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Gonadal development and mode of sexuality in a coral-reef damselfish, Dascyllus trimaculatus
- Kazue Asoh, Maiko Kasuya
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- Journal:
- Journal of Zoology / Volume 256 / Issue 3 / March 2002
- Published online by Cambridge University Press:
- 14 February 2002, pp. 301-309
- Print publication:
- March 2002
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The process of early gonadal development and the mode of sexuality were examined in the three-spot damselfish Dascyllus trimaculatus. All gonads developed an ovarian lumen and primary-growth-stage oocytes after an initially undifferentiated state. From this ovarian state or from more developed ovaries, some gonads redifferentiated into testes. None of 130 individuals examined had a gonad containing degenerating vitellogenic oocytes and proliferating spermatogenic tissue. Nineteen individuals, however, had a gonad containing degenerating cortical–alveolus stage oocytes and proliferating spermatogenic tissue. The size of these individuals overlapped with the female size range in which most females were still in the middle of the maturation process or the smallest female size class in which most females had vitellogenic oocytes. This suggests that the transition toward maleness in these individuals is likely to have occurred after oocytes in their gonads attained cortical–alveolus stage, but before their final maturation and spawning as females, and that the protogynous pattern of gonadal development in D. trimaculatus is non-functional. The observed protogynous pattern seems to have been present in the common ancestor of the genus and have been retained in this descendant species.
Adaptive explanation in socio-ecology: lessons from the Equidae
- WAYNE L. LINKLATER
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- Journal:
- Biological Reviews / Volume 75 / Issue 1 / February 2000
- Published online by Cambridge University Press:
- 01 February 2000, pp. 1-20
- Print publication:
- February 2000
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Socio-ecological explanations for intra- and interspecific variation in the social and spatial organization of animals predominate in the scientific literature. The socio-ecological model, developed first for the Bovidae and Cervidae, is commonly applied more widely to other groups including the Equidae. Intraspecific comparisons are particularly valuable because they allow the role of environment and demography on social and spatial organization to be understood while controlling for phylogeny or morphology which confound interspecific comparisons. Feral horse (Equus caballus Linnaeus 1758) populations with different demography inhabit a range of environments throughout the world. I use 56 reports to obtain 23 measures or characteristics of the behaviour and the social and spatial organization of 19 feral horse populations in which the environment, demography, management, research effort and sample size are also described. Comparison shows that different populations had remarkably similar social and spatial organization and that group sizes and composition, and home range sizes varied as much within as between populations. I assess the few exceptions to uniformity and conclude that they are due to the attributes of the studies themselves, particularly to poor definition of terms and inadequate empiricism, rather than to the environment or demography per se. Interspecific comparisons show that equid species adhere to their different social and spatial organizations despite similarities in their environments and even when species are sympatric. Furthermore, equid male territoriality has been ill-defined in previous studies, observations presented as evidence of territoriality are also found in non-territorial equids, and populations of supposedly territorial species demonstrate female defence polygyny. Thus, territoriality may not be a useful categorization in the Equidae. Moreover, although equid socio-ecologists have relied on the socio-ecological model derived from the extremely diverse Bovidae and Cervidae for explanations of variation in equine society, the homomorphic, but large and polygynous, and monogeneric Equidae do not support previous socio-ecological explanations for relationships between body size, mating system and sexual dimorphism in ungulates. Consequently, in spite of the efforts of numerous authors during the past two decades, functional explanations of apparent differences in feral horse and equid social and spatial organization and behaviour based on assumptions of their current utility in the environmental or demographic context remain unconvincing. Nevertheless, differences in social cohesion between species that are insensitive to intra- and interspecific variation in habitat and predation pressure warrant explanation. Thus, I propose alternative avenues of inquiry including testing for species-specific differences in inter-individual aggression and investigating the role of phylogenetic constraints in equine society. The Equidae are evidence of the relative importance of phylogeny and biological structure, and unimportance of the present-day environment, in animal behaviour and social and spatial organization.