Most research on ontogenetic niche shifts has focused on changes in habitat or resource use related to food resource distribution and heterospecific size‐limited predation. Cannibalism, an intraspecific interaction, can also affect habitat selection or resource use by vulnerable size classes. Morphological defenses, such as spines, increase the effective size of an individual, making it more difficult to consume. The importance of such defense structures in affecting niche shifts in early life history stages is unclear. Using a combination of field observations and experiments in aquaria and wading pools, we examined the relative roles of cannibalism and morphology in determining juvenile habitat use in two populations of threespine stickleback that differ in pelvic spine morphology. Juveniles were categorized into three size classes: small (5–10 mm), medium (11–15 mm), and large (15–25 mm). In experiments assessing the relative vulnerability of juveniles to cannibalism by adults, we documented a significant difference among size classes in the number of juveniles eaten such that more large juveniles were eaten from the population lacking pelvic spines. The natural distribution of small and large juveniles in two distinct littoral microhabitats, open water and vegetation, was determined in each lake. In both populations, small juveniles were more abundant in vegetation. In the population with pelvic spines, a greater proportion of large juveniles was observed in open water than in vegetation. In the population without pelvic spines, the proportion of large juveniles did not differ between the two habitats. Experiments comparing juvenile habitat use in the presence or absence of adult conspecifics suggest that differences in habitat use may not only depend on the size of the individual, or the size of the individual relative to the size of the adult predator, but also on the degree of development or expression of defensive structures.

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ABSTRACT.-Over 50 species of Neotropical birds have been recorded foraging for animal prey in bromeliads. Of these bird species, Pseudocolaptes lawrencii is one of the most specialized. At a montane rainforest site in Costa Rica, 74% of its documented foraging efforts were in epiphytic bromeliads. P. lawrencii selected large bromeliads and foraged for arthropods within leaf litter and organic debris trapped in the plants. Based on our analyses of the bromeliad prey base and bird stomach contents, P. lawrencii was an opportunistic predator of the litter-inhabiting arthropods. Birds consumed dermapterans, orthopterans, arachnids, and coleopterans in proportions equal to the prey's availability and did not select for prey size. However, P. lawrencii avoided isopods. P. lawrencii did not consume aquatic insect larvae, which were the largest component of the bromeliad prey base and occurred in 80% of bromeliads sampled.RESUMEN.-Mas de 50 especies de aves Neotropicales han sido estudiadas mientras forrajean por animates en bromelias. De estas especies de aves, Pseudocolaptes lawrencii es una de las mas especializadas. En un bosque tropical montanoso en Costa Rica, se encontro que el 74% de los atentos de forrajeo fueron en bromelias epifiticas. Pseudocolaptes lawrencii selecciono bromelias grandes y forrajeo en las hojas y los escombros organicos atrapados en las bromelias. Basado en nuestro analisis de los animates encontrados en las bromelias y el contenido estomacal de las aves, concluimos que P. lawrencii es un predador oportunistico de los artropodos que viven en los escombros organicos. Las aves consumieron dermapteros, orthopteros, aracnidos y coleopteros en proporciones identicas a la disponibilidad de estas presas. No se encontro seleccion de presas, sin embargo, P. lawrencii evito isopodos. No consumi6 larvas de insectos acuaticos, la presa mas abundante y se encontrd en un 80% de las bromelias estudiadas.Among the masses of epiphytes that give Neotropical montane forests their "fantastic appearance" (Slud 1964:205), bromeliads are often the most conspicuous plants. Bromeliads increase the structural complexity of forests and create additional microhabitats for birds and their animal prey. Indeed, a diverse fauna exists within the impounded water and detritus of tank bromeliads (e.g., Picado 1911, Pittendrigh 1948, Laessle 1961, Diesel 1989, Paoletti et al. 1991, consisting of two primary components: animals living within the aquatic medium (e.g. dipteran larvae, frogs) and animals typically associated with soil and organic debris (e.g., earwigs [Dermaptera], roaches [Orthoptera], isopods). Thus, bromeliads can enhance opportunities for resource subdivision and specialization by birds in Neotropical forests. Foraging specialization on unique tropical resources, such as bromeliads, is thought to be one mechanism responsible for the high bird species diversity of the Neotropics relative to the Temperate Zone (Schoener 1968;Orians 1969;Karr 1971;Terborgh 1980;Remsen 1985). At least 51 Neotropical bird species h...

In a population of the common freshwater copepod Leptodiaptomus minutus Lilljeborg, 7.3% of adult females exhibited male secondary sexual characteristics. Although these masculinized or intersex females had the internal morphology of normal females, they possessed one or rarely two normal geniculate male antennules, and in a few instances male fifth legs. The sex ratio of normal adult males to females in the population was 1:1. No differences in body size among normal males, normal females, and masculinized females were found. Antennule lengths of males and masculinized females were not significantly different from one another; however, normal females had longer antennules than either males or masculinized females. Mating experiments indicated that masculinized females were functionally female and they were able to mate with normal males to produce offspring that were normal or masculinized. The occurrence of intersex copepods in lakes of North America may be more widespread than we now recognize because they can be easily overlooked or mistaken for normal adults.

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