Papers by Sybrand van Sittert
Journal of Morphology, 2015
Giraffes have remarkably long and slender limb bones, but it is unknown how they grow with regard... more Giraffes have remarkably long and slender limb bones, but it is unknown how they grow with regard to body mass, sex, and neck length. In this study, we measured the length, mediolateral (ML) diameter, craniocaudal (CC) diameter and circumference of the humerus, radius, metacarpus, femur, tibia, and metatarsus in 10 fetuses, 21 females, and 23 males of known body masses. Allometric exponents were determined and compared. We found the average bone length increased from 340 ± 50 mm at birth to 700 ± 120 mm at maturity, while average diameters increased from 30 ± 3 to 70 ± 11 mm. Fetal bones increased with positive allometry in length (relative to body mass) and in diameter (relative to body mass and length). In postnatal giraffes bone lengths and diameters increased iso- or negatively allometric relative to increases in body mass, except for the humerus CC diameter which increased with positive allometry. Humerus circumference also increased with positive allometry, that of the radius and tibia isometrically and the femur and metapodials with negative allometry. Relative to increases in bone length, both the humerus and femur widened with positive allometry. In the distal limb bones, ML diameters increased isometrically (radius, metacarpus) or positively allometric (tibia, metatarsus) while the corresponding CC widths increased with negative allometry and isometrically, respectively. Except for the humerus and femur, exponents were not significantly different between corresponding front and hind limb segments. We concluded that the patterns of bone growth in males and females are identical. In fetuses, the growth of the appendicular skeleton is faster than it is after birth which is a pattern opposite to that reported for the neck. Allometric exponents seemed unremarkable compared to the few species described previously, and pointed to the importance of neck elongation rather than leg elongation during evolution. Nevertheless, the front limb bones and especially the humerus may show adaptation to behaviors such as drinking posture.
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African Zoology, 2013
Giraffe are thought to have excellent vision.We measured eye size, orbit orientation and retina s... more Giraffe are thought to have excellent vision.We measured eye size, orbit orientation and retina surface area in 27 giraffes of both sexes ranging in age from neonates to mature adults (>10 yrs), to assess how it changes with growth, whether their eye anatomy correlates with their apparently excellent vision and lifestyle, and we have compared our findings with those for other large mammals to assess whether giraffe eye anatomy is unique.We found that giraffe eye volume increases from 33 cm3 at birth to approximately 65 cm3 in adults. The focal (axial) length increases from c. 40 to 48 mm in adults and retina surface area from c. 3000 mm2 at birth to 4320 mm2 in adults. The orbital axis angle at birth is c. 73° and the horizontal visual
field mainly monocular and panoramic.With age the axis angle becomes more acute to c. 50° in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.
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Journal of the South African Veterinary Association, 2013
Controlled-release albendazole capsules (CRCs) are currently registered for use in Australia and ... more Controlled-release albendazole capsules (CRCs) are currently registered for use in Australia and New Zealand as anthelmintic treatment in sheep. However, reports on the efficacy of such products on resistant parasite populations are sometimes controversial. This is the first study to report on the efficacy of such products under South African field conditions in sheep harbouring a population of Haemonchus contortus with known multiple anthelmintic resistance, including to albendazole. Treatment groups were comprised of CRC-treated and single dose albendazole-treated sheep, as well as negative controls. Groups were compared by using faecal egg count reduction tests, FAMACHA© anaemia scoring, conception rates and comparative weight gains over three and a half months. Based on a comparison of faecal egg counts, no advantage could be found using CRCs. Moreover, the use of the product actually decreased weight gain when compared with the control group animals.
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This is the first case of African horse sickness (AHS) in a dog where there was no apparent inges... more This is the first case of African horse sickness (AHS) in a dog where there was no apparent ingestion of horse meat. Significantly, the dog was part of a colony that resides in a Good Clinical Practice and Good Laboratory Practice accredited facility where complete history, weather and feeding records are maintained. The dog died after a week-long illness despite therapy. The principal post-mortem findings were severe hydrothorax and pulmonary consolidation (red hepatisation of the lungs). Histopathology revealed severe oedema and congestion of the lungs, hyaline degeneration of the myocardium and congestion of the liver sinusoids. Immunohistochemistry detected AHS-positive staining granules in the myocardium, whilst a real-time reverse transcription quantitative Polymerase chain reaction assay of tissue samples was strongly positive for African horse sickness virus nucleic acid. Other dogs on the property showed a 43%seroconversion rate to AHS.
