Allocation of parental investment among individual offspring in the European beewolf Philanthus triangulum F. (Hymenoptera: Sphecidae)

Optimal allocation of parental resources is an important life history trait. However, it has been rarely investigated empirically. We tested aspects of optimal allocation theory in a digger wasp, the European beewolf. Investment allocation theory assumes (1) a trade‐off between investment per offspring and offspring number and (2) a convex relationship between investment per offspring and fitness returns. From mis relationship an optimum amount of investment per offspring can be derived and parents are predicted to provide each offspring with this optimum amount of investment. We used the number of bees in a brood cell as a measure of parental investment. Offspring fitness was quantified as both survival until emergence and success as adults. There is evidence for a trade‐off between current and future reproduction, suggesting that the first assumption is met. In contradiction to the second assumption, one mortality factor, parasitism, increased proportionally with the number of bees in a brood cell. However, overall mortality until emergence significantly decreased with the number of bees in a brood cell as assumed by the theory. The determination of the optimum amount of investment per offspring is complicated because the sexes possibly differ in their relationship between amount of investment and fitness. Individual males received considerably fewer bees (2.2 ± 0.8) than females (3.8 ± 0.5). Two independent estimates of the investment specific survival suggested that sons with two bees had the highest fitness returns per single bee and, consistent with the prediction, most sons were provisioned with two bees. For daughters, four bees is probably the optimum amount and most daughters were provisioned with this number. In both sexes the variation of investment per offspring was less than expected by a Poisson distribution with the same mean. These findings support the view that parental investment is allocated in a way that optimizes the trade‐off between offspring number and investment per offspring. However, variation contradicting the hypothesis still occurred. This might be explained either by adaptive variation in the amount of investment per offspring, constraints in the adjustment of the optimum amount of investment, or problems in measuring parental investment.     

Seasonal changes in resource allocation within an individual offspring of the wolf spider, Pardosa pseudoannulata (Araneae: Lycosidae)

Abstract.  Seasonal changes in resource investment into individual offspring are well documented, but no attention has been paid to the allocation of the invested resource among the body parts of the offspring. In the present study, seasonal changes in the absolute and relative sizes in a spiderling of the wolf spider Pardosa pseudoannulata Boes. et Str. are investigated, and the relationship between spiderling body size and the ability to moult during a food shortage is clarified. Spiderlings that emerge in November have a significantly larger cephalothorax and abdomen than those that emerge in June. In addition, the abdomen–cephalothorax size ratio is significantly greater in November than in June offspring. Under limited food availability conditions, only 40% of spiderlings moult. Nymphs that do moult have a significantly larger cephalothorax, abdomen and abdomen–cephalothorax size ratio than nymphs that do not moult. Thus, both the quantity of resources invested in the cephalothorax and abdomen of a spiderling and the proportional allocation of resources between the two body parts change seasonally in Pa. pseudoannulata ; alteration of the resource allocation occurs in late autumn. Larger spiderlings of Pa. pseudoannulata that emerge in late autumn would be able to develop into advanced instars even during food shortage, and therefore may have better overwintering survival rates.     

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.     

The positive correlation between maternal size and offspring size: fitting pieces of a life‐history puzzle

The evolution of investment per offspring (I) is often viewed through the lens of the classic theory, in which variation among individuals in a population is not expected. A substantial departure from this prediction arises in the form of correlations between maternal body size and I, which are observed within populations in virtually all taxonomic groups. Based on the generality of this observation, we suggest it is caused by a common underlying mechanism. We pursue a unifying explanation for this pattern by reviewing all theoretical models that attempt to explain it. We assess the generality of the mechanism upon which each model is based, and the extent to which data support its predictions. Two classes of adaptive models are identified: models that assume that the correlation arises from maternal influences on the relationship between I and offspring fitness [w(I)], and those that assume that maternal size influences the relationship between I and maternal fitness [W(I)]. The weight of evidence suggests that maternal influences on w(I) are probably not very general, and even for taxa where maternal influences on w(I) are likely, experiments fail to support model predictions. Models that assume that W(I) varies with maternal size appear to offer more generality, but the current challenge is to identify a specific and general mechanism upon which W(I) varies predictably with maternal size. Recent theory suggests the exciting possibility that a yet unknown mechanism modifies the offspring size–number trade‐off function in a manner that is predictable with respect to maternal size, such that W(I) varies with size. We identify two promising avenues of inquiry. First, the trade‐off might be modified by energetic costs that are associated with the initiation of reproduction (‘overhead costs’) and that scale with I, and future work could investigate what specific overhead costs are generally associated with reproduction and whether these costs scale with I. Second, the trade‐off might be modified by virtue of condition‐dependent offspring provisioning coupled with metabolic factors, and future work could investigate the proximate cause of, and generality of, condition‐dependent offspring provisioning. Finally, drawing on the existing literature, we suggest that maternal size per se is not causatively related to variation in I, and the mechanism involved in the correlation is instead linked to maternal nutritional status or maternal condition, which is usually correlated with maternal size. Using manipulative experiments to elucidate why females with high nutritional status typically produce large offspring might help explain what specific mechanism underlies the maternal‐size correlation.     

