Monthly Archives: March 2014
Attendees for the latest Science Drinks session at the William Wallace included:
Lilly started the session by introducing a paper on allometry in cervids by Lemaître and colleagues recently published in Biology Letters. Allometry is the study of scaling relationships, and is typically quantified by measuring the slope of the line fit between a log-transformed “body size” index and log-transformed trait values. But a review by MacLeod (2010) pointed out several problems with standard assumptions by scientists studying allometry, including that the best-fit line is straight. Lemaître and colleagues show that the allometric relationship between (log) body mass and (log) antler length in cervids is curved, with the increase in antlers leveling off once mass reaches about 110 kg. they argue that this curvilinearity may be explained by energetic constraints on investment in antlers among the largest species., and caution that researchers should be mindful of possible nonlinearities when assessing scaling relationships.
Adam shared a different comparative study, this one focusing on the lengths of telomeres in chimps and humans and recently published in the American Journal of Human Biology. Telomeres are a section of repetitive DNA that caps the end of each chromosome, and that tend to shorten progressively as an animal ages because with every cell replication, a tiny bit of the telomere at the very end of the chromosome cannot be copied. The enzyme telomerase can restore lost telomere, but because this kind of repair mechanism is costly, the length of telomeres may reveal both individual-level information about biological age and susceptibility to degenerative diseases, as well as species-level investment in somatic repair that has evolved in conjunction with maximum longevity (see Monaghan 2010 for a great review of telomere dynamics in a life history context). Tackney and colleagues (2014) predicted that because the maximum longevity of humans exceeds that of chimps, chimps might have shorter telomeres or faster rates of telomeric loss than humans. The computed the relative signals of telomeric and single copy gene signals (T/S ratios) in white blood cells, and found that while their estimates of telomeric loss for chimps (approx. 73 nucleotides per year lost) exceeded that of humans (approximately 40 nucleotides per year), these rates were not statistically distinguishable. Moreover, the chimp telomeres were actually longer, having a T/S ratio around twice that of humans. It’s unclear what the implications of this finding are, but it raises many intriguing questions about the complex relationship between longevity and investment in repair that concerns all students of life history.
Toby showed some new figures, hot off the printer, illustrating some of his Hons research findings about the relationships between eye facet size divergence (which involves adaptations for photosensitivity) and the angles between facets (which may limit acuity) in dance flies. Claudia showed off some new plots and tables of coefficients of her own, involving the study of courtship and mating as a function of diet and social environment in crickets. I don’t want to step on imminent punch-lines: both Toby and Claudia will submit their dissertations in a few short weeks, and we’ll have a chance to see summaries of their work then. Stay tuned!
I (Luc) wrapped up the session by calling attention to a paper just published as an “early-view” manuscript on the journal Evolution‘s homepage by Walker (2014). This paper deals with a problem that is well-known but still underappreciated: when dealing with observation data (unmanipulated experimentally), coefficients of regression analyses will be biased upwards if any of the predictors are correlated with other causal factors that have been omitted from the model through ignorance or negligence. Walker uses simulations to show that this upward bias can be substantial, and argues that the problem is especially acute if one seeks to interpret coefficients as evidence for causal relationships. He further argues that the problem is not necessarily avoided by having prior knowledge of the “right traits” to include in one’s model, and carefully measuring candidate rival traits that one can include as covariates. He writes,
“forty percent of the time an additional covariate is added, even using omniscient prior knowledge of effect size, the error in the estimate of the effect is worse than if the confounder had been left unmeasured”.
This is deeply unsettling research for someone like me who spends a lot of time trying to find patterns of covariance in observational data (much of it by necessity since some of our work is on organisms that do little in the lab except die). Walker does point to directed acyclical graphs (structural equation models) as promising some clarity concerning causal relationships in observational data, which is satisfying news given my recent obsession with them, but overall the MS is more cautionary than prescriptive:
“Unless a functional model generates very specific predictions of effect magnitude, observational data is better used for testing model assumptions and not the mere presence of an effect. For studies where experimental manipulation or functional modeling is not an option, we may have to be content with simply not knowing the magnitude of effects very precisely.”
Science Drinks kicked off with Gregor explaining exciting progress he has made in his honours project work. Gregor is using mapping techniques (GIS) to examine the phenology of dance flies across the UK, trying to find whether there is a difference in emergence times across dance fly species which display both conventional and reversed sex roles.
Luc then presented to the group a paper recently accepted in Evolution looking at age-dependent performance and senescence in sport (Lailvaux, Wilson & Kasumovic 2014). The authors used an extensive dataset on male and female professional basketball players to investigate sex differences in ageing and performance. The main results included a trend for earlier senescence in males, and evidence that different male performance traits showed varying rates of senescence. The Science Drinks group discussed the paper at length, especially how aspects of the data and game were controlled for in the analyses.
