Skye F. Cameron

Sex-specific trade-offs and compensatory mechanisms: bite force and sprint speed pose conflicting demands on the design of geckos ( Hemidactylus frenatus )

SUMMARY One of the more intuitive viability costs that can result from the possession of exaggerated sexually selected traits is increased predation pressure as a result of reduced locomotor capacity. Despite mixed empirical support for such locomotor costs, recent studies suggest that such costs may be masked by compensatory traits that effectively offset any detrimental effects. In this study, we provide a comprehensive assessment of the locomotor costs associated with improved male–male competitive ability by simultaneously testing for locomotor trade-offs and potential compensatory mechanisms in territorial male and non-territorial female geckos. Fighting capacity and escape performance of male Asian house geckos (Hemidactylus frenatus) are likely to pose conflicting demands on the optimum phenotype for each task. Highly territorial and aggressive males may require greater investment in head size/strength but such an enhancement may affect overall escape performance. Among male geckos, we found that greater biting capacity because of larger head size was associated with reduced sprint performance; this trade-off was further exacerbated when sprinting on an incline. Females, however, showed no evidence of this trade-off on either flat or inclined surfaces. The sex specificity of this trade-off suggests that the sexes differ in their optimal strategies for dealing with the conflicting requirements of bite force and sprint speed. Unlike males, female H. frenatus had a positive association between hind-limb length and head size, suggesting that they have utilised a compensatory mechanism to alleviate the possible locomotor costs of larger head sizes. It appears that there is greater selection on traits that improve fighting ability (bite force) for males, but it is viability traits (sprint speed) that appear to be of greater importance for females. Our results emphasise that only by examining both functional trade-offs and potential compensatory mechanisms is it possible to discover the varied mechanisms affecting the morphological design of a species.

Moving in complex environments: a biomechanical analysis of locomotion on inclined and narrow substrates

Characterisation of an organisms’ performance in different habitats provides insight into the conditions that allow it to survive and reproduce. In recent years, Northern quolls (Dasyurus hallucatus)—a medium-sized semi-arboreal marsupial native to northern Australia—have undergone significant population declines within open forest, woodland and riparian habitats, but less so in rocky areas. To explore this decline, we quantified the biomechanical performance of wild Northern quolls over inclined narrow (13 mm pole) and inclined wide (90 mm platform) substrates. We predicted that quolls may possess biomechanical adaptations to increase stability on narrow surfaces, which are more common in rocky habitats. Our results display that quolls have some biomechanical characteristics consistent with a stability advantage on narrow surfaces. This includes the coupled use of limb pairs and high grip torques (Max = 68.9 N.m, Min = −69.9 N.m), highlighting their ability to generate corrective forces to counteract the toppling moments commonly encountered during gait on narrow surfaces. However, unlike other arboreal specialists such as primates, speed was constrained on narrow surfaces, and quolls did not adopt diagonal sequence gaits. Quolls appear to adopt similar performance characteristics to cats and dogs which may limit their ability to outperform these key predators (invasive or otherwise) in particular habitats. This overlap may contribute to the declining population of Northern quolls on mainland Australia.

Evaluation of risks for two native mammal species from feral cat baiting in monsoonal tropical northern Australia

Abstract Context. Feral cats are a significant threat to native wildlife and broad-scale control is required to reduce their impacts. Two toxic baits developed for feral cats, Curiosity® and Hisstory®, have been designed to reduce the risk of baiting to certain non-target species. These baits involve encapsulating the toxin within a hard-shelled delivery vehicle (HSDV) and placing it within a meat attractant. Native animals that chew their food more thoroughly are predicted to avoid poisoning by eating around the HSDV. This prediction has not been tested on wild native mammals in the monsoonal wet–dry tropics of the Northern Territory. Aim. The aim of this research was to determine whether northern quolls (Dasyurus hallucatus) and northern brown bandicoots (Isoodon macrourus) would take feral cat baits and ingest the HSDV under natural conditions on Groote Eylandt. Methods. We hand-deployed 120 non-toxic baits with a HSDV that contained a biomarker, Rhodamine B, which stains animal whiskers when ingested. The species responsible for bait removal was determined with camera traps, and HSDV ingestion was measured by evaluating Rhodamine B in whiskers removed from animals trapped after baiting. Key results. During field trials, 95% of baits were removed within 5 days. Using camera-trap images, we identified the species responsible for taking baits on 65 occasions. All 65 confirmed takes were by native species, with northern quolls taking 42 baits and northern brown bandicoots taking 17. No quolls and only one bandicoot ingested the HSDV. Conclusion. The use of the HSDV reduces the potential for quolls and bandicoots to ingest a toxin when they consume feral cat baits. However, high bait uptake by non-target species may reduce the efficacy of cat baiting in some areas. Implications. The present study highlighted that in the monsoonal wet–dry tropics, encapsulated baits are likely to minimise poisoning risk to certain native species that would otherwise eat meat baits. However, further research may be required to evaluate risks to other non-target species. Given the threat to biodiversity from feral cats, we see it as critical to continue testing Hisstory® and Curiosity® in live-baiting trials in northern Australia.

