Complex selection on life-history traits and the maintenance of variation in exaggerated rostrum length in acorn weevils

Bonal R., Espelta J.M., Vogler A.P. (2011) Complex selection on life-history traits and the maintenance of variation in exaggerated rostrum length in acorn weevils. Oecologia. 167: 1053-1061.
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Doi: 10.1007/s00442-011-2036-7

Resum:

Trophic interactions can trigger the development of exaggerated specialized characters and promote morphological diversification. For example, acorn weevils (genus Curculio) present strikingly long rostrums, which are used by females to perforate oviposition holes through the seed coat. Species exhibiting longer rostrums are known to exploit larger acorns, and therefore rostrum length is thought to be subject to selection to match the preferred acorn type. However, rostrum length is strongly correlated with body size, and morphological divergence could result from either selection on rostrum length for optimal food exploitation or from other pressures acting on body size. We collected infested acorns at oak forests where the large Curculio elephas and the small-bodied Curculio glandium co-occur. There were no interspecific differences in adult female body size to rostrum length allometric relationships, and rostrum length is equally correlated with body size in either species. MtDNA-based species identification showed that C. glandium larvae were present within acorns of all sizes, whereas C. elephas larvae were restricted to acorns above a minimum size, irrespective of oak species. Hence, exploitation of large acorns can hardly have triggered rostrum enlargement, as the small sized C. glandium adults (with short rostrums) could perforate and oviposit in both small and large acorns. Rather, increased rostrum length is probably a by-product of the larger body sizes of individuals emerging from bigger acorns, which allow increased larval size and enhance larval survival likelihood. Summarizing, when exaggerated feeding traits co-vary with other body features, interspecific morphological variability may result from contrasting selective pressures acting on these correlated characters. © 2011 Springer-Verlag.

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Male barn swallows use different signalling rules to produce ornamental tail feathers

Muñoz A., Aparicio J.M., Bonal R. (2011) Male barn swallows use different signalling rules to produce ornamental tail feathers. Evolutionary Ecology. 25: 1217-1230.
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Doi: 10.1007/s10682-011-9512-8

Resum:

The evolution of secondary sexual characters is the subject of controversial debate between those defending their role as 'viability indicators' and those arguing that ornaments are purely 'attractive traits' selected by females. Recent theoretical studies suggest that these hypotheses are not mutually exclusive, as both viability and attractiveness can contribute to improve the reproductive success of progeny and could thus simultaneously underlie female choices. If that is the case, strategies of cheaper advertisement, allowing the expression of larger ornaments for the same cost, could proliferate even in species in which honest signalling of viability prevails. Under this scenario, different males could invest a different amount of resources per ornament unit of expression, thus using different signalling rules. We studied the relationship between tail feather length (a trait that is the subject of a female mate preference) and feather mass (a measure of investment in feather production) in a barn swallow Hirundo rustica population. Different males used different and consistent signalling rules when developing ornamental feathers. That is, to produce a feather of a given length, each male used a constant amount of resources across different years, but this amount varied between males. Although the amount of material invested in feathers (feather mass) is a condition-dependent trait, the organization of this material in ornamental feathers (i. e. the signalling rules) was not. Neither survival nor risk of feather breakage was related to the signalling rules. Thus, these results suggest that both 'viability' and 'runaway' mechanisms are independent determinants of the evolution of ornamental sexual feathers in the barn swallow. A preference for long tails will ensure that females either obtain a sire with high viability, or one transferring the capability to produce longer and more attractive tails at a lower cost of production to its offspring. © 2011 Springer Science+Business Media B.V.

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Linking seed dispersal to cache protection strategies

Muñoz A., Bonal R. (2011) Linking seed dispersal to cache protection strategies. Journal of Ecology. 99: 1016-1025.
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Doi: 10.1111/j.1365-2745.2011.01818.x

Resum:

1.The spatial distribution of dispersed seeds results from the combined action of the caching strategies followed by different granivores. Hence, it is essential to study the factors that influence seed predation and caching decisions to achieve a better understanding of the dispersal process. 2.In this study, we document how seed dispersal and the spatial patterns of natural recruitment are linked to the strategies used by granivores to protect their cached seeds from pilferage. We present a theoretical model showing that those strategies may convey benefits for both seed cachers and plants. 3.We studied the relationships among seed production, seed predation/caching, cache pilferage and plant recruitment in a savanna-like landscape of oaks dispersed by scatter-hoarding rodents. 4.Our results show that acorn-dispersing rodents were concentrated under the canopies of scattered oaks, where the theft of cached acorns increased by 77% as compared to that of the surrounding open landscape. Acorns were thus cached selectively in the open areas to reduce pilferage; in fact, none of the few seeds cached beneath tree canopies survived predation by granivores (pilferage+recovery). Meanwhile, some acorns cached in the surrounding open areas were neither pilfered nor recovered and then recruited successfully. Accordingly, natural recruitment of newly emerged seedlings was higher outside than under canopies, suggesting that rodent caching strategies have direct implications for the directed dispersal of oaks. 5.Synthesis. The spatial patterns of seed dispersal shape the fitness of both the plant because they influence dispersal and recruitment efficiency, and the granivores that cache and predate its seeds because they influence their foraging efficiency. Cache protection strategies reduce pilferage significantly and enhance seed recovery rates by the cache owner. At the same time, more seeds remain dispersed and unrecovered. Thus, cache protection strategies can provide net benefits to the plant in terms of effective directed dispersal. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.

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