suggesting that intra-brood agonistic behaviour affects access to food for smaller fry. Therefore, increased predation and reduced growth are two negative effects that act on fry of donor parents.     

Temporal Variation in Decisions about Parental Care in Bluegill, Lepomis macrochirus

Parental investment theory states that an individual will trade‐off present and future reproductive potential to maximize lifetime reproductive success. Only when parental care is costly in terms of reduced future reproductive potential should individuals be sensitive to changes in the value of current offspring and adjust their care. Here, we examine temporal variation in parental care decision‐making in bluegill (Lepomis macrochirus), in which care is provided by males called ‘parentals’. Previous research has shown that parentals that nest early in the breeding season are in higher energetic condition than those that nest later, and early nesting males appear not to pay an opportunity cost to their care in terms of reduced future reproductive potential. Early nesting males also may have higher paternity in their broods than later nesting males. To examine the parental care decisions made by early and mid‐season nesting parentals, we experimentally reduced males’ perceived paternity by swapping eggs between nests. We found that experimental males that nested early in the breeding season adjusted their brood defence behaviour similarly to control males, which had sham egg swaps performed. Conversely, experimental males that nested mid‐season significantly decreased their brood defence behaviour after the manipulation as compared with control males. Thus, unlike mid‐season nesting males, early nesting males appear relatively insensitive to changes in brood value (paternity), possibly because early nesting males pay little cost in terms of reduced future reproductive potential to providing full care or because these males have a predisposition to high paternity.     

Filial cannibalism in an assassin bug

Filial cannibalism has been described in many fish species with male parental care, and has typically been explained as a response to high energetic costs of brood defence and decreased feeding opportunities during the period of care. We investigated filial cannibalism in an insect, the assassin bug Rhinocoris tristis. In this species, males guard eggs of a number of females, cannibalizing some of their offspring within the brood. We monitored guarding males in both the field and the laboratory. Males typically ate eggs around the periphery of the brood, which were those most likely to have been parasitized by wasps. However, cannibalism persisted in the laboratory in the absence of parasites, and the number of cannibalized eggs was related to the length of care and overall brood size, suggesting that males use eggs as an alternative source of food. This conclusion was further supported by the fact that males in the field did not lose weight while guarding, despite being unable to forage efficiently while caring. Males were also observed to adopt broods, but in a laboratory experiment did not eat more eggs from adopted than from their own broods.     

Filial cannibalism in teleost fish

This review summarizes information on filial cannibalism (the act of eating one's own offspring) in teleost fish. Cannibalistic parents can either consume their whole brood (total filial cannibalism), or eat only some of the eggs in the nest (partial filial cannibalism). Offspring consumption has been argued to be adaptive under the assumption that offspring survival is traded against feeding, and that offspring can act as an alternative food source for the parents. The evidence supporting the basic predictions formulated under these assumptions is summarized for both total and partial filial cannibalism. These two forms of cannibalism differ significantly since the former represents an investment only in future reproductive success, whereas the latter can affect both present and future reproductive success. Despite a few inconsistencies in the data from laboratory and field studies, the energy-based explanation appears valid for both forms of cannibalism. Alternative non-energy-based explanations are considered, but they are unable to account for the wide distribution of this behaviour in teleosts. The intersexual conflict arising from attempts of the non-cannibal sex to minimize the cost of filial cannibalism is also discussed, together with the potential effect of this behaviour on the operational sex ratio at a population level.     

