Functional relations between locomotor performance traits in spiders and implications for evolutionary hypotheses.

BACKGROUND: Locomotor performance in ecologically relevant activities is often linked to individual fitness. Recent controversy over evolution of extreme sexual size dimorphism (SSD) in spiders centres on the relationship between size and locomotor capacity in males. Advantages for large males running over horizontal surfaces and small males climbing vertically have been proposed. Models have implicitly treated running and climbing as functionally distinct activities and failed to consider the possibility that they reflect common underlying capacities. FINDINGS: We examine the relationship between maximum climbing and running performance in males of three spider species. Maximum running and climbing speeds were positively related in two orb-web spiders with high SSD (Argiope keyserlingi and Nephila plumipes), indicating that for these species assays of running and climbing largely reveal the same underlying capacities. Running and climbing speeds were not related in a jumping spider with low SSD (Jacksonoides queenslandica). We found no evidence of a performance trade-off between these activities. CONCLUSIONS: In the web-spiders A. keyserlingi and N. plumipes good runners were also good climbers. This indicates that climbing and running largely represent a single locomotor performance characteristic in these spiders, but this was not the case for the jumping spider J. queenslandica. There was no evidence of a trade-off between maximum running and climbing speeds in these spiders. We highlight the need to establish the relationship between apparently disparate locomotor activities when testing alternative hypotheses that yield predictions about different locomotor activities. Analysis of slopes suggests greater potential for an evolutionary response on performance in the horizontal compared to vertical context in these spiders.

Control of copula duration and sperm storage by female Queensland fruit flies.

Copula duration and sperm storage patterns can directly or indirectly affect fitness of male and female insects. Although both sexes have an interest in the outcome, research has tended to focus on males. To investigate female influences, we compared copula duration and sperm storage of Queensland fruit fly females that were intact, or had been incapacitated through decapitation or abdomen isolation. We found that copulations were far longer when females had been incapacitated, indicating that constraints imposed on copula duration by intact females had been relaxed. Repeatability of copula duration for males was very low regardless of female treatment, and this is also consistent with strong female influence. Number of sperm in the spermathecae was not influenced by female treatment, suggesting that female abdominal ganglia control the transport of sperm to these long-term storage organs. However, more sperm were found in the ventral receptacles of incapacitated females compared to intact females. Overall, results implicate cephalic ganglia in regulation of copula duration and short-term sperm storage in the ventral receptacle and abdominal ganglia in regulation of long-term sperm storage in the spermathecae.

Lethal and sublethal toxicity of ammonia to native, invasive, and parasitised freshwater amphipods.

In lethal and sublethal ammonia toxicity tests, we examined differences in tolerance of three species of freshwater amphipods, one native and two invasive in Ireland. The native Gammarus duebeni celticus was slightly less tolerant to ammonia than the invasive G. pulex (96 h LC50= 1.155 and 1.544 mg l(-1), respectively), while another invader, Crangonyx pseudogracilis, had the lowest tolerance (LC50= 0.36 mg l(-1)). Parasitism of G. pulex by the acanthocephalan Echinorhynchus truttae greatly reduced the tolerance of the invader to ammonia (LC50= 0.381 mg l(-1)). Further, precopula pair disruption tests indicated that G. d. celticus was more sensitive to ammonia than G. pulex at sublethal levels. We discuss these results in the context of the ecological replacements of native by invader amphipods.

Roles of parasites in animal invasions.

Biological invasions are global threats to biodiversity and parasites might play a role in determining invasion outcomes. Transmission of parasites from invading to native species can occur, aiding the invasion process, whilst the 'release' of invaders from parasites can also facilitate invasions. Parasites might also have indirect effects on the outcomes of invasions by mediating a range of competitive and predatory interactions among native and invading species. Although pathogen outbreaks can cause catastrophic species loss with knock-on effects for community structure, it is less clear what impact persistent, sub-lethal parasitism has on native-invader interactions and community structure. Here, we show that the influence of parasitism on the outcomes of animal invasions is more subtle and wide ranging than has been previously realized.

SEXUAL SIZE DIMORPHISM AND REPRODUCTIVE INVESTMENT BY FEMALE SPIDERS: A COMPARATIVE ANALYSIS.

We investigate the association between female reproductive investment, absolute size, and sexual size dimorphism in spiders to test the predictions of the fecundity-advantage hypothesis. The relationships between absolute size and sexual size dimorphism and aspects of female reproductive output are examined in comparative analyses using phylogenetically independent contrasts. We provide support for the idea that allometry for sexual dimorphism is the result of variation in female size more so than male size. Regression analyses suggest selection for increased fecundity in females. We argue that fecundity selection provides the only general explanation for the evolution of sexual size dimorphism in spiders.