Abstract Burrow-associated diagenetic alteration and eventual reservoir quality parameters such as porosity and permeability may be altered due to reorganization of the sediment fabric associated with animal burrowing, or result from heterogeneous cement distribution influenced by the bioturbate texture. Petrographic analysis has significant application in recognizing burrow-associated porosity characteristics in marine sandstones. Petrographic analysis can provide mineral identification due to diagenetic chemical alterations and textural evidence regarding cementation history that can lead to more accurate hydrocarbon target interpretations. Overlooking burrow structures may lead to misinterpretations of permeability streaks in hydrocarbon reservoirs. This may be extremely important for reservoirs where slight permeability variations have an effect on hydrocarbon reserve calculations. Understanding biogeochemical reactions and burrow-associated diagenesis that ultimately control reservoir quality is necessary if production from ancient bioturbated marine sandstone reservoirs is to be optimized.

No description available.

The Gopher Tortoise is an important component of a number of upland ecological communities throughout the southeastern U.S., but populations have experienced significant declines over the past century, largely in conjunction with the loss of longleaf pine (Pinus palustris) forests. Military installations have preserved large tracts of longleaf pine, often while implementing prescribed fires that mimic natural lightning-ignited fires (frequent low intensity fire is a necessary component of longleaf pine communities), which in turn has provided refuges for many imperiled longleaf associates, including the Gopher Tortoise. Eglin Air Force Base in the western Florida panhandle presents a unique situation in which large tracts of longleaf pine sandhill (suitable Gopher Tortoise habitat) are available, but tortoise sub-populations on base are small, and many tortoises currently inhabit treeless military testing and training ranges (test ranges) rather than typical forested sandhill. My objectives were therefore to identify factors that may have been influencing use of test ranges as habitat by gopher tortoises and that might explain observed differences in burrow densities among sites. In Chapter 1, I compared vegetation structure, composition, and burrow site selection among sites and between forested and test range vegetation types. I also attempted to identify relationships between vegetation characteristics and variation in burrow densities (a proxy for abundance within a given area) among sites. In Chapter 2, I distributed a questionnaire to other military installations throughout the southeast to identify common management techniques used to maintain testing and training areas at other tortoise-occupied military installations, as these techniques likely affect their suitability as tortoise habitat. In Chapter 3, I compared surface activity patterns of juvenile Gopher Tortoises between forested and test range vegetation types, as surface activity in these vulnerable, but important size classes may affect survival rates. I found that test ranges generally had greater herbaceous vegetation cover than forested sites (greater forage availability), were highly species diverse in terms of groundcover plants, and had herbaceous communities that shared a number of common sandhill plant species with forested sites, but also were unique in a number of ways. I also found that adult tortoises (burrow site selection) and juvenile tortoises (surface activity) may have exhibited different behaviors in novel test range vegetation types compared to individuals inhabiting more natural longleaf pine sandhill on base. However, I did not find strong evidence that current vegetation structure or composition was related to observed differences in burrow densities among sites and that other factors might have played a greater role in structuring Eglin's remnant tortoise sub-populations. / Master of Science / The Gopher Tortoise is an important component of a number of upland ecological communities throughout the southeastern U.S., but populations have experienced significant declines over the past century, largely in conjunction with the loss of longleaf pine (Pinus palustris) forests. Military installations have preserved large tracts of longleaf pine, often while implementing prescribed fires that mimic natural lightning-ignited fires (frequent low intensity fire is a necessary component of longleaf pine communities), which in turn has provided refuges for many imperiled longleaf associates, including the Gopher Tortoise. Eglin Air Force Base in the western Florida panhandle presents a unique situation in which large tracts of longleaf pine sandhill (suitable Gopher Tortoise habitat) are available, but tortoise sub-populations on base are small, and many tortoises currently inhabit treeless military testing and training ranges (test ranges) rather than typical forested sandhill. My objectives were therefore to identify factors that may have been influencing use of test ranges as habitat by gopher tortoises and that might explain observed differences in burrow densities among sites. In Chapter 1, I compared vegetation structure, composition, and burrow site selection among sites and between forested and test range vegetation types. I also attempted to identify relationships between vegetation characteristics and variation in burrow densities (a proxy for abundance within a given area) among sites. In Chapter 2, I distributed a questionnaire to other military installations throughout the southeast to identify common management techniques used to maintain testing and training areas at other tortoise-occupied military installations, as these techniques likely affect their suitability as tortoise habitat. In Chapter 3, I compared surface activity patterns of juvenile Gopher Tortoises between forested and test range vegetation types, as surface activity in these vulnerable, but important size classes may affect survival rates. I found that test ranges generally had greater herbaceous vegetation cover than forested sites (greater forage availability), were highly species diverse in terms of groundcover plants, and had herbaceous communities that shared a number of common sandhill plant species with forested sites, but also were unique in a number of ways. I also found that adult tortoises (burrow site selection) and juvenile tortoises (surface activity) may have exhibited different behaviors in novel test range vegetation types compared to individuals inhabiting more natural longleaf pine sandhill on base. However, I did not find strong evidence that current vegetation structure or composition was related to observed differences in burrow densities among sites and that other factors might have played a greater role in structuring Eglin’s remnant tortoise sub-populations.

