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Overcoming environmental and morphological constraints: egg size and pelvic kinesis in the smallest tortoise, Homopus signatus

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Abstract:

The small tortoises of southern Africa include the only testudinid taxa that produce single-egg clutches. This group includes the world's smallest tortoise, Homopus signatus (Gmelin, 1789), which inhabits a harsh, arid environment. Climate and body size may influence reproductive output, so we hypothesized that the east–west aridity gradient in southern Africa affects egg and clutch size of the small indigenous tortoises, and that the morphology of H. signatus constrains egg size, preventing the formation of optimal eggs. Here we show that aridity and unpredictable rainfall determine which of these tortoise taxa produce single-egg clutches. Taxa in less predictable environments produce larger eggs relative to body size than do taxa in more predictable environments. Homopus signatus produces the largest egg relative to body size, probably to enhance offspring survival in its harsh environment. Body size, pelvic aperture size, and the narrow anal gap of H. signatus appear to constrain egg size. Despite these constraints, females produce rigid-shelled eggs larger than the pelvic canal and use pelvic kinesis to pass eggs at oviposition; both features are unknown in other chelonians and emphasize the selective advantage of large eggs to H. signatus.

Les petites tortues du sud de l'Afrique contiennent les seuls taxons de testudinidés qui produisent une seule couvée. Ce groupe comprend les plus petites tortues du monde, Homopus signatus (Gmelin, 1789), qui habitent des milieux rudes et arides. Comme le climat et la taille corporelle influencent vraisemblablement le rendement reproductif, nous avons émis l'hypothèse que le gradient est–ouest d'aridité dans le sud de l'Afrique affecte la taille des oeufs et celle des couvées des petites tortues indigènes et que la morphologie d'H. signatus limite la taille des oeufs, empêchant ainsi la formation d'oeufs optimaux. Nous démontrons que l'aridité et les précipitations imprévisibles déterminent lesquels de ces taxons produisent une seule couvée. Les taxons qui vivent dans les milieux moins prévisibles produisent des oeufs plus gros relativement à leur taille que les taxons des milieux plus prévisibles. Homopus signatus produit les oeufs les plus gros, compte tenu de sa taille, probablement afin d'améliorer la survie des ses rejetons dans un environnement rude. La taille du corps, la dimension de l'ouverture pelvienne et l'étroitesse du passage anal semblent restreindre la taille des oeufs. Malgré ces contraintes, les femelles produisent des oeufs à coquille rigide plus grands que leur canal pelvien et utilisent la cinétique du pelvis pour faire passer les oeufs lors de la ponte. Ces caractéristiques sont inconnues chez les autres chéloniens, ce qui souligne l'avantage sélectif des gros oeufs chez H. signatus.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 1, 2005

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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