Time filter

Source Type

South Africa

Bekker A.,University of Cape Town | Bekker A.,Stellenbosch University | Cloete T.J.,University of Cape Town | Chinsamy-Turan A.,Private Bag X3 | And 3 more authors.
Materials Science and Engineering C | Year: 2014

Cortical bone is a visco-elastic material which implies that strain rate will affect its response. Although the Split-Hopkinson Pressure Bar is an accepted technique for determining the dynamic compressive properties of cortical bone it has been shown that the strain rate of compression does not remain constant throughout the duration of a classical experiment with a uniform striker. This raises concerns as to the measurement of smeared responses. This paper presents a shaped striker technique whereby the incident pulse can be shaped to attain a constant strain rate experiment for bovine bone. Shaped strikers offer benefits such as re-usability and increased test repeatability. A comparison of the stress-strain-strain rate responses attained through classical and constant strain rate experiments shows that the shape of the stress-strain curves from conventional experiments is adversely affected in the portion where the strain rate varies. The dynamic response corridors for the two tests are similar, however the ultimate properties are affected. It is concluded that the strain rate history should be presented with dynamic stress-strain responses since the instantaneous strain rate is a likely contributor to potential constitutive models. © 2014 Elsevier B.V. All rights reserved.

De Vos A.,Private Bag X3 | O'Riain M.J.,Private Bag X3
Behavioral Ecology | Year: 2013

The selfish herd hypothesis predicts that animals can reduce their relative predation risk by moving toward their neighbors. However, several computer simulation studies have found that smaller herds, rather than large, compact aggregations form when animals move in this manner and that larger herds are only achieved by following more complex movement rules, thought to be unrealistic for real biological systems. Despite much theoretical work, predictions on how animals move to reduce their predation risk have seldom been tested in natural systems. Here we investigate the movement patterns of fur seals (Arctocephalus pusillus pusillus) within 7 groups as they move through a zone of high risk to the relative safety of their foraging grounds. We assess different movement rules against all the individuals in the study and identify whether such an individual's movement at a snapshot in time plausibly reflects a follow (1) of a rule or not (0). Our results suggest that seals traversing high predation risk areas use simple movement rules, rather than complex averaging rules, to reduce their domains of danger. Simple movement rules that serve to decrease an individual seals' domain of danger resulted in the formation of compact groups as predicted by the selfish herd hypothesis. Importantly, individuals dropped these simple movement rules where predation risk is low, which coincided with a reduction in mean group compaction. Despite our small sample size, our results provide empirical support for the central predictions of the selfish herd hypothesis. © 2012 The Author.

De Vos A.,Private Bag X3 | Justin O'Riain M.,Private Bag X3 | Meyer M.A.,Branch Oceans and Coasts | Kotze P.G.H.,Branch Oceans and Coasts | Kock A.A.,Private Bag X3
Marine Mammal Science | Year: 2015

The marked differences in predation risk posed by white sharks (Carcarodon carcarias) at island rookeries of Cape fur seals (Arctocephalus pusillus pusillus) offer a quasi-experimental design within a natural system for exploring how prey adjust their behavior in response to temporal variation in predation risk. Here we compare movement of juvenile and adult Cape fur seals at a high risk (Seal Island) and low risk (Egg Island) rookery. We further compare juveniles and adults at Seal Island in low and high risk seasons and at low and high risk times of day within those seasons. Adult fur seals at Seal Island avoided traversing the zone of high white shark predation risk during the high risk period (0700-0959) in the season of high risk (winter), but not during the low risk season (summer). By contrast, adult fur seals at Egg Island showed no temporal discretion in either season. Unlike juvenile fur seals at Egg Island, juveniles at Seal Island adjusted their temporal movement patterns to more closely mimic adult seal movement patterns. This suggests that exposure to predators is the primary driver of temporal adjustments to movement by prey species commuting from a central place. © 2015 Society for Marine Mammalogy.

Discover hidden collaborations