EVOLUTION OF OBLIGATE SIBLICIDE IN BOOBIES. 2: FOOD LIMITATION AND PARENT–OFFSPRING CONFLICT

Proximate limitation on parental food delivery has long been invoked to explain the evolution of single-chick broods of pelagic seabirds such as masked boobies (Sula dactylatra). A second possible proximate limit on brood size is siblicide driven by genetic parent–offspring conflict (POC) over brood size, if siblicidal offspring can reduce brood size to one even if the parents' optimal brood size is greater than one. I tested these two hypotheses by experimentally suppressing obligate siblicide in masked booby broods and comparing breeding parameters of these broods with unmanipulated single-chick control broods. Per capita mortality rate of experimental nestlings was higher than that of controls, but this deficit was more than made up by larger brood size. Parents of experimental broods brought more food to offspring, had higher fledging success, and apparently incurred no additional major short-term cost of reproduction, relative to parents of control broods, thus refuting the food limitation hypothesis. Estimates of inclusive fitness of chicks in experimental broods were higher than were those of control nestlings, a result inconsistent with the POC hypothesis that the siblicidal offspring's optimal brood size is one while the parents' optimum is greater than one. This discrepency between natural brood size and apparent brood size optima might be resolved in several ways: experimental artifacts may give misleading estimates of optimal brood size; experimental and control offspring may have different reproductive values at the time of fledging; nestling masked boobies may face a special frequency-dependent case of POC in which the high risk of sharing a nest with a siblicidal sibling makes invasion of other behavioral genotypes difficult even when offspring and parent inclusive fitnesses are higher from a nonsiblicidal brood of two than from a brood of one.     

Sex ratio in relation to timing of breeding, and laying sequence in a dimorphic seabird

When the cost of rearing sons and daughters differs and the subsequent survival and reproductive success of one sex is more dependent than the other, on the amount of parental investment, adult females tend to produce more chicks of the more dependent sex if the females are in good condition themselves. One method of varying the total investment in each sex is through modifying the sex ratio of offspring produced. This study shows that in broods of European Shags Phalacrocorax aristotelis , the sex ratio varied with laying date. Presumably in this species, the lifetime reproductive success of males is more dependent on the level of parental investment. Early breeders are in better condition, the brood sex ratio of early broods was male biased (0.63), while that of late broods was female biased (0.36). The overall difference in sex ratio found between early and late nests could be attributed to manipulation of sex in the first laid egg. In early broods, 77% of the first hatched chicks were male but only 30% of the first hatched chicks in late broods were male. The sex combination of the first two chicks in a brood significantly affected growth as measured by asymptotic mass.     

The effect of brood size and age on partial filial cannibalism in the scissortail sergeant

Within the size range found in the field for the scissortail sergeant Abudefduf sexfasciatus , there was no correlation between the number of cannibalized eggs and total brood size. Very small broods were fully cannibalized. In a manipulation experiment, males were provided with broods of one of three standardized sizes: at the two extremes and in the middle of the range of naturally occurring broods. Brood size had no effect on partial filial cannibalism, but parental effort increased with increasing brood size. Field correlates and the manipulation experiment showed that the cost of cannibalism in terms of current reproductive success decreased significantly with increasing brood size. Contrary to expectations, there was no relationship between male size and the incidence of cannibalism. No preference for younger eggs was found either from field correlates or from a manipulative experiment in which males were provided with an equal number of young and old eggs at the beginning of the parental phase.     

Shared or Unshared Parental Care in Overwintering Brent Geese (Branta bernicla hrota)

We examine brood size effects on the behaviour of wintering parent and juvenile brent geese (Branta bernicla hrota) to test predictions of shared and unshared parental care models. The behaviour of both parents and offspring appear to be influenced by declining food availability over the winter. Parental vigilance increased with brood size and may be explained by vigilance having functions in addition to antipredator behaviour where the benefits are shared among the brood. There was no increase in parental aggression with brood size and this does not fit the prediction of shared care. Nevertheless, large families are able to monopolize better feeding areas compared with smaller families and large families static feed more but walk feed less than do small families, the former apparently being the preferred mode. The presence of additional young, rather than increasing the amount of parental aggression, seems to enhance the family's competitive ability. Because parents with large broods benefit from enhanced access to resources there is likely to be no additional significant cost in the parental care of larger broods (sensu Trivers 1972 ).     