Many amphibian populations are rapidly disappearing throughout the world. An important issue for ecologists is why some amphibian species are more susceptible to decline than others. Here I present five experiments that compare the performance of an endangered (Rana sevosa) and a common (Rana sphenocephala) frog in changing habitats, to determine why these two species differ in their persistence. I include additional studies investigating the habitat requirements and behavior of R. sevosa. I found that habitat change in the form of canopy closure over breeding ponds negatively affects both species, making them smaller as tadpoles and at metamorphosis. The magnitude of size differences was greater for R. sevosa and this species was less likely to survive in closed canopy ponds. Larval survival was not affected in R. sphenocephala and this is likely a key reason for the persistence of this species in habitats where R. sevosa has been extirpated. The introduction of fish to breeding ponds would also differentially affect the two species. R. sevosa did not display behavioral defenses to the threat of fish predation while R. sphenocephala did. R. sevosa displayed a preference for certain characteristics in its habitat including open canopy ponds, grassy terrestrial habitats and an abundance of burrows. An association with the chemical cues of burrow making organisms suggests that these organisms may be important for R. sevosa. Therefore, specialized habitat requirements and behaviors which may be contributing to its decline. I found some evidence of asymmetric competition in the larval stage with R. sevosa negatively affecting R. sphenocephala. It does not appear that larval competition with R. sphenocephala has contributed to the decline of R. sevosa. The primary difference between the two species was in responses to habitat change. R. sevosa appears to be rigid in its habitat requirements and behavior and its inability to respond and adapt to change is a key component of its rarity. By contrast R. sphenocephala showed an ability to cope with habitat changes. Conservation of R. sevosa will require suitable management of the aquatic and terrestrial habitats, primarily through the instigation of an appropriate fire regime.

Soft-sediment ecosystems can be degraded through anthropogenic development, leading to reduced habitat suitability for biological communities. On the North Coast of British Columbia, Canada, intensive industrial activity and coastal development has occurred, specifically around the Skeena and Kitimat River Estuaries. In addition to current development, both regions have the potential for further development, while also undergoing passive restoration from historical disturbances. Therefore, I aimed to broaden our understanding of passive restoration and non-invasive monitoring of intertidal soft-sediment ecosystems, by carrying out experiments at mudflats in both estuaries during the summer of 2017. Specifically, I aimed to expand the use of a non-invasive population assessment technique to novel species in soft-sediment ecosystems. Relationships between burrowing decapod abundance and burrow openings have been successfully used to estimate population sizes, but this technique has yet to be applied to large burrowing polychaetes, bivalves, or in regions of high macrofaunal diversity. As such, I assessed mudflats in regions of low (n = 1 species) and high (n = 8 species) biodiversity to determine if macrofauna abundances could be estimated from burrow openings on the sediment surface. Where only one burrowing bivalve species was present, a relationship between burrow openings and population abundance was not feasible, but burrow openings were useful in estimating total macrofaunal community abundance at a high diversity mudflat. This suggests that monitoring through burrow opening counts has the ability to detect overall changes in population abundance. Next, I examined the infaunal community, sediment conditions, and nutrient availability at one intertidal mudflat in the Skeena River Estuary following the cessation of heavy industrial activities (i.e. a salmon cannery and pulp mill) to determine the capacity for passive restoration. Sediment conditions varied spatiotemporally, and nutrient availability showed temporal variation but trends were difficult to relate to historical or current potential disturbances. The legacy of past development is still apparent on the infaunal community in the form of patchy distributions of disturbance-indicating taxa, but the mudflat appears to be in an overall healthy state with a diverse and functioning food web, indicating community recovery from historical activities. Results from these studies indicate passive restoration can be appropriate for estuarine soft-sediment ecosystems, while monitoring population abundance through burrow openings could be a method of detecting disturbances or tracking recovery of macrofaunal populations. / Graduate / 2020-06-28

