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CURATOR
A pinboard by
Ariel Marcy

PhD Student, University of Queensland

PINBOARD SUMMARY

How did they do it? In just under 5 million years, rodents evolved into every major Aussie habitat!

Australian native rodents provide a fascinating case study. They are relatively new to the continent: about 5 million years (which is very short in evolutionary terms), yet they are the most species-rich family of mammals in Australia, beating out the marsupials despite their 100+ million years history on the continent. Australian rodents come in all sizes from the very small 6 gram delicate mouse to the enormous 1 kg giant white tailed rat. They thrive in all the major habitats from the desert-dwelling hopping mouse to the tropical prehensile-tail rat to the carnivorous water rat. They have also evolved unusual behaviors like the pebble-mound building pebble-mound mice and the nest-crafting cave-dwelling stick-nest rat.

How can I capture all of this diversity in a feasible PhD project? I will focus on the skulls because it encodes information about the individual’s size, ecology, behavior, as well as the location it was captured. I visit lots of museum collections and take a 3D digital photograph of each skull with a 3D scanner. My methodology is called 3D Geometric Morphometrics and it’s a process borrowed from engineering: first, I place points, called landmarks, all over the skull on places that can be reproduced for each specimen (like the tip of the front tooth) then, I use software to compare the coordinates of those landmarks to each other and across specimens. The results allow me to visualize how individuals and species differ from one another and from there I can make inferences about why their shapes are different. I can also visualize how shape changes with size, a major contributor to shape change, since shape doesn’t usually scale 1:1 so increasing size is often how species evolves a new shape. I can also look at what shape changes don’t occur with size and see if they correspond with say certain ecologies or certain behaviors (like burrowing) - here we can infer adaptation and compare across species in these same ecologies and/or behaviors.

Whew! Basically I want to know what happened when a small band of placental mammals arrived on a new continent with no placental mammals. I hope my thesis will contribute to the larger literature of shape evolution and to shape analysis in general since no one has looked at these species with the type of high-tech methods I’m using. In the future I hope to do more direct comparisons between the marsupial mammals and the placental mammals on the continent and ask why their patterns are so different.

3 ITEMS PINNED

Life in Burrows Channelled the Morphological Evolution of the Skull in Rodents: the Case of African Mole-Rats (Bathyergidae, Rodentia)

Abstract: Abstract African mole-rats are fossorial rodents that consist of five chisel-tooth digging genera (Heterocephalus, Heliophobius, Georychus, Fukomys, and Cryptomys) and one scratch digger (Bathyergus). They are characterized by striking physiological, morphological, and behavioral adaptations intimately related to their subterranean life. The influence of their mode of life in shaping the cranial morphology has yet to be evaluated in comparison to other Ctenohystrica, especially fossorial genera, which include the subterranean genera Spalacopus and Ctenomys. In our study, we seek to determine to what extent subterranean life affects the morpho-functional properties of the skull among fossorial ctenohystricans. 3D geometric morphometric analyses were performed on 277 skulls, encompassing 63 genera of Ctenohystrica, and complemented by biomechanical studies. African mole-rats and other subterranean Ctenohystrica, especially chisel-tooth diggers, have a short snout, a wide cranium with enlarged zygomatic arches, and a strongly hystricognathous mandible. Even if convergences are also manifest between most fossorial Ctenohystrica, subterranean rodents departed from the main ctenohystrican allometric trends in having a skull shape less size-dependent, but under stronger directional selection with intense digging activity as a major constraint. African mole-rats, notably chisel-tooth diggers, show important mechanical advantage for the temporalis muscles favoring higher forces at the bite point, while mechanical advantage of the superficial masseter muscles is lower compared to other Ctenohystrica. If subterranean species can be clearly discriminated based on their skull morphology, the intrinsic mosaic of anatomical characters of each genus (e.g., skull, teeth, and muscles) can be understood only in the light of their ecology and evolutionary history.AbstractAfrican mole-rats are fossorial rodents that consist of five chisel-tooth digging genera (Heterocephalus, Heliophobius, Georychus, Fukomys, and Cryptomys) and one scratch digger (Bathyergus). They are characterized by striking physiological, morphological, and behavioral adaptations intimately related to their subterranean life. The influence of their mode of life in shaping the cranial morphology has yet to be evaluated in comparison to other Ctenohystrica, especially fossorial genera, which include the subterranean genera Spalacopus and Ctenomys. In our study, we seek to determine to what extent subterranean life affects the morpho-functional properties of the skull among fossorial ctenohystricans. 3D geometric morphometric analyses were performed on 277 skulls, encompassing 63 genera of Ctenohystrica, and complemented by biomechanical studies. African mole-rats and other subterranean Ctenohystrica, especially chisel-tooth diggers, have a short snout, a wide cranium with enlarged zygomatic arches, and a strongly hystricognathous mandible. Even if convergences are also manifest between most fossorial Ctenohystrica, subterranean rodents departed from the main ctenohystrican allometric trends in having a skull shape less size-dependent, but under stronger directional selection with intense digging activity as a major constraint. African mole-rats, notably chisel-tooth diggers, show important mechanical advantage for the temporalis muscles favoring higher forces at the bite point, while mechanical advantage of the superficial masseter muscles is lower compared to other Ctenohystrica. If subterranean species can be clearly discriminated based on their skull morphology, the intrinsic mosaic of anatomical characters of each genus (e.g., skull, teeth, and muscles) can be understood only in the light of their ecology and evolutionary history.HeterocephalusHeliophobiusGeorychusFukomys,CryptomysBathyergusSpalacopusCtenomys

