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A. David M. Latham, M. Cecilia Latham, Janet M. Wilmshurst, David M. Forsyth, Andrew M. Gormley, Roger P. Pech, George L. W. Perry and Jamie R. Wood (2019)
A refined model of body mass and population density in flightless birds reconciles extreme bimodal population estimates for extinct moa.
Ecography 42: 1-2 (advance online publication)
doi:
https://doi.org/10.1111/ecog.04917https://onlinelibrary.wiley.com/doi/10.1111/ecog.04917Free pdf:
https://onlinelibrary.wiley.com/doi/pdf/10.1111/ecog.04917Flightless birds were once the largest and heaviest terrestrial fauna on many archipelagos around the world. Robust approaches for estimating their population parameters are essential for understanding prehistoric insular ecosystems and extinction processes. Body mass and population density are negatively related for extant flightless bird species, providing a method for quantifying densities and population sizes of extinct flightless species. Here we assemble an updated global data set of body mass and population densities for extant flightless birds and estimate the relationship between these variables. We use generalised least squares models that account for phylogenetic relatedness and incorporate the effects of limiting factors (e.g. habitat suitability) on population density. We demonstrate the applicability of this allometric relationship to extinct species by estimating densities for each of the nine species of moa (Dinornithiformes) and generating a combined spatially explicit map of total moa density across New Zealand. To compare our density estimates with those previously published, we summed individual species' abundances to generate a mean national density of 2.02â9.66 birds kmâ2 for lowâ and highâdensity scenarios, respectively. Our results reconcile the extreme bimodality of previous estimates (< 2 birds kmâ2 and > 10 birds kmâ2) and are comparable to contemporary densities of large herbivorous wild mammals introduced into New Zealand about 150 yr ago. The revised moa density has little effect on the harvest rates required to bring about extinction within 150â200 yr, indicating that rapid extinction was an inevitable response to human hunting, irrespective of the initial population of moa.
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Michael C. Granatosky, Eric J. McElroy, Pierre Lemelin, Stephen M. Reilly, John A. Nyakatura, Emanuel Andrada, Brandon M. Kilbourne, Vivian R. Allen, Michael T. Butcher, Richard W. Blob & Callum F. Ross (2019)
Variation in limb loading magnitude and timing in tetrapods.
Journal of Experimental Biology: jeb.201525 (advance online publication)
doi: 10.1242/jeb.201525
https://jeb.biologists.org/content/early/2019/11/26/jeb.201525
Comparative analyses of locomotion in tetrapods reveal two patterns of stride cycle variability. Tachymetabolic tetrapods (birds and mammals) have lower inter-cycle variation in stride duration than bradymetabolic tetrapods (amphibians, lizards, turtles, and crocodilians). This pattern has been linked to the fact that birds and mammals share enlarged cerebella, relatively enlarged and heavily myelinated Ia afferents, and Î-motoneurons to their muscle spindles. Tachymetabolic tetrapod lineages also both possess an encapsulated Golgi tendon morphology, thought to provide more spatially precise information on muscle tension. The functional consequence of this derived Golgi tendon morphology has never been tested. We hypothesized that one advantage of precise information on muscle tension would be lower and more predictable limb bone stresses, achieved in tachymetabolic tetrapods by having less variable substrate reaction forces than bradymetabolic tetrapods. To test this hypothesis, we analyzed hindlimb substrate reaction forces during locomotion of 55 tetrapod species in a phylogenetic comparative framework. Variation in species-means of limb loading magnitude and timing confirm that, for most of the variables analyzed, variance in hindlimb loading and timing is significantly lower in species with encapsulated versus unencapsulated Golgi tendon organs. These findings suggest that maintaining predictable limb loading provides a selective advantage for birds and mammals by allowing for energy-savings during locomotion, lower limb bone safety factors, and quicker recovery from perturbations. The importance of variation in other biomechanical variables in explaining these patterns, such as posture, effective mechanical advantage, and center-of-mass mechanics, remains to be clarified.
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https://www.nature.com/articles/s41598-019-53329-5.pdfWhile running, small animals frequently encounter large terrain variations relative to their body size, therefore, terrain variations impose important functional demands on small animals. Nonetheless, we have previously observed in lizards that running specialists can maintain a surprisingly good running performance on very uneven terrains. The relatively large terrain variations are offset by their capacity for leg adjustability that ensures a âsmooth rideâ of the centre of mass (CoM). The question as to how the effect of an uneven terrain on running performance and locomotor costs differs between species exhibiting diverse body build and locomotor specializations remains. We hypothesise that specialized runners with long hind limbs can cross uneven terrain more efficiently than specialized climbers with a dorso-ventrally flattened body and equally short fore and hind limbs. This study reports 3D kinematics using high-speed videos (325âHz) to investigate leg adjustability and CoM movements in two lacertid lizards (Acanthodactylus boskianus, running specialist; Podarcis muralis, climbing specialist). We investigated these parameters while the animals were running on a level surface and over a custom-made uneven terrain. We analysed the CoM dynamics, we evaluated the fluctuations of the positive and negative mechanical energy, and we estimated the overall cost of transport. Firstly, the results reveal that the climbers ran at lower speeds on flat level terrain but had the same cost of transport as the runners. Secondly, contrary to the running specialists, the speed was lower and the energy expenditure higher in the climbing specialists while running on uneven terrain. While leg movements adjust to the substratesâ variations and enhance the stability of the CoM in the running specialist, this is not the case in the climbing specialist. Although their legs are kept more extended, the amplitude of movement does not change, resulting in an increase of the movement of the CoM and a decrease in locomotor efficiency. These results are discussed in light of the respective (micro-)habitat of these species and suggest that energy economy can also be an important factor for small vertebrates.