Relative investment in egg load and poison sac in fig wasps: Implications for physiological mechanisms underlying seed and wasp production in figs

Fig pollinating wasps and most non-pollinator wasps apply secretions from their poison sacs into oviposited flowers that appear necessary to the formation of the galls that their developing offspring consume. Thus, both eggs and poison sac secretions appear to be essential for wasp reproduction, but the relative investment in each is unknown. We measured relative investment in poison sac and egg production in pollinating and non-pollinating wasps associated with seven species of monoecious Panamanian figs representing both active and passive pollination syndromes. We then collected similar data for four fig hosts in China, where some wasp species in the genus Eupristina have lost the ability to pollinate (“cheaters”). All wasps examined possessed large poison sacs, and we found a strong positive correlation between poison sac size and absolute egg production. In the Panamanian species, the relative poison sac to egg investment was highest in the externally ovipositing non-pollinator wasps, followed by active pollinators, then by passive pollinators. Further, pollinator wasps of fig species with demonstrated host sanctions against “cheating” wasps showed higher investment in the poison sac than wasps of species without sanctions. In the Chinese samples, relative investment in the poison sac was indistinguishable between pollinators and “cheaters” associated with the same fig species. We suggest that higher relative investment in poison sac across fig wasp species reflects higher relative difficulty in initiating formation of galls and subsequently obtaining resources from the fig. We discuss the implications for the stability of the fig–wasp mutualism, and for the ability of non-pollinators to exploit this mutualism.     

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.     

Influences of Parental Care and Food Deprivation on Regulation of Body Mass in a Burying Beetle

Changes in adult body mass during breeding can reveal how parents prepare energetically for care, the stress of care, and the need to terminate care in a state conducive for future reproduction. Interpreting changes in parent mass can be difficult, however, because temporal variation in body mass may reflect a constraint imposed by the stress of care, revealing conflict within the family, or a shift to a new body mass optimum adaptive for a different stage of the breeding cycle. Here, we examined the effect of food deprivation and parenting on variation in female body mass of Nicrophorus orbicollis, an insect in which parents and offspring share a common food resource (a prepared carrion ball). Female parents demonstrated a remarkable degree of regulation of body mass: Despite varied periods of food deprivation (0–8 d) prior to discovery of a carcass, females attained a similar body mass (108.3–109.2% of pre‐deprivation mass) at the time of larval hatching. Females attained a greater body mass in anticipation of rearing a greater number of young. Mothers lost mass during active parental care, and mass at the end of caregiving was less in mothers that reared more and heavier young. Body mass at the end of care was less than the preferred mass for females searching for a carcass, indicating that the mothers sacrificed self‐maintenance and future reproductive potential for their current brood. Contrary to prediction, pre‐breeding food deprivation had no effect on offspring size or on female condition at the end of care. We conclude that there is a limited degree of conflict over the sharing of food among N. orbicollis parents and offspring, but that this conflict is not exacerbated by food deprivation prior to breeding.     

Parental investment and quality of insurance offspring in an obligate brood-reducing species, the American white pelican

The last-hatched chick, or B-offspring, of American white pelicanstypically survives only as "insurance" when its elder siblingfails. Life-history theory suggests that parents should investrelatively less in these disadvantaged insurance offspring.For an insurance strategy to be effective, however, reducedinvestment may be constrained by the need to maintain potentialinsurance offspring in a viable condition until at least 3–6days of age, after which they are rarely needed. In agreementwith the life-history prediction, egg size, resultant hatchingmass, and growth rates at two-chick nests were significantlylower for B-offspring. When hatched in the laboratory, B-eggswere also slightly but significantly less efficient at convertingegg size into hatching mass. Despite these differences, B-chicksthat were reared as singles, free from sibling competition fromhatching onward, showed no decrement in survival or growth rate.When A-chicks were removed from nests with underweight 3- or6-day-old B-chicks, a minority (21%) of B-chicks failed to recover,but mean growth rates of survivors increased rapidly to controllevels. Results suggest that although parental investment inB-offspring is reduced, it is usually adequate to produce andmaintain potential insurance offspring in viable condition duringthe time that they are most likely to be needed as replacementsfor failed elder siblings.     