Effects of a low oxygen environment on parental effort and filial cannibalism in the male sand goby, Pomatoschistus minutus

In fish, brood cycling parental males sometimes eat some orall of their eggs, a behavior termed filial cannibalism. Wetested predictions of filial cannibalism models related to thecost of parental care in the male sand goby, Pomatoschistusminutus, by increasing the parental effort (fanning expenditure)through reduced levels of dissolved oxygen to 39% in an experimentalgroup, whereas a control group had fully saturated water. Malesshowed both full-clutch cannibalism and partial-clutch cannibalismin both treatments. Giving the males one to three females tospawn with, we found that small clutches were completely eatenmore often than were larger ones, whereas partial-clutch cannibalismwas not affected by clutch size. Although treatment did notaffect filial cannibalism, it did affect a male's energy statesuch that males in the low oxygen treatment lost more body fat,indicating a greater fanning effort. This shows that males inthe low oxygen treatment allocated more energy to the presentbrood, potentially at the expense of future reproductive success.Our study strongly suggests that filial cannibalism in malesand gobies represents a strategic life-history decision asan investment in future reproductive success, and is not triggeredby a proximate need for food necessary for the male's own survival.Furthermore, males in the low oxygen treatment built nests withlarger entrances, and were less likely to rebuild their nestsafter destruction. Presumably, this makes fanning easier butthe nest more vulnerable to predators, suggesting a trade-offbetween fanning and nest defense.     

Brood size and the economy of brood defence: examining Lack's hypothesis in a biparental cichlid fish

Synopsis We tested the explanatory value of two hypotheses reviewed by Lack (1954) in the maintenance of brood size in free-ranging convict cichlidsCichlasoma nigrofasciatum: (1) physiological constraints on egg production, and (2) behavioural constraints imposed by brood defence. Number of free-swimming young in 13 experimental (E) broods was augmented to the upper limit of the size distribution of natural broods (150 young); 18 control (C) broods were handled in the same way but brood size was not changed (mean ± SE = 69.5 ± 11.0). E and C brood sizes were measured at 5 day intervals. At day 20 (just before independence from parental care), 50.3 ± 9.4 (n = 9) young remained in E broods and 30.8 ± 7.8 (n = 8) young remained in C broods (p> 0.05). Offspring number did not differ significantly (p> 0.05) between C and E broods after day 10. Mean growth rate of offspring was significantly lower in E broods than in C broods, perhaps in response to increased density of young in the former. Both the convergence of offspring number in E and C broods and suppression of growth in E broods support a behavioural constraint; that during the first 10 days in which the young are free swimming, two parents are unable to defend large broods as successfully as small broods. A trade-off exists in parental investment between current and future reproduction. Extra-parental investment in current reproduction (eggs) does not result in an increased number of young at independence, therefore a behavioural constraint during brood defence should stabilize the evolution of clutch size.     

Effects of among-offspring relatedness on the origins and evolution of parental care and filial cannibalism

Parental care is expected to increase the likelihood of offspring survival at the cost of investment in future reproductive success. However, alternative parental behaviours, such as filial cannibalism, can decrease current reproductive success and consequently individual fitness. We evaluate the role of among-offspring relatedness on the evolution of parental care and filial cannibalism. Building on our previous work, we show how the evolution of care is influenced by the effect of among-offspring relatedness on both the strength of competition and filial cannibalism. When there is a positive relationship between among-offspring competition and relatedness, parental care will be favoured when among-offspring relatedness is relatively low, and the maintenance of both care and no-care strategies is expected. If the relationship between among-offspring competition and relatedness is negative, parental care is most strongly favoured when broods contain highly related offspring. Further, we highlight the range of conditions over which the level of this among-offspring relatedness can affect the co-occurrence of different care/no care and cannibalism/no cannibalism strategies. Coexistence of multiple strategies is independent of the effects of among-offspring relatedness on cannibalism but more likely when among-offspring relatedness and competition are positively associated.     