Sex-specific thermal sensitivities of performance and activity in the asian house gecko, Hemidactylus frenatus

Studies of sexual selection primarily focus on morphological traits such as body size and secondary trait dimorphism, with less attention been given to the functional differences between the sexes and even more so their thermal performance capacities. Each sex may benefit from possessing different thermal performance capacities that would allow them to maximise their fitness relative to their different reproductive roles; especially when performances are closely related to reproductive success. Here, we examine sexual divergence in thermal sensitivities of performance across three populations of the Asian house gecko (Hemidactylus frenatus) over an extensive latitudinal cline. Using analyses of the thermal sensitivity of routine activity, bite force and sprint speed, we explored whether: (i) males and females differed in their optimal temperatures for performance, (ii) the sexes differed in their thermal sensitivities of performance, and (iii) the degree of sexual divergence in thermal sensitivity varied among the populations. Because male H. frenatus are highly aggressive and frequently engage in combat to gain territories and mating opportunities, we expected males would be active over a wider range of temperatures than females and this would favour broad thermal sensitivity curves for males. In addition, we expected a greater divergence between the sexes in thermal sensitivities for the temperate populations that experience greater daily and seasonal thermal variation. We found that males were more active, and had greater bite forces and faster sprint speeds than females, independent of body size. In addition, we found differences between the sexes in thermal sensitivities for the tropical population; female H. frenatus were less active and possessed lower sprint speeds at higher temperatures than males. Although H. frenatus from the most variable thermal environments also displayed the broadest thermal performance range, it was the more stable tropical population that exhibited the greatest divergence between the sexes in thermal sensitivity of performance. The divergence in thermal physiology that we detected between the sexes of H. frenatus is consistent with the idea that males will derive mating and territorial advantages for maintaining function over a broader range of temperatures.

Multidimensional analyses of physical performance reveal a size‐dependent trade‐off between suites of traits

Animal movement is multidimensional and complex, and to understand the motor system of wild animals in the context of their natural ecology, we must analyse how suites of performance traits both mutualistically and antagonistically affect function-a necessity highlighted by previous work on performance trade-offs. Evidence from some studies of human athletes using multidimensional analyses of performance suggests that overall quality among individuals can mask functional trade-offs within them, yet no studies have tested this idea using wild animals. In this study, we investigated the possible mutualistic and antagonistic associations among eight different whole-animal performance traits in male and female northern quolls (Dasyurus hallucatus). We detected trade-offs between pairs of performance traits when conducted on raw standardized data using both Pearson product moment correlations and partial correlation analyses. For example, grasp strength was negatively associated with beam-running speed using both analyses, suggesting that morphological designs that enhance grasp strength simultaneously compromise an animals motor control or stability on a narrow beam. In addition, we detected a trade-off between two distinct sets of performance traits; grasp strength, bite force and maximum oxygen consumption were negatively associated with jump acceleration and beam-running speed. This trade-off between sets of performance traits accounted for around one-third of the total variance in performance among individuals and was primarily driven by the effects of body size on both groups of traits. Larger body sizes improved grasp strength, bite forces and maximum oxygen consumption rates but decreased jump accelerations and beam-running speeds. Because the first component of a principal component analysis based on all eight performance traits (PC1) did not load in the same direction for all traits, PC1 did not represent an overall metric of motor performance-which differs from previous multivariate analyses of human physical performance. Our study highlights the importance of studying suites of traits when exploring the functional phenotype of organisms rather than just one or two dimensions of performance. A plain language summary is available for this article.

Manganese contamination affects the motor performance of wild northern quolls (Dasyurus hallucatus)

Neuromotor deficits are an important sign of manganese (Mn) toxicity in humans and laboratory animals. However, the impacts of Mn exposure on the motor function of wild animals remains largely unknown. Here, we assessed the impact of chronic exposure to Mn from active mining operations on Groote Eylandt, Australia on the motor function of the semi-arboreal northern quoll (Dasyurus hallucatus), an endangered species. The three motor tests conducted-maximum sprint speed on a straight run, manoeuvrability around a corner, and motor control on a balance beam-showed that elevated Mn body burden did not diminish performance of these traits. However, quolls with higher Mn body burden approached a corner at a significantly narrower range of speeds, due to a significantly lower maximum approach speed. Slower speeds approaching a turn may reduce success at catching prey and avoiding predators. Given that maximum sprint speed on a straight run was not affected by Mn body burden, but maximum speed entering a corner was, slower speeds approaching a turn may reflect compensation for otherwise impaired performance in the turn.