The social use of space : aspects of ecology, ethology and endocrinology in the ghost crabs Ocypode ceratophthalmus (Pallas) and Ocypode laevis Dana Ecology, ethology and endocrinology in the ghost crabs

Date
1977
Authors
Lighter, Frederick John
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Abstract
This study concerns the social use of space in two species of ghost crabs, Ocypode ceratophthalmus (Pallas) and Ocypode laevis Dana. The work involves four major aspects: the spatial patterns of the burrows of both species in nature, the space-related behaviors of the two species, a test of a mathematical model of social interaction for O. ceratophthalmus, and the influence of the androgenic gland on burrowing behavior and growth in O. ceratophthalmus. To determine the natural spatial patterns of the two species, nearest neighbor, 'mean crowding' and 'patchiness' measurements are made for thirteen populations of O. ceratophthalmus and seven populations of O. laevis. Measurements of density, 'mean crowding I and 'patchiness' fail to show a close association with dispersion pattern as measured by nonparametric regressions. Rank correlations demonstrate no significant association between size of individual and distance to nearest neighbor. All individual O. laevis interact to produce a total population dispersion pattern. In contrast, the reproductively active male O. ceratophthalmus, through behavioral competition for space, exert the major influence in determining population dispersion. Three categories of space-related behaviors are observed in the field and laboratory: burrow defense, defense of an area surrounding the burrow, and defense of individual distance. Nine modal action patterns are utilized by O. ceratophthalmus in its space-related behavioral repertoire; six are found in O. laevis. O. ceratophthalmus defends space in order to guarantee ownership of a burrow, to reduce competitive social stimulation between neighbors, and to provide for an area for sexual display. O. laevis defends space in order to provide ownership of a burrow and to reduce competitive social stimulation between neighbors. Crowding experiments with O. ceratophthalmus show that, as density increases, defensive behavior shifts from burrow and area defense to the maintenance of individual distance. Work with models demonstrates that the presence of an adult male within a defended area elicits Threat by a resident reproductively-active male O. ceratophthalmus. An intruder which exhibits Threat elicits a higher intensity response from the resident than a nonthreat intruder. Calhoun's (1957, 1963) mathematical model of social interaction is tested on field and laboratory data for O. ceratophthalmus. The data do not follow the prediction of the model that the frequency of social contacts per unit time increases as group size increases. Rather, the data indicate that the relationship is bimodal, with peak frequencies at 2.5 and 4.5 individuals/m^2. As an alternative to Calhoun's model, I conclude that increased density produces a period of intensified social competition for space. After this period, O. ceratophthalmus enters an "extended refractory period," which is reflected in periods of low frequency of social activity. The influence of the androgenic gland on burrowing behavior and growth of O. ceratophthalmus is investigated by implantation and removal experiments. No obvious differences in the growth of carapace width, pollex length, abdomen length and abdomen width are found between implanted individuals, individuals with the androgenic gland gland removed, and sham-operated control animals. Changes in burrowing behavior are found in implanted juvenile females and an implanted juvenile male. In these individuals, burrowing behavior is changed toward that of an adult, reproductively-active male. Evidence suggests that androgenic gland secretory activity, and thus male reproductive activity, is synchronized within populations to coincide with the period of the full moon, when time of burrow exposure is greatest. Implanted juvenile females show a masculinization in the value of the abdomen width/ carapace width ratio, while implanted females do not. For females, a critical period exists beyond which androgenic gland implantation no longer produces secondary male sexual characteristics.
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Typescript.
Bibliography: leaves 142-149.
Microfiche.
xi, 149 leaves ill
Keywords
Ghost crabs, Crabs -- Geographical distribution
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