Population density, body size, and phenotypic plasticity of brood size in a burying beetle

Theory predicts that organisms living in heterogeneous environmentswill exhibit phenotypic plasticity. One trait that may be particularlyimportant in this context is the clutch or brood size becauseit is simultaneously a maternal and offspring characteristic.In this paper, I test the hypothesis that the burying beetle,Nicrophorus orbicollis, adjusts brood size, in part, in anticipationof the reproductive environment of its adult offspring. N. orbicollisuse a small vertebrate carcass as a food resource for theiryoung. Both parents provide parental care and actively regulatebrood size through filial cannibalism. The result is a positivecorrelation between brood size and carcass size. Adult bodysize is an important determinant of reproductive success forboth sexes, but only at higher population densities. I testthree predictions generated by the hypothesis that beetles adjustbrood size in response to population density. First, averageadult body size should vary positively with population density.Second, brood size on a given-sized carcass should be larger(producing more but smaller young) in low-density populationsthan in high-density populations. Third, females should respondadaptively to changes in local population density by producinglarger broods when population density is low and small broodswhen population density is high. All three predictions weresupported using a combination of field and laboratory experiments.These results (1) show that brood size is a phenotypically plastictrait and (2) support the idea that brood size decisions arean intergenerational phenomenon that varies with the anticipatedcompetitive environment of the offspring.     

Seasonal pattern of filial cannibalism by Apogon doederleini mouthbrooding males

In temperate waters of southern Japan, mouthbrooding males of the cardinal fish Apogon doederleini sometimes ate their entire brood within a day of spawning. In spite of annual variation in age structure and length of the breeding season, however, the annual cannibalism rate (broods cannibalized to total broods spawned) was nearly constant, 12–16%. Fish 1 year old frequently cannibalized early broods of the season, especially the first brood, whereas the cannibalism rate by 2 year-old fish did not vary within the season. In contrast, the cannibalism by older males increased late in the season. This tendency was attributed to their different allocation patterns to growth and reproduction. For 1 year-old fish, which can enhance future reproductive success by growth and cannibalizing on early broods instead of mouthbrooding (at relatively low temperatures), this may be a tactic for investing energy in growth. For fish 3 years, which have more breeding cycles and grow little, cannibalizing late broods may be energetic compensation for their poor somatic condition. No seasonal variation of cannibalism by 2 year-old fish can be explained by their intermediate nature of growth and reproduction. Multiple mate availability is one factor facilitating the cannibalism by 2 and 3 + year-old fish whose operational sex ratio is female-biased.     

Filial cannibalism in fishes: Why do parents eat their offspring?

Filial cannibalism (the eating of one's own offspring) occurs in a variety of taxa, but is especially prevalent in fishes with parental care. Recent research supports a central tenet of parental-investment theory; that is, parents consume their offspring when it maximizes their lifetime reproductive success. This review outlines the theoretical framework used to explain the adaptive significance of filial cannibalism, evaluates experimental studies to test some predictions of this theory and discusses how the occurrence of filial cannibalism affects other aspects of a species' reproductive ecology.     

Cascading costs of reproduction in female house wrens induced to lay larger clutches

In many species, females produce fewer offspring than they are capable of rearing, possibly because increases in current reproductive effort come at the expense of a female's own survival and future reproduction. To test this, we induced female house wrens (Troglodytes aedon) to lay more eggs than they normally would and assessed the potential costs of increasing cumulative investment in the three main components of the avian breeding cycle – egg laying, incubation and nestling provisioning. Females with increased clutch sizes reared more offspring in the first brood than controls, but fledged a lower proportion of nestlings. Moreover, nestlings of experimental females were lighter than those of control females as brood size and prefledging mass were negatively correlated. In second broods of the season, when females were not manipulated, experimental females laid the same number of eggs as controls, but experienced an intraseasonal cost through reduced hatchling survival and a lower number of young fledged. Offspring of control and experimental females were equally likely to recruit to the breeding population, although control females produced more recruits per egg laid. The reproductive success of recruits from broods of experimental and control females did not differ. The manipulation also induced interseasonal costs to future reproduction, as experimental females had lower fecundity than controls when breeding at least 2 years after having their reproductive effort experimentally increased. Finally, females producing the modal clutch size of seven eggs in their first broods had the highest lifetime number of fledglings.     

