[Show abstract][Hide abstract] ABSTRACT: The energy currencies used by foraging animals are expected to relate to the energy costs and benefits of resource collection. However, actual costs of foraging are rarely measured. We measured the ratio of energetic benefit relative to cost (B/C) during foraging for the giant tropical ant, Paraponera clavata. The B/C ratio was 3.9 for nectar-foragers and 67 for insect prey foragers. In contrast, the B/C ratio during foraging for seed harvester ants (Pogonomyrmex occidentalis) is over 1000, demonstrating that the B/C ratio can vary widely among ants. The B/C ratio was 300 times lower for nectar-foraging Paraponera than for the seed-harvesting Pogonomyrmex because of: (1) a 5-fold lower energetic benefit per trip, (2) a 10-fold greater cost due to longer foraging distances, and (3) a 6-fold greater energy cost per meter due to larger body size. For Paraponera daily colonial energy intake rates are similar to expeditures and may limit colony growth and reproduction. In contrast, for Pogonomyrmex energy intake rates are an order of magnitude higher than estimated costs, suggesting that Pogonomyrmex colonies are unlikely to be limited by short-term energy intake. We suggest that variation in individual B/C ratios may explain why the foraging behavior of Paraponera but not Pogonomyrmex appears sensitive to foraging costs.
[Show abstract][Hide abstract] ABSTRACT: Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting
pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious
plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility,
or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically
be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual
specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy
(males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization
may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant
Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves
more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent
on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability.
Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or
nonexistent—the reproductive mode recognized as dioecy.
Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates,
sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a
particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode
of pollination (insector wind-pollinated) and other ecological factors.
The Botanical Review 12/1996; 63(1):65-92. · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The flowers of Justicia aurea, morphologically characteristic of ornithophily, attracted a diverse array of foragers where they occurred as a dense stand in the tropical forests at La Selva, Costa Rica, and so provided an arena for this study of competition and coexistence. Two hummingbird species (Phaethornis superciliosus and Campylopterus hemileucurus) visited the flowers legally early in the morning, and defended the nectar resource; a third smaller bird (P. longuemareus) foraged for nectar throughout the day but collected it illegally by piercing the corollas. In addition, nectar was harvested illegally by four species of stingless bee (Trigona) and by ants, creating a further drain on the limited floral resources.Consideration of the diurnal patterns of foraging activities in combination with a spatial axis (defined here in terms of microclimate and insolation) nevertheless showed a good separation of flower visits for the different nectarivores. Hummingbirds visited flowers in zones where the reward was highest, while insects foraged to minimise their energetic costs; each of these factors could in turn be related to microclimatic considerations. A comprehensive scheme of resource utilisation could therefore be extracted from the field observations and interpreted in these terms. The limited area of niche overlap thus revealed corresponded closely with the situations where direct interference competition was observed, between hummingbird species or between bees and aggressive ants.
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