<p>Southern Africa harbours one-third of the world&rsquo / s Testudinid species, many of which inhabit arid or semi-arid areas, but ecological information on these species is scant. I studied the habitat, morphology and ecology of Kalahari tent tortoises over 13 months in semi-arid Savanna at Benfontein farm, Northern Cape Province, South Africa. In order to allow continuous monitoring of individuals, I attached radiotransmitters to males and females, split equally between two habitats, sites E (east) and W (west), with apparent differences in vegetation structure. Results of the study were based on data obtained from 27 telemetered tortoises and 161 individuals encountered opportunistically. Female Kalahari tent tortoises were larger than males and the sex ratio did not differ from 1:1. Based on person-hours to capture tortoises, the population appeared to have a low density, with more time required to capture a juvenile (35 hours) than an adult (10-11 hours). The frequency distribution of body size ranges was indicative of recruitment. Relative age, based on annuli counts, suggested that males were younger than females, perhaps because males as the smaller sex are more predation-prone than females. Linear relationships between annuli counts and shell volume indicated that, after reaching sexual maturity, female body size increased faster in volume than did male body size, possibly because a larger volume may enhance female reproductive success. Body condition differed between sites, sexes and among seasons. The hot and dry summer may account for low summer body condition, whereas vegetation differences and size effects, respectively, may account for the low body condition of tortoises in site W and in males. Site E was sandy with grasses, particularly Schmidtia pappophoroides, being the prevalent growth form. This habitat resembled a Savanna vegetation type Schmidtia pappophoroides &ndash / Acacia erioloba described for a neighbouring reserve. Site W was stonier, dominated by shrubs, and was reminiscent of Northern Upper Karoo vegetation (NKu3). Neither site resembled Kimberley Thornveld (SVk4), the designated vegetation type of the area. Differences in substrate and grazing intensity may have contributed to site vegetation differences. Rainfall had an important influence on seasonal vegetation. Short grass abundance correlated with rainfall and annual plants sprouted after spring rain. Refuge use changed according to season and sex. Males selected denser refuges than females did, perhaps because males were smaller and more vulnerable to predation and solar heat. Tortoises selected sparse, short grass as refuges in cool months, probably to maximise basking whilst remaining in protective cover. During hot periods, mammal burrows were preferred to vegetation as refugia. The smaller males spent more time in cover than females, which may be related to predator avoidance or thermoregulation.&nbsp / Females spent more time basking than males, perhaps due to their larger size and to facilitate reproductive processes. Tortoises did not brumate, but through a combination of basking, and orientation relative to the sun in their refuges, managed to attain body temperatures that allowed small bouts of activity. Body temperature for active tortoises was similar among seasons, and was higher for more specialised active behaviours, such as feeding and socialising, than for walking. Increased activity by males in spring could relate to mating behaviour while females were more active in autumn, when they foraged more than males, perhaps due to the high cost of seasonal reproductive requirements. Males displaced further per day than did females, but home range estimates did not differ between sexes. Annual home range estimates varied substantially among individuals: 0.7&ndash / 306 ha for minimum convex polygons and 0.7&ndash / 181 ha for 95% fixed kernel estimates. The ability to&nbsp / cover large areas would assist tortoises in finding resources, e.g., food, in an area where resource distribution may be patchy. Differences among seasonal home ranges and movements probably reflect seasonal climatic change / activity areas shrinking when temperatures were extreme. In order to assess the effects of a semi-arid environment on the morphology of P. oculifer, I compared its morphology to that of its &lsquo / cool-adapted&rsquo / sister taxon Psammobates geometricus, using live and museum specimens. Both P. oculifer and P. geometricus are sexually dimorphic and differences between the two species could indicate environmental or sexual selection effects, or a combination of the two. The shorter bridge length, which allowed more leg space, and wider front feet in P. oculifer cohorts probably represent traits for manoeuvring in a sandy habitat, while wider heads in P. oculifer possibly relate to interspecific differences in diet. The flatter shell in female P. oculifer, relative to P. geometricus, may represent a trade-off between space for reproductive structures, e.g., eggs, and the need to fit into small refuges, e.g., mammal burrows. Male P. oculifer had wider shells, more space around their hind legs, and wider hind feet than P. geometricus males had, all characteristics which may assist males to fight and mate in a sandy environment.</p>

