Allocation Of Parental Investment Research Articles

Cooperative Begging in Banded Mongoose Pups

Vivid begging displays are common in species with parental care [1, 2]. They are usually seen as the way that rival offspring selfishly compete over parental investment [3], and individuals are expected to respond to the begging of rivals by increasing their own begging intensity [4, 5]. Here I show the opposite - that potential rivals gain direct benefits from begging by littermates, so that begging behavior becomes a collective enterprise, similar to other cooperative activities. I investigate begging in communally breeding banded mongooses (Mungos mungo), where each pup forms an exclusive relationship with a single helper (its "escort"), minimizing competition over food allocation. Escorts were influenced by the total signal emanating from a litter, so that pups who begged at low rates received more food as litter size increased. Focal pups increased their begging when litters were experimentally reduced or littermates were induced to beg at low rates, but they received food at similar rates and showed reduced weight gain - indicating that they were paying a higher cost for a similar reward. These results suggest that offspring can benefit from companions despite conflicts over the allocation of parental investment [6, 7]. Such benefits provide an explanation for observed variation in the expression of parent-offspring conflict.

JUVENILE MORTALITY INCREASES WITH CLUTCH SIZE IN A NEOTROPICAL BIRD

The change in avian clutch size with latitude is a celebrated example of geographic variation in a vertebrate life-history trait. Alternative hypotheses for this pattern invoke nest predation, limited food for nestlings, or post-fledging juvenile mortality as selection pressures leading to small clutch size of tropical birds. We manipulated the clutch size of Spotted Antbirds (Hylophylax naevioides) in central Panama to test these hypotheses. We observed that rates of nest predation were not influenced by parental activity at nests and parents could successfully feed nestlings in enlarged broods. Although larger broods produced the most fledged juveniles, these individuals were less likely to survive to dispersal than were juveniles that fledged from smaller broods. Consequently, nest productivity did not vary with clutch size. Post-fledging mortality was not related to nestling mass two to three days prior to fledging. Rather, differences in the allocation of parental investment per juvenile among differently sized broods appeared to influence juvenile survival probability. These results identify parental care during the post-fledging period as a potential key factor influencing the evolution of small clutch size in tropical birds.

Sex-specific energy requirements in nestlings of an extremely sexually size dimorphic bird, the European sparrowhawk (Accipiter nisus)

Allocation of parental investment is predicted to be equal at the population level between both sexes of offspring, and should lead to sex ratio biases in species that exhibit a sex-difference in parental care. Sex-differences in parental care are rarely quantified. We measured daily energy expenditure in free-living nestlings of the extremely sexually size dimorphic European sparrowhawk (Accipiter nisus), using the doubly labelled water method. These data were combined with measured growth characteristics to estimate daily and total metabolised energy intake of male and female young during the nestling stage. Females reached an asymptotic body mass 1.6 times higher than males. This resulted in a total metabolised energy an estimated 1.4 times higher for the nestling stage. Furthermore, we observed a decline in daily metabolised energy with an increase in brood size, which was significantly stronger for females than for males. These results are discussed in the context of Fisher's equal allocation theory. Empirical evidence of a sex ratio bias at the end of parental care, with an overall excess of males, is lacking in this species. Consequently, our data do not support the idea of equal allocation between the sexes. The observed sex difference in daily metabolised energy in response to brood size may give scope for sex ratio bias at the level of the individual brood.

Parent-Offspring Conflict in Primates

Parent‐offspring conflict (POC) theory (Trivers, 1974) has stimulated controversy in evolutionary biology and behavioral ecology. The theory has been criticized by some primate behavioral researchers on both conceptual and empirical grounds. First, it has been argued that it would be more advantageous to mothers and offspring to agree over the allocation of parental investment and to cooperate rather than to disagree and engage in conflict. Second, some studies have provided data suggesting that primate mothers and offspring engage in behavioral conflict over the scheduling of their activities rather than parental investment. In reality, parent‐offspring interactions are likely to involve both cooperation and conflict, and the hypothesis that mothers and infants squabble over the scheduling of their activities is not incompatible with POC theory. Furthermore, the predictions of POC theory are supported by a number of empirical studies of primates. POC theory has enhanced our understanding of the dynamics of parent‐offspring relationships in many animal species, and it is very likely that future studies of primates will continue to benefit from using POC theory as an explanatory framework.

Antipredator defense as a limited resource: unequal predation risk in broods of an insect with maternal care

The allocation of parental investment is a potential source of conflict within broods whenever offspring are able obtain differential access to the parental resource. Unlike the provisioning of food, parental antipredator behavior is usually considered a resource that benefits all offspring simultaneously. In the thornbug treehopper (Umbonia crassicornis), offspring form aggregations in exposed positions on host-plant stems. They are subject to intense predation, and maternal defense is their primary means of protection. I examined the distribution of risk within these offspring groups, using natural variation in the outcome of more than 500 predation attempts (324 recorded on videotape) by vespid wasps (Pseudopolybia compressa )o n 18U. crassicornis aggregations. I found three influences on an individual offspring’s risk of predation. The first was the presence of a defending female: as expected, offspring were much more likely to survive contact with a wasp if the female was present than if the female had disappeared. The second influence was position relative to other offspring: when wasps were successful in removing an individual, they almost always removed it from the edge of the group. The third influence was distance from the female: the closer an offspring was to the female at the time it was contacted by a wasp, the higher its likelihood of survival. The distribution of risk is determined largely by the behavior of defending females and the prey-searching behavior of wasps. The nature of risk within these aggregations sets the stage for two forms of sibling rivalry: selfish herd behavior and competition for access to maternal defense. It also raises the question of how a parent should allocate defense among offspring when it is unable to defend them all simultaneously. Key words: parental care, predation, Pseudopolybia compressa, sibling rivalry, Umbonia crassicornis. [Behav Ecol 13:125–133 (2002)]

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 this 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.

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 this 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.

An Experimental Study of Parental Investment and Polygyny in Male Blackbirds

Despite experimental reduction of the value of parental effort, male Brewer's blackbirds did not reallocate resources into mating effort; they did not attempt to mate polygynously. Although it was selectively advantageous for members of this population to do so, apparently no strong environmental potential for polygyny existed. Male yellow-headed blackbirds, when faced with a reduction of the marginal value of their 10 broods, reallocated investment toward their 20 broods but would not redirect feeding effort toward their 30 broods. Allocation of parental investment by these yellowheads was based on clutch size, nestling age and possible nestling sex.

Reproductive Value and Rank Relations Among Macaque Sisters

The ontogeny of social dominance relations among female macaques follows a highly predictable sequence. Females rank just below their mothers. By the time a female attains 4 yr of age, she surpasses older sisters in rank. Thus, as adults, rank among sisters is inversely correlated with age. Such regularity cannot be attributed to phenotypic differences in individual prowess. We believe that asymmetries in the network of agonistic support confer rank and that the inverse correlation of rank with age among adult sisters may in part be a consequence of mothers favoring daughters with the highest reproductive value which are always the youngest among adult daughters. Reproductive values are computed from a macaque life table and the predictions of our model are discussed with respect to existing data on the allocation of parental investment and the generated reproductive value curve.