Mechanisms of homing in the fiddler crab Uca rapax. 2. Information sources and frame of reference for a path integration system

Fiddler crabs Uca rapax are central-place foragers, making feeding excursions of up to several meters from their burrows. This study investigates the sources of directional and distance information used by these crabs when returning to their burrows. We tested the spatial frame of reference (egocentric or exocentric), and the source of spatial information (idiothetic or allothetic) used during homing. We also tested which components of their locomotion they integrated (only voluntary, or voluntary plus reflexive). Fiddler crabs in their natural mudflat habitat were passively rotated during normal foraging behavior using experimenter-controlled disks, before they returned home. Crabs resisted passive rotations on the disk by counter-rotating when the disk turned, which was a compensatory response to unintended movement. Crabs were usually situated eccentrically on the disk, and therefore were also subjected to a translation when the disk rotated. No crab actively compensated for this translation. Crabs that fully compensated for disk rotation made no directional homing error. Crabs that did not fully compensate homed in a direction that reflected their new body orientation. In other words, if we succeeded in reorienting a crab (i.e. it undercompensated for disk rotation), its homing error was equal to the angle by which it had been reoriented, regardless of the magnitude of the optomotor compensation. Computer-modelled crabs, each equipped with a path integrator utilizing different combinations of external (allothetic) and path-related (idiothetic) input, traversed the digitized paths of the real crabs. The home vector computed by the model crab was then compared to the homing direction observed in the real crab. The model home vector that most closely matched that of the real crab was taken to comprise the path integration mechanism employed by fiddler crabs. The model that best matched the real crab gained direction and distance idiothetically (from internal sources such as proprioceptors), and integrated only voluntary locomotory information. Crabs were also made to run home across a patch of wet acetate, on which they slipped and were thus forced to take more steps on the homeward path than theoretically required by the home vector. Crabs whose running velocity across the patch was unusually low also stopped short of their burrow before finding it. Crabs whose running velocity was not impeded by the patch did not stop short, but ran straight to the burrow entrance, as did control crabs that ran home with no slippery patch. We interpret this to mean that the velocity of some crabs was impeded because of slipping, and these therefore stopped short of their burrow after having run out their homing vector. This is positive evidence in support of the hypothesis that path integration is mediated either by leg proprioceptors or by efferent commands, but our data do not allow us to distinguish between these two possibilities.

Mechanisms of homing in the fiddler crab Uca rapax. 1. Spatial and temporal characteristics of a system of small-scale navigation

Fiddler crabs Uca rapax are central-place foragers, making feeding excursions of up to 2 m from their burrows. We describe the natural feeding excursions of path-integrating fiddler crabs and analyze their paths for signs of significant systematic or random navigation errors. No signs of any systematic errors are evident. Random errors are small, probably due to a combination of the short length and low sinuosity of the foraging paths, as well as the fiddler crabs' unique method of locomotion that allows them to remain oriented to their burrows throughout the foraging path and to minimize large body turns. We further examined the extent to which their body orientation during foraging (transverse body axis pointing more or less towards home) accurately represented their stored home vector. By examining sequences of fast escape, we have shown that crabs can correct for deviations of their transverse body axis from home during their escape path. Thus their stored home vector is independent of their moment-to-moment body orientation. Crabs were subjected to passive translational displacements and barrier obstructions. Responses to translational displacements were identical to those observed by previous authors, namely that crabs returned in the correct egocentric direction and distance as though no displacement had occurred. Covering the burrow entrance resulted in crabs returning to the correct position of the burrow, and then beginning to search. When a barrier was placed between foraging crabs and their burrow, crabs oriented their bodies toward the burrow as accurately as with no barrier.

Assessments of the effects of nutrition on mental development and behaviour in Jamaican studies

Methodological issues and problems in studies of the effects of protein-energy malnutrition (PEM) and child development are discussed with reference to our experience in Jamaica. The usefulness of defining PEM by stunting, wasting, and edema is stressed. Difficulties in achieving reliable and valid measurements of mental development and behavior in third world children, and allowing for the confounding and possibly interacting effects of social background, are discussed. Problems associated with different approaches are outlined. (AU)

A longitudinal study of the mental development of severely malnourished children who received psychosocial stimulation - abstract

Children who suffer from severe malnutrition in early childhood have lower levels of school achievement and intelligence quotients, and more behavioural problems than their peers or siblings for many years. Few studies have attempted intervention, and these have been short-term and had only transient benefits. We previously reported a programme of two years intervention, which had marked benefits. We now report the childrens' progress over a third year of intervention and three years following its cessation. The development of 16 children , who were in hospital with severe malnutrition and participated in a home visiting programme of psychosocial stimulation, was compared with that of two other groups who were also in hospital but received standard medical care only; another severely malnourished group (n=18) and an adequately nourished one (n = 20). All groups were assessed regularly on the Griffiths Mental Development Scales and then the Stanford Binet tests. Both malnourished groups were markedly behind the adequately nourished group on admission to hospital and the non-intervened group showed little signs of catching up. The intervened group caught up to the adequately nourished group in two years and showed no further improvement in the third year of intervention. Over the following three years, they showed a decline in three of the five Griffiths subscales. They, however, retained a marked advantage over the non-intervened malnourished group on the Standard Binet test to the end of follow-up, showing no further decline in the last year. Both malnourished groups failed to catch up to the adequately nourished group in height. It was concluded that relatively simple intervention can benefit the development of severely malnourished children (AU)

A new look at the asssessment of mental development in young children recovering from severe malnutrition

Previous studies of malnourished children is hospital have shown that their developmental quotients (DQs) improve as their nutritional status improves, but they have not taken into account the possible effects of being in hospital and of test practice. The present study assessed development of malnourished and adequately nourished children in hospital and found that mean DQs of each group rose to a similar extent during recovery from illness. It is concluded that DQs on admission to hospital were lowered by the children's discomfort and unhappiness at being in hospital, and/or by illness, and that rises in DQ shown by children recovering from malnutrition are not necessarily attributable to improving nutritional status. (AU)