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

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 rigidshelled 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. Résumé :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, empê-chant 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 caractéristi-ques sont inconnues chez les autres chéloniens, ce qui souligne l'avantage sélectif des gros oeufs chez H. signatus.[Traduit par la Rédaction] Hofmeyr et al. 1352

Growing and shrinking in the smallest tortoise, Homopus signatus signatus: the importance of rain

“…H. s. signatus lives in rocky terrain, where it retreats in crevices (Loehr 2002a), and has a herbivorous diet (Loehr 2006). Female H. s. signatus produce one egg at a time and egg size is strongly correlated to body size and body condition (Hofmeyr et al 2005). Body condition in turn is inXuenced by winter rainfall patterns (V.J.T.…”

Cardio‐pulmonary anatomy in theropod dinosaurs: Implications from extant archosaurs

“…The larger pelvic foramen of birds is often assumed to be correlated to the size of the egg produced (Romer and Parsons, 1977;Carpenter, 2000), and clearly having wider, open pelves allow for larger eggs (and embryos). However, in many turtles, eggs are significantly smaller than predicted by the pelvic aperture size constraint (Bowden et al, 2004;Hofmeyr et al, 2005) and there are locomotor and postural factors beyond reproduction that influence avian pelvic dimensions (Naples et al, 2002;Hertel and Campbell, 2007). We suggest that in addition to the previously addressed factors, the enlarged pelvic space of modern birds is, in part, tightly linked to the presence of extensive abdominal air-sacs in these taxa.…”

Population ecology ofPsammobates oculiferin a semi-arid environment

“…Regressions of BM on SV provide the best surrogate measure of body condition (BC) in chelonians (Hofmeyr et al 2005;Loehr et al 2007a). We used the mean BM per season for the relevant individual when doing seasonal analyses because we measured tortoises' BM more than once per season for radio-tracked and recaptured animals.…”

“…Seasonal changes in temperature and rainfall can influence body condition (Nagy & Medica 1986;Henen 1997;Loehr et al 2007a) and hence fitness parameters such as reproductive output (Hofmeyr et al 2005;Loehr et al 2007a) and survival (Shine et al 2001). Geographic variation in environmental productivity can influence population density, with lower densities in arid than mesic regions (see Branch 1984;Mason et al 2000).…”

“…Regular weighing allowed a consistent evaluation of seasonal changes in body condition of telemetered tortoises. We measured straight carapace length (SCL, mm), shell width at the middle (SWM, at the sixth marginal scute; mm) and shell height (SHM, at the apex of the highest dorsal scute; mm) for each tortoise with vernier callipers (9 0.01 mm; Kanon, Tokyo) to calculate shell volume (SV, cm 3 ) using a modified formula for an ellipsoid: SV =p * SCL * SHM * SW/6000 (Loehr et al 2004;Hofmeyr et al 2005;Loehr et al 2007a). Body mass and SV were used to calculate body condition as a ratio of mass to shell volume (Loehr et al 2004) and as mass scaled to shell volume (Loehr et al 2007a).…”