<p>Southern Africa harbours one-third of the world&rsquo / s Testudinid species, many of which inhabit arid or semi-arid areas, but ecological information on these species is scant. I studied the habitat, morphology and ecology of Kalahari tent tortoises over 13 months in semi-arid Savanna at Benfontein farm, Northern Cape Province, South Africa. In order to allow continuous monitoring of individuals, I attached radiotransmitters to males and females, split equally between two habitats, sites E (east) and W (west), with apparent differences in vegetation structure. Results of the study were based on data obtained from 27 telemetered tortoises and 161 individuals encountered opportunistically. Female Kalahari tent tortoises were larger than males and the sex ratio did not differ from 1:1. Based on person-hours to capture tortoises, the population appeared to have a low density, with more time required to capture a juvenile (35 hours) than an adult (10-11 hours). The frequency distribution of body size ranges was indicative of recruitment. Relative age, based on annuli counts, suggested that males were younger than females, perhaps because males as the smaller sex are more predation-prone than females. Linear relationships between annuli counts and shell volume indicated that, after reaching sexual maturity, female body size increased faster in volume than did male body size, possibly because a larger volume may enhance female reproductive success. Body condition differed between sites, sexes and among seasons. The hot and dry summer may account for low summer body condition, whereas vegetation differences and size effects, respectively, may account for the low body condition of tortoises in site W and in males. Site E was sandy with grasses, particularly Schmidtia pappophoroides, being the prevalent growth form. This habitat resembled a Savanna vegetation type Schmidtia pappophoroides &ndash / Acacia erioloba described for a neighbouring reserve. Site W was stonier, dominated by shrubs, and was reminiscent of Northern Upper Karoo vegetation (NKu3). Neither site resembled Kimberley Thornveld (SVk4), the designated vegetation type of the area. Differences in substrate and grazing intensity may have contributed to site vegetation differences. Rainfall had an important influence on seasonal vegetation. Short grass abundance correlated with rainfall and annual plants sprouted after spring rain. Refuge use changed according to season and sex. Males selected denser refuges than females did, perhaps because males were smaller and more vulnerable to predation and solar heat. Tortoises selected sparse, short grass as refuges in cool months, probably to maximise basking whilst remaining in protective cover. During hot periods, mammal burrows were preferred to vegetation as refugia. The smaller males spent more time in cover than females, which may be related to predator avoidance or thermoregulation.&nbsp / Females spent more time basking than males, perhaps due to their larger size and to facilitate reproductive processes. Tortoises did not brumate, but through a combination of basking, and orientation relative to the sun in their refuges, managed to attain body temperatures that allowed small bouts of activity. Body temperature for active tortoises was similar among seasons, and was higher for more specialised active behaviours, such as feeding and socialising, than for walking. Increased activity by males in spring could relate to mating behaviour while females were more active in autumn, when they foraged more than males, perhaps due to the high cost of seasonal reproductive requirements. Males displaced further per day than did females, but home range estimates did not differ between sexes. Annual home range estimates varied substantially among individuals: 0.7&ndash / 306 ha for minimum convex polygons and 0.7&ndash / 181 ha for 95% fixed kernel estimates. The ability to&nbsp / cover large areas would assist tortoises in finding resources, e.g., food, in an area where resource distribution may be patchy. Differences among seasonal home ranges and movements probably reflect seasonal climatic change / activity areas shrinking when temperatures were extreme. In order to assess the effects of a semi-arid environment on the morphology of P. oculifer, I compared its morphology to that of its &lsquo / cool-adapted&rsquo / sister taxon Psammobates geometricus, using live and museum specimens. Both P. oculifer and P. geometricus are sexually dimorphic and differences between the two species could indicate environmental or sexual selection effects, or a combination of the two. The shorter bridge length, which allowed more leg space, and wider front feet in P. oculifer cohorts probably represent traits for manoeuvring in a sandy habitat, while wider heads in P. oculifer possibly relate to interspecific differences in diet. The flatter shell in female P. oculifer, relative to P. geometricus, may represent a trade-off between space for reproductive structures, e.g., eggs, and the need to fit into small refuges, e.g., mammal burrows. Male P. oculifer had wider shells, more space around their hind legs, and wider hind feet than P. geometricus males had, all characteristics which may assist males to fight and mate in a sandy environment.</p>