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Journal of Experimental Zoology Part B-molecular and Developmental Evolution, 2010
As mammalian cervical vertebral count is almost always limited to seven, the vertebral column of ... more As mammalian cervical vertebral count is almost always limited to seven, the vertebral column of the giraffe (Giraffa camelopardalis) provides an interesting study on scaling and adaptation to shape in light of these constraints. We have defined and described the growth rates of the lengths, widths, and heights of the vertebrae from fetal through neonatal life to maturity. We found that the disproportionate elongation of the cervical vertebrae is not a fetal process but occurs after birth, and that each cervical (C2-C7) vertebrae elongates at the same rate. C7 is able to specialize toward elongation as its function has been shifted to T1. We concluded that T1 is a transitional vertebra whose scaling exponent and length is between that of the cervical and thoracic series. Despite its transitional nature, T1 is still regarded as thoracic, as it possesses an articulating rib that attaches to the sternum. The other dimensions taken (width, height, and spinous process length) show that giraffe vertebral morphology exhibit adaptations to biomechanical strain, and we have underlined the importance of the thoracic spinous processes in supporting the head and neck. J. Exp. Zool. (Mol. Dev. Evol.) 314B:469–479, 2010. © 2010 Wiley-Liss, Inc.
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Transactions of The Royal Society of South Africa, 2010
Darwin's theory for the evolution of the long neck of giraffes is that height confers access to b... more Darwin's theory for the evolution of the long neck of giraffes is that height confers access to browse free of competition from smaller browsers. The theory predicts that survivors of a drought will be the tallest animals in a population. All studies so far have tested this hypothesis by analysis of feeding patterns and behaviour. We have studied it by analysing the demography of deaths in a drought. Using skeletal material from 26 giraffes that died as a result of a drought in southeastern Zimbabwe in 2008, we established the body mass, height, and age of the dead giraffes using allometric equations developed from culled animals. Typical giraffe populations consist of 55% adults (>6 years old), 15% young adults (3–6 years old), 15% juveniles (1–3 years old), and 15% neonates (<1 year old). Skeletons came from 54% adults, 14% young adults, and 32% juveniles. No neonatal skeletons were found. More juveniles died than expected because they have to compete with other browsers for nutrients. Most adult deaths occurred in the tallest and largest males because their daily requirements for browse are highest and could not be met by the amounts available at any level. Thus the survivors of this drought were young adults, a finding contrary to the predictions of Darwin's feeding hypothesis.
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Journal of Zoology, 2009
The evolutionary origin of the long neck of giraffes is enigmatic. One theory (the ‘sexual select... more The evolutionary origin of the long neck of giraffes is enigmatic. One theory (the ‘sexual selection’ theory) is that their shape evolved because males use their necks and heads to achieve sexual dominance. Support for this theory would be that males invest more in neck and head growth than do females. We have investigated this hypothesis in 17 male and 21 female giraffes with body masses ranging from juvenile to mature animals, by measuring head mass, neck mass, neck and leg length and the neck length to leg length ratio. We found no significant differences in any of these dimensions between males and females of the same mass, although mature males, whose body mass is significantly (50%) greater than that of mature females, do have significantly heavier (but not longer) necks and heavier heads than mature females. We conclude that morphological differences between males and females are minimal, that differences that do exist can be accounted for by the larger final mass of males and that sexual selection is not the origin of a long neck in giraffes.
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South African Journal of Wildlife Research, 2009
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Papers by Sybrand van Sittert
field mainly monocular and panoramic.With age the axis angle becomes more acute to c. 50° in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.
field mainly monocular and panoramic.With age the axis angle becomes more acute to c. 50° in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.