RESUMEN

Avian brood parasites depend on other species, the hosts, to raise their offspring. During the breeding season, parasitic cowbirds (Molothrus sp.) search for potential host nests to which they return for laying a few days after first locating them. Parasitic cowbirds have a larger hippocampus/telencephalon volume than non-parasitic species; this volume is larger in the sex involved in nest searching (females) and it is also larger in the breeding than in the non-breeding season. In nature, female shiny cowbirds Molothrus bonariensis search for nests without the male's assistance. Here we test whether, in association with these neuroanatomical and behavioural differences, shiny cowbirds display sexual differences in a memory task in the laboratory. We used a task consisting of finding food whose location was indicated either by the appearance or the location of a covering disk. Females learnt to retrieve food faster than males when food was associated with appearance cues, but we found no sexual differences when food was associated with a specific location. Our results are consistent with the view that parasitism and its neuroanatomical correlates affect performance in memory tasks, but the effects we found were not in the expected direction, emphasising that the nature of avian hippocampal function and its sexual differences are not yet understood.

RESUMEN

To test the hypothesis that selection for spatial abilities which require birds to locate and to return accurately to host nests has produced an enlarged hippocampus in brood parasites, three species of cowbird were compared. In shiny cowbirds, females search for host nests without the assistance of the male; in screaming cowbirds, males and females inspect hosts' nests together; in bay-winged cowbirds, neither sex searches because this species is not a brood parasite. As predicted, the two parasitic species had a relatively larger hippocampus than the non-parasitic species. There were no sex differences in relative hippocampus size in screaming or bay-winged cowbirds, but female shiny cowbirds had a larger hippocampus than the male.

Asunto(s)

RESUMEN

We studied the foraging behaviour of guinea pigs (Cavia porcellus) in laboratory environments with a single patch type. Six experiments were designed to test predictions of the marginal value theorem (MVT) for various foraging problems in a constant physical setting. In Part one (experiments I, I', and II) we used patches with resource depression. In experiment I there were two treatments, which differed in the function relating cumulative food gain to time in the patch. Experiment I' was a replicate of experiment I with greater differences in patch quality between treatments. In experiment II the treatments differed in the travel requirement between patches, while the patch gain function remained the same throughout. In experiments III, IV and V (Part two) there were patches with linear gain function and sudden exhaustion, and two treatments in each experiment. The treatments differed in prey encounter rate, maximal number of prey per patch, and travel time, respectively. In the six experiments the MVT predictions for prey per patch visit were qualitatively supported by the experimental results, and in most cases the quantitative fit was also good. Giving up times were longer than predicted. We conclude that the hypothesis of rate maximization, in spite of failing to predict some aspects of the results, provides a suitable framework for examining the foraging behaviour of this species.

RESUMEN

In nineteen rats the cardiovascular responses to the electrical stimulation of the lateral mesencephalic reticular formation (MRF) were recorded before, during and after different doses of sodium pentobarbital injected intravenously. Before pentobarbital, stimulation of the MRF induced cardiovascular changes in 100% of cases. The following four patterns were observed: a) hypertension plus bradycardia (N = 11; 57.9%); b) hypertension without chronotropic effects (N = 4; 21%); c) hypertension plus tachycardia (N = 3; 15.8%), and d) hypotension without chronotropic effects (N = 1;5.3%). Administration of small doses of sodium pentobarbital induced drastic variations in the incidence of the aforementioned patterns of response as follows: a) N = 3; 15.8%; b) N = 2; 10.4%; c) N = 11; 58%; d) N = 0. Other two complementary patterns appeared: e) no changes in blood pressure plus bradycardia N = 1; 5.2%, and f) lack of response: N = 2; 10.4%. When the cumulative doses were over than 19 mg/kg no systematization was possible due to the extreme variability of the responses. This change in the characteristics of the cardiovascular effects of the stimulation of the MRF is supposed to be due to a demasking effect of barbiturates which depressing reticular neurons allows a cardioacceleratory component of the response to become apparent.

Asunto(s)

RESUMEN

In nineteen rats the cardiovascular responses to the electrical stimulation of the lateral mesencephalic reticular formation (MRF) were recorded before, during and after different doses of sodium pentobarbital injected intravenously. Before pentobarbital, stimulation of the MRF induced cardiovascular changes in 100

), and d) hypotension without chronotropic effects (N = 1;5.3

). Administration of small doses of sodium pentobarbital induced drastic variations in the incidence of the aforementioned patterns of response as follows: a) N = 3; 15.8

; d) N = 0. Other two complementary patterns appeared: e) no changes in blood pressure plus bradycardia N = 1; 5.2

, and f) lack of response: N = 2; 10.4

. When the cumulative doses were over than 19 mg/kg no systematization was possible due to the extreme variability of the responses. This change in the characteristics of the cardiovascular effects of the stimulation of the MRF is supposed to be due to a demasking effect of barbiturates which depressing reticular neurons allows a cardioacceleratory component of the response to become apparent.

RESUMEN

In nineteen rats the cardiovascular responses to the electrical stimulation of the lateral mesencephalic reticular formation (MRF) were recorded before, during and after different doses of sodium pentobarbital injected intravenously. Before pentobarbital, stimulation of the MRF induced cardiovascular changes in 100

of cases. The following four patterns were observed: a) hypertension plus bradycardia (N = 11; 57.9

); b) hypertension without chronotropic effects (N = 4; 21

); c) hypertension plus tachycardia (N = 3; 15.8

), and d) hypotension without chronotropic effects (N = 1;5.3

). Administration of small doses of sodium pentobarbital induced drastic variations in the incidence of the aforementioned patterns of response as follows: a) N = 3; 15.8

; b) N = 2; 10.4

; c) N = 11; 58

; d) N = 0. Other two complementary patterns appeared: e) no changes in blood pressure plus bradycardia N = 1; 5.2

, and f) lack of response: N = 2; 10.4

. When the cumulative doses were over than 19 mg/kg no systematization was possible due to the extreme variability of the responses. This change in the characteristics of the cardiovascular effects of the stimulation of the MRF is supposed to be due to a demasking effect of barbiturates which depressing reticular neurons allows a cardioacceleratory component of the response to become apparent.