Cold seep ecosystems in the deep sea are fuelled by chemosynthetic processes based on methane emission to the sediment surface from gas hydrate disassociation, methanogenesis or thermogenic processes. While cold seep ecosystems have been studied in the last three decades worldwide, little is known about New Zealand’s cold seep habitats and associated fauna. A joint German-New Zealand cruise to the Hikurangi Margin in early 2007 enabled biological and sediment sampling to investigate the biological and sedimentological relationships and variability of seeps and their faunal diversity. Multi-disciplinary approaches were employed that included Xray radiography, stratigraphic descriptions, lebensspuren traces analysis, sediment grain size analysis, determination of total organic content, carbonate content and its stable isotopic composition, and analysis of benthic invertebrate assemblages of seep habitats. The results of this study revealed three distinctive habitats and associated fauna based on the sediment characteristics and faunal type. Habitat 1 includes all sites pertaining to Omakere Ridge, a seep-related habitat comprised of layers of very poorly sorted, sandy silt, shell hash and bands of methane-derived authigenic aragonitic carbonate nodules with low total organic content (TOC). Due to the characteristics of the sediments and death assemblages of molluscs, it is inferred that Habitat 1 methane seepage is actively diffusive, waning or dormant. Habitat 2 describes sites that are either non-seep or relic and applies to those at Bear’s Paw and Kaka. Habitat 2 constituted of shell hash overlain with very poorly sandy silt, and low carbonates content and low to medium TOC. Habitat 3 describes non-seep related habitats, and includes all sites of the Wairarapa region and one reference site from Kaka also falls into this category. Sediments for Habitat 3 constituted poorly sorted silt with high TOC and low carbonate content which can be explained by their close proximity to land and converging sea currents. The mineral components of the background siliciclastic sediments for all sites studied originated in the Tertiary mudstone of the East Coast Basin. The characteristics of seep habitats of the Hikurangi Margin were comparable to that of the Northern Hemisphere modern seep counterparts, although the abundance and distributions of seep fauna were low. Results from this research have enhanced our understanding on the spatial and variability of methane fluxes and their affects on the duration of cold seep ecosystems, especially for New Zealand. However, more such studies are essential to increase our understanding of seep sediments and explain disturbance-sediment-benthic invertebrate interactions.

Cold seep ecosystems in the deep sea are fuelled by chemosynthetic processes based on methane emission to the sediment surface from gas hydrate disassociation, methanogenesis or thermogenic processes. While cold seep ecosystems have been studied in the last three decades worldwide, little is known about New Zealand’s cold seep habitats and associated fauna. A joint German-New Zealand cruise to the Hikurangi Margin in early 2007 enabled biological and sediment sampling to investigate the biological and sedimentological relationships and variability of seeps and their faunal diversity. Multi-disciplinary approaches were employed that included Xray radiography, stratigraphic descriptions, lebensspuren traces analysis, sediment grain size analysis, determination of total organic content, carbonate content and its stable isotopic composition, and analysis of benthic invertebrate assemblages of seep habitats. The results of this study revealed three distinctive habitats and associated fauna based on the sediment characteristics and faunal type. Habitat 1 includes all sites pertaining to Omakere Ridge, a seep-related habitat comprised of layers of very poorly sorted, sandy silt, shell hash and bands of methane-derived authigenic aragonitic carbonate nodules with low total organic content (TOC). Due to the characteristics of the sediments and death assemblages of molluscs, it is inferred that Habitat 1 methane seepage is actively diffusive, waning or dormant. Habitat 2 describes sites that are either non-seep or relic and applies to those at Bear’s Paw and Kaka. Habitat 2 constituted of shell hash overlain with very poorly sandy silt, and low carbonates content and low to medium TOC. Habitat 3 describes non-seep related habitats, and includes all sites of the Wairarapa region and one reference site from Kaka also falls into this category. Sediments for Habitat 3 constituted poorly sorted silt with high TOC and low carbonate content which can be explained by their close proximity to land and converging sea currents. The mineral components of the background siliciclastic sediments for all sites studied originated in the Tertiary mudstone of the East Coast Basin. The characteristics of seep habitats of the Hikurangi Margin were comparable to that of the Northern Hemisphere modern seep counterparts, although the abundance and distributions of seep fauna were low. Results from this research have enhanced our understanding on the spatial and variability of methane fluxes and their affects on the duration of cold seep ecosystems, especially for New Zealand. However, more such studies are essential to increase our understanding of seep sediments and explain disturbance-sediment-benthic invertebrate interactions.