Pub.: 01 Jun '16, Pinned: 27 Jul '17

Getting a head in hard soils: Convergent skull evolution and divergent allometric patterns explain shape variation in a highly diverse genus of pocket gophers (Thomomys).

Abstract: High morphological diversity can occur in closely related animals when selection favors morphologies that are subject to intrinsic biological constraints. A good example is subterranean rodents of the genus Thomomys, one of the most taxonomically and morphologically diverse mammalian genera. Highly procumbent, tooth-digging rodent skull shapes are often geometric consequences of increased body size. Indeed, larger-bodied Thomomys species tend to inhabit harder soils. We used geometric morphometric analyses to investigate the interplay between soil hardness (the main extrinsic selection pressure on fossorial mammals) and allometry (i.e. shape change due to size change; generally considered the main intrinsic factor) on crania and humeri in this fast-evolving mammalian clade.Larger Thomomys species/subspecies tend to have more procumbent cranial shapes with some exceptions, including a small-bodied species inhabiting hard soils. Counter to earlier suggestions, cranial shape within Thomomys does not follow a genus-wide allometric pattern as even regional subpopulations differ in allometric slopes. In contrast, humeral shape varies less with body size and with soil hardness. Soft-soil taxa have larger humeral muscle attachment sites but retain an orthodont (non-procumbent) cranial morphology. In intermediate soils, two pairs of sister taxa diverge through differential modifications on either the humerus or the cranium. In the hardest soils, both humeral and cranial morphology are derived through large muscle attachment sites and a high degree of procumbency.Our results show that conflict between morphological function and intrinsic allometric patterning can quickly and differentially alter the rodent skeleton, especially the skull. In addition, we found a new case of convergent evolution of incisor procumbency among large-, medium-, and small-sized species inhabiting hard soils. This occurs through different combinations of allometric and non-allometric changes, contributing to shape diversity within the genus. The strong influence of allometry on cranial shape appears to confirm suggestions that developmental change underlies mammalian cranial shape divergences, but this requires confirmation from ontogenetic studies. Our findings illustrate how a variety of intrinsic processes, resulting in species-level convergence, could sustain a genus-level range across a variety of extrinsic environments. This might represent a mechanism for observations of genus-level niche conservation despite species extinctions in mammals.

Pub.: 12 Oct '16, Pinned: 27 Jul '17