John H. Carothers

Time as a Niche Difference: The Role of Interference Competition

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Ecological, Morphological, and Bioenergetic Correlates of Hunting Mode in Hawks and Owls

We examined four correlates of hunting mode, dichotomized as active search (AS) and sit-and-wait (SW), in five assemblages of sympatric hawks and owls. Examining twenty-four hawk and ten owl species we found that: (1) Linearized wing loadings are light for kites, harriers, and eagles (which are primarily AS), and heavy for buteonines, falcons, and accipiters (which are mixed-mode hunters using both AS and SW techniques). Primarily AS-owls (mainly strigines and tytonines) have light linearized wing loadings, whereas SW-owls (mainly bubonines) are more heavily loaded. AShawks do not differ significantly from AS-owls, nor do SW-hawks from SW-owls; (2) Among hawks, diet breadth increases from accipiters through harriers, to falcons, eagles, buteonines, and to kites; and among owls, from strigines through bubonines to tytonines. No significant differences in diet breadth are detected between AS and SW hawks (or owls), nor between single-mode and mixed-mode hawks (or owls); (3) Among hawks, the ratio prey mass/body mass (PM/BM) increases from harriers through falcons to buteonines, accipiters, eagles, and to kites, and from tytonines through strigines to bubonines, among owls. No significant differences in the ratios PM/BM are found between AS and SW hawks (or owls); (4) Energetic costs incurred per prey capture are greater with the AS than with the SW mode in three out of four cases analyzed, but information on energetic gains are available for only one. Here, AS rendered a larger difference between benefit and cost than did SW; in the other three cases that differential was not known, but several advantages associated to AS were claimed to offset its high energetic cost. Our main conclusions are: (1) Calculation of linearized wing loading of a hawk (or owl) enables prediction of its main hunting mode; (2) Although use of AS or SW techniques (singly or in combination) seems not to constrain access of raptors to prey resources (in terms of diversity and size), further studies focusing on single subfamilies are likely to detect differences in prey use by raptors using different techniques; (3) Little can be predicted about the extent of hunting modes used by different raptors, until the associated energetic gains are evaluated.

Dominance and competition in an herbivorous lizard

SummaryA study of social organization in an herbivorous lizard Ctenosaura hemilopha investigated the role of dominance in group member behavior. Attention focused on a very populous (16 ind.) colony to examine causes and effects of crowding with respect to competitive factors. Lizards were colonial, with a top-rank male, one or more adult females, and various subadults and juveniles. Top-rank males defended harems, with colonies female-biased (1:4 in focal colony). Dominance hierarchy was observed, with females more aggressive than males, except for the top-rank male. Group awareness facilitated adaptive responses to threatening dominants and predators. Strong correlations existed among individual size, rank, and aggression. Top-rank male aggressiveness was partly explained by harem defense. Food resource competition, which causes aggression in female insectivorous lizards, did not explain female C. hemilopha behavior. Response to predators, predator fecal pellet analysis, and tail break frequencies implicate crevice escape sites for predator avoidance as a prime controller of social and population structure in these lizards.

Size-related activity patterns in an herbivorous lizard

A study of the pre-breeding season behavior of a colony (N=16) of Ctenosaura hemilopha revealed significant differences among the four size classes with respect to time of activity. Comparisons of lizard size classes showed that the smaller lizards emerged earlier, had earlier activity periods, and usually fed earlier than the larger lizards (all P<0.01). The two smaller sizes were also aggressive earlier in the day than the two larger classes (P<001).Members of all size classes ate the same food items, but food competition was absent. Thus, the differences in activity patterns among the size classes are not due to avoidance of food competition, an explanation which has been invoked for insectivorous lizards. These patterns probably result from the fact that thermal inertia increases with body size. Differences in activity patterns of insectivorous species can also originate from thermoregulatory constraints, and could provide the variation upon which selection for reduced competition may have acted to increase separation along the time axis of niche space.

Altitudinal zonation among lizards of the genus Liolaemus: questions answered and unanswered questions

Revisamos los factores que influyen sobre las distribuciones de Liolaemus en los Andes del centro de Chile y sugerimos futuras avenidas de investigacion. Nuestros estudios previos revelan que los parasitos (garrapatas y acaros ectoparasitos y el endoparasito Plasmodium) no influyen sobre los limites altitudinales de Liolaemus. Las tolerancias termicas tampoco parecen limitar dichas distribuciones altitudinales, aunque las temperaturas ambientales frias determinan que solo las especies viviparas puedan sobrevivir arriba de los 2.400 m de altura. Tres especies de Liolaemus se especializan en microhabitats de distribucion restringida en el gradiente altitudinal. Liolaemus tenuis se encuentra exclusivamente en la base de arboles, los cuales se distribuyen tipicamente debajo de los 1.800 m. Liolaemus leopardinus se especializa en rocas grandes que se encuentran solo en altitudes mayores. Liolaemus monticola usa rocas pequenas: la oviparidad determina su limite distribucional superior en dos de los tres transectos examinados, y en el tercero esta especie y su habitat rocoso virtualmente desaparecen arriba de los 1.500 m. La competencia interespecifica entre Liolaemus aparentemente no se relaciona con su distribucion altitudinal. No encontramos ejemplos de distribuciones parapatricas entre pares de especies de Liolaemus con similares requerimientos de nicho. Otros investigadores han encontrado que la depredacion no se correlaciona con la altitud: su papel en determinar las distribuciones altitudinales de especies depende tanto de las densidades de predadores como de las presas. Concluimos que los factores determinantes de los limites de distribucion de las especies de Liolaemus varian dependiendo de sus circunstancias individuales en el tiempo y el espacio: si bien la fisiologia puede ser relevante en un transecto, la disponibilidad del microhabitat preferido puede ser importante en otro

Vocalization as a Response to Capture in the Central Chilean Lizard Liolaemus chiliensis (Tropiduridae)

Among 11 Liolaemus lizard species examined in central and south-central Chile only L. chiliensis responded to capture and handling by vocalizing. Vocalizations sounded more like a squeak than a hiss, and volume was sufficient to be heard at a distance of ~3 m. The vocalization is probably produced by the larynx and may be an antipredator response. Similarities of L. chiliensis to the anguid lizard Elgaria multicarinata in California are discussed.

Parasite loads and altitudinal distribution of Liolaemus lizards in the central Chilean Andes

Este estudio compara las distribuciones de diez especies de lagartijas Liolaemus en los Andes de Chile central, con las distribuciones de cuatro tipos de parasitos: Plasmodium causantes de malaria, nematodos intestinales, garrapatas y acaros. Quisimos verificar si los numeros de parasitos pudieran ser un factor determinante de los limites de distribucion de las lagartijas. Encontramos que no habia evidencia de infestacion por malaria en las lagartijas; que las garrapatas estaban casi ausentes; que los numeros de acaros mas a menudo decrecian que aumentaban en los limites distribucionales de las lagartijas; y que los nematodos intestinales confinados a las lagartijas herbivoras en nuestra muestra bien podian ser beneficiosos antes que daninos. Mas bien que parasitismo, otras interacciones bioticas (e.g., depredacion o competencia) son candidatos mas probables como factores que influyen las distribuciones altitudinales de las lagartijas, ademas de caracteristicas abioticas tales como la disponibilidad de microhabitats y los factores termicos