Delayed benefits of paternal care in the burying beetle Nicrophorus vespilloides

Burying beetles, Nicrophorus spp., inter the carcasses of small vertebrates as a food source for their offspring. Females can bury a carcass and rear a brood on it alone, but are frequently assisted by a male whose presence reduces the risk of the carcass being taken over by other beetles. However, the male often stays for longer than the carcass is vulnerable to take-over, and he cares for the brood without conferring any further benefits on it. In a laboratory experiment using N. vespilloides, we found that, in the absence of competitors, male assistance conferred no advantages on the brood for which he was caring, but significantly increased the subsequent reproductive success of his mate, in terms of the mass and rate of development of a second brood, reared alone. We suggest that this is due to a reduced parental effort of assisted females, who spent less time feeding offspring and more time resting than unassisted females whilst rearing their first broods. In the field, a female is unlikely to pair with the same male for consecutive broods, so we discuss the possible benefits a male may accrue from increasing his mate's reproductive success. We also discuss the relevance of these results to our understanding of the evolution of biparental care in birds. Copyright 2000 The Association for the Study of Animal Behaviour.     

Evolution of single-chick broods in the Swallow-tailed Gull Creagrus furcatus

Swallow-tailed Gulls lay single-egg clutches, and so raise single-chick broods. As they are pelagic seabirds, this small brood size is expected to relate to proximate food limitation owing to infrequent food deliveries. However, a previous brood doubling experiment detected an 82% increase in fledging success from experimentally doubled broods compared to controls. We repeated the brood doubling experiment, and found that none of 50 enlarged broods produced more than one independent offspring. Control and experimental parents produced fledglings of similar body size, which also had indistinguishable rates of fledging and subsequent survival and reproduction. A variety of parameters estimating survival and breeding costs of reproduction showed no treatment effect. Since two-chick broods yield dramatically higher fledging rates at some times, apparently without excess costs of reproduction, selection on brood size appears to favour a two-chick brood. However, selection may not favour a two-egg clutch if egg production is very costly. Additionally, our estimates of reproductive success do not incorporate the performance of experimental and control offspring as adults, which could differ, since growth of chicks differed slightly by treatment.     

Parental physiological condition and reproductive success in chinstrap penguins (Pygoscelis antarctica)

Recent studies suggest that parental resource allocation may be the most important factor explaining differences in reproductive output among parents. That said at least two different hypotheses of balance between parental foraging effort and resource allocation have been proposed. First, parents with high foraging effort have high reproductive success. Second, parents with higher allocation of resources to offspring have high reproductive success. We tested the second hypothesis using chinstrap penguins (Pygoscelis antarctica) as a model. We evaluated nutritional condition of the parents using blood urea, uric acid, creatine kinase, and cholesterol levels. We evaluated reproductive success according to total mass of the brood and asymmetries inside the brood. We measured the degree of asymmetry using weight and culmen length. Generalized linear models were used to examine relationships between adult plasma urea levels with year, nest position, and degree of asymmetry in chicks. Our results demonstrate that lighter broods were more asymmetric and associated with lower values of adult plasma urea, uric acid, and creatine kinase. We interpret these findings as evidence that the birds allocate fewer resources to their chicks than adults with more symmetric broods are.     

Convict cichlids benefit from close proximity to another species of cichlid fish

The coexistence of species with overlapping resource use is often thought to involve only negative fitness effects as a consequence of interspecific competition. Furthermore, the scarce empirical research on positive species interactions has predominantly focused on sessile organisms. Here, I experimentally assessed the effect of close proximity of a potential brood predator and competitor on reproductive success of a neotropical cichlid fish. I demonstrate that convict cichlid (Archocentrus nigrofasciatus) broods have a higher survival rate near territories of the Nicaragua cichlid (Hypsophrys nicaraguensis), and that escape from predation and nest takeovers is the most likely explanation for the decreased offspring mortality.