Whilst the paper made me realise how very little I know about basketball it prompted a long discussion about the use of sport stats (of which there are apparently huge repositories for some sports) in scientific analyses. Luc explained his ongoing interest in analysing sumo wrestling statistics and then went on to describe a paper he co-authored in 2004. This paper used data from cricket (the sport not the insect) to show that there was evidence of negative-frequency dependent success of left-handed batsman in the 2003 cricket World Cup. After the group chatted about cricket for a while I added both cricket and sumo wrestling to the quickly growing list of sports I know absolutely nothing about! The discussion then moved on to the group pondering what other sports may have large and detailed datasets, collected and published by enthusiasts, that could be used to answer biological questions.
Adam was next to speak and gave us a very interesting introduction to his work. His main interests are senescence and menopause in mammals and he currently uses a large dataset of human birth and death records to answer questions in this field. This dataset was collected from pre-industrial Finnish church records that are apparently extensive and very detailed. He is currently using the dataset to try and find the effect that number of children has on maternal fitness and survival. An issue that he has found in this system is that if a mother died, her offspring often died soon after, meaning the causal relationship is reversed (a lack of maternal care affects child survival, rather than the birth of children affecting mothers). The challenge of disentangling complex causal relationships appears to be a persistent problem for life history research.
Finally, Stuart talked us through some thoughts he was having in his own field of study using Daphnia to look at host-parasite coevolution. A major interest of his currently surrounds the idea that not every parasite will successfully infect a host and will instead simply pass through the host’s digestive system unharmed. He is looking into the cost of a failed infection on the parasite and how this affects both host and parasite population dynamics and coevolution.
Edit Apr 7: added cool pictures from Ellie.
Research group alumnus Ellie Rotheray (who defended her PhD last year) has recently had yet another of her thesis chapters published in the Journal of Insect Conservation. The experiment in question involved a lot of painstaking field work marking and recapturing the rare aspen hoverfly, Hammerschmidtia ferruginea, and observing the dispersal patterns and territorial behaviour of adults.
Our conclusions are sometimes necessarily tentative, but in spite of this I think they provide invaluable natural history details of the kind so rarely found in modern scientific studies, but which are crucial for both fundamental life history research and applied conservation efforts. I’ve included a sample (in the form of our Fig 3) below. Comments or requests for early view reprints are most welcome!
This announcement is quite late, in part because in the post-submission haze and good cheer I forgot to let the rest of the world know via blog: Tom Houslay submitted his PhD thesis with dozens of minutes to spare at the end of last month. Congratulations Tom! This was a rather heroic effort. More news to follow once the viva preparations are in place.
Attendees: Luc Bussiere, Elizabeth Herridge, Toby Hector, Claudia Santori, Gregor Hogg
To start with, Luc discussed ongoing struggles he is having in presenting some statistical concepts to first year students. It turns out that lecturers often have just as much trouble writing lecture material as we do understanding it! We debated the issue of making the material engaging enough to keep everyone interested in a difficult subject, and considered the contrasting pressure of getting all the information across when the lecture is a pivotal point in the course. Luc will report back on how his lectures went later on….
Next onto science as Toby presented a paper that has found a novel method of looking at depth perception using jumping spiders (Depth perception from image defocus in a jumping spider, 2012). The authors report evidence for the first known example of an animal (the jumping spider) that uses defocused images as a primary mechanism for depth perception.
Claudia then shared an interesting paper on the convergent evolution between Cane Toads and the Madagascan plant Mother of Millions (Interacting Impacts of Invasive Plants and Invasive Toads on Native Lizards, 2012). In a case study on the blue tongue lizard (Tiliqua scinoides) it was observed that while these omnivorous lizards are threatened by the invasion of toads in north western Australia, conspecifics from other areas of Australia are less affected by the poison of the toads, including where the toads have yet to invade. Researchers noticed that this pattern was consistent with the geographic occurrence of an ornamental plant from Madagascar – Mother of Millions (Bryophyllum spp.), introduced in the continent around the same time as the cane toads.
This seems to be a remarkable case of convergent evolution, where the toxins produced by the Mother of Millions are chemically extremely similar to the bufotoxins produced by the toads. The lizards that were found to have evolved resistance to the plant toxins also turned out to be tolerant to the poison of the cane toad. Different individuals from various geographical areas were collected and injected with a sublethal dose of toxin, and changes in their locomotor performance were then observed. Lizards located in areas where neither Mother of Millions nor Cane Toads were present were found to have to lowest tolerance to the toxins. This supports the idea that both the Mother of Millions and the Cane Toads impose selection on bufadienolide resistance.
Finally Lilly discussed some of her ongoing work on sexual selection in dance flies!