Philosophiae Doctor - PhD (Biodiversity and Conservation Biology) / Southern Africa harbours one-third of the world's Testudinid species, many of which inhabit arid or semi-arid areas, but ecological information on these species is scant. I studied the habitat, morphology and ecology of Kalahari tent tortoises over 13 months in semi-arid Savanna at Benfontein farm, Northern Cape Province, South Africa. In order to allow continuous monitoring of individuals, I attached radiotransmitters to males and females, split equally between two habitats, sites E (east) and W (west), with apparent differences in vegetation structure. Results of the study were based on data obtained from 27 telemetered tortoises and 161 individuals encountered opportunistically. Female Kalahari tent tortoises were larger than males and the sex ratio did not differ from 1:1. Based on person-hours to capture tortoises, the population appeared to have a low density, with more time required to capture a juvenile (35 hours) than an adult (10-11 hours). The frequency distribution of body size ranges was indicative of recruitment. Relative age, based on annuli counts, suggested that males were younger than females, perhaps because males as the smaller sex are more predation-prone than females. Linear relationships between annuli counts and shell volume indicated that, after reaching sexual maturity, female body size increased faster in volume than did male body size, possibly because a larger volume may enhance female reproductive success. Body condition differed between sites, sexes and among seasons. The hot and dry summer may account for low summer body condition, whereas vegetation differences and size effects, respectively, may account for the low body condition of tortoises in site W and in males. Site E was sandy with grasses, particularly Schmidtia pappophoroides, being the prevalent growth form. This habitat resembled a Savanna vegetation type Schmidtia pappophoroides – Acacia erioloba described for a neighbouring reserve. Site W was stonier, dominated by shrubs, and was reminiscent of Northern Upper Karoo vegetation (NKu3). Neither site resembled Kimberley Thornveld (SVk4), the designated vegetation type of the area. Differences in substrate and grazing intensity may have contributed to site vegetation differences. Rainfall had an important influence on seasonal vegetation. Short grass abundance correlated with rainfall and annual plants sprouted after spring rain. Refuge use changed according to season and sex. Males selected denser refuges than females did, perhaps because males were smaller and more vulnerable to predation and solar heat. Tortoises selected sparse, short grass as refuges in cool months, probably to maximise basking whilst remaining in protective cover. During hot periods, mammal burrows were preferred to vegetation as refugia. The smaller males spent more time in cover than females, which may be related to predator avoidance or thermoregulation. Females spent more time basking than males, perhaps due to their larger size and to facilitate reproductive processes. Tortoises did not brumate, but through a combination of basking, and orientation relative to the sun in their refuges, managed to attain body temperatures that allowed small bouts of activity. Body temperature for active tortoises was similar among seasons, and was higher for more specialised active behaviours, such as feeding and socialising, than for walking. Increased activity by males in spring could relate to mating behaviour while females were more active in autumn, when they foraged more than males, perhaps due to the high cost of seasonal reproductive requirements. Males displaced further per day than did females, but home range estimates did not differ between sexes. Annual home range estimates varied substantially among individuals: 0.7–306 ha for minimum convex polygons and 0.7–181 ha for 95% fixed kernel estimates. The ability to cover large areas would assist tortoises in finding resources, e.g., food, in an area where resource distribution may be patchy. Differences among seasonal home ranges and movements probably reflect seasonal climatic change; activity areas shrinking when temperatures were extreme. In order to assess the effects of a semi-arid environment on the morphology of P. oculifer, I compared its morphology to that of its ‘cool-adapted’ sister taxon Psammobates geometricus, using live and museum specimens. Both P. oculifer and P. geometricus are sexually dimorphic and differences between the two species could indicate environmental or sexual selection effects, or a combination of the two. The shorter bridge length, which allowed more leg space, and wider front feet in P. oculifer cohorts probably represent traits for manoeuvring in a sandy habitat, while wider heads in P. oculifer possibly relate to interspecific differences in diet. The flatter shell in female P. oculifer, relative to P. geometricus, may represent a trade-off between space for reproductive structures, e.g., eggs, and the need to fit into small refuges, e.g., mammal burrows. Male P. oculifer had wider shells, more space around their hind legs, and wider hind feet than P. geometricus males had, all characteristics which may assist males to fight and mate in a sandy environment. / South Africa