Effects of male genetic contribution and paternal investment to egg and hatchling size in the cricket,Gryllus firmus

Egg provisions represent the complete energy supply of oviparous organisms from fertilization until hatching, and egg size is generally correlated with initial offspring size and a suite of other early fitness related traits. Since egg size is determined by the mother, little attention has been given to potential sources of paternal effects on either egg size or initial offspring size. This study considers two processes by which the sire can affect the egg size and/or initial body size of his offspring, in the multiply mated cricket, Gryllus firmus. The first is a paternal genetic effect, whereby differences in offspring genotype result in differences in the efficiency of metabolising available resources. The second is a paternal environmental effect, whereby the quality or size of paternal investment varies among male phenotypes and this is correlated with the size of eggs the females subsequently produce. Using a one-locus two-allele recessive mutation for pale eye colour as a marker, a mating experiment was designed which enabled the discrimination between eggs fertilized by two males mated simultaneously to a single female. The results of this experiment suggest that sire effects on egg and initial body size occur through both processes. Eggs fertilised by the two males were significantly different at day ten of development, suggesting that the genetic contribution of the sire is affecting embryonic body size. Further, a negative correlation was found between the head size of the pale eyed male and the size of all the eggs that the female laid, suggesting an effect of male size of the amount of nutrients the female receives from her mates. The results of this study suggest that paternal effects may be both more common and more profound than previously thought, and that studies examining early fitness traits that are correlated to egg size or initial body size, in oviparous animals, should consider the possibility and importance of the paternal contribution.     

Expected future reproductive success and paternal behaviour in the sand goby, Pomatoschistus minutus (Pisces, Gobiidae)

We studied egg care and guarding behaviour in the paternal sand goby, Pomatoschislus minutus , in relation to egg number and age. A male's expected reproductive success is assumed to increase with larger egg masses and older eggs. Non-shareable care, i.e. fanning, was higher in larger and older egg masses. Egg age explained 65% and egg mass size 22% of the total variation in the amount of fanning performed by a male. Contrary to expectation, egg removal did not affect a male's fanning behaviour, Males that were approached stayed longer in the nest when they guarded larger egg masses. Males with older eggs were also more reluctant to leave the nest. The time since a male had been chased away from his nest until he returned decreased as the season progressed. The results are discussed in relation to tradeoffs between present and future reproductive events.     

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.     

Egg presence, egg loss, and female mate preferences in the sand goby (Pomatoschistus minutus)

We studied the effect of egg presence on female mate choicein a fish with paternal care. Females who were allowed a freechoice between two males mated within a shorter time than femaleswho were randomly assigned to a particular male. When a secondfemale was allowed to choose among the males, she preferredthe same male as the previous female. This result shows thatfemales are concordant in their mate choice. When the initialfemale was randomly assigned to mate with one of two males (forcedchoice), the second female mated randomly with respect to thefirst one. Thus females do not prefer males with eggs. If theinitial female was given a free choice, but the eggs were removedfrom the chosen male, the test female mated randomly. When boththe males initially had mated but one randomly determined male'seggs were removed, the test female preferred the male who wasstill guarding eggs. These experiments show that females avoidspawning in unsuccessful nests. When the females in the freechoice/egg removal experiment mated with the unsuccessful malethere was a considerably bigger size difference in favor ofthis male than when the females mated with the other male. Weconclude that female sand gobies show clear mate preferences,but that they do not prefer males with eggs over males withouteggs. They do, however, avoid mating with males guarding unsuccessfulnests. We therefore suggest that egg loss could be an importantfactor selecting for egg preference.     

Musical improvisation skill in a prospective partner is associated with mate value and preferences,consistent with sexual selection and parental investment theory: Implications for the origin of music

Music is a human universal, which suggests a biological adaptation. Several evolutionary explanations have been proposed, covering the entire spectrum of natural, sexual, and group selection. Here we consider the hypothesis that musical behaviour constitutes a reliable or even costly signal of fitness, and thus may have evolved as a human trait through sexual selection. We experimentally tested how musical performance quality (MPQ), in improvisations on the drums, saxophone, and violin, affects mate values and mate preferences perceived by a prospective partner. Swedish student participants (27 of each sex) saw a face of a person of the opposite sex and heard a piece of improvised music being played. The music occurred in three levels of MPQ and the faces in three levels of facial attractiveness (FA). For each parametric combination of MPG and FA, the participants rated four mate value scales (intelligence, health, social status, and parenting skill) and four mate preference scales (date, intercourse, and short- and long term relationship). Consistent with sexual selection theory, mate value ratings were generally increased by MPQ for raters of both sexes. Consistent with more specific hypotheses that follow from combining sexual selection and parental investment theory, women’s but not men’s preference for a long-term, but not short-term, relationship was significantly increased by MPQ, MPQ generally affected women’s ratings more than men’s, FA generally affected men’s ratings more than women’s, and women’s ratings of intelligence were even more influenced by MPQ than by FA.