Author Spotlight: How soil features are shaping the bite force and skull morphology in subterranean rodents

17 01 2017

The role of soil features in shaping the bite force and related skull and mandible morphology in the subterranean rodents of genus Ctenomys (Hystricognathi: Ctenomyidae)

L.R. BorgesR. MaestriB.B. KubiakD. GalianoR. Fornel4 and T.R.O. Freitas

Tuco-tucos (genus Ctenomys) are subterranean rodents widespread in the southern cone of South America. They are members of the caviomorph lineage (e.g. the guinea pig and their relatives), which arrived in South America ~50 million years ago via transoceanic dispersal from Africa. Today, more than 60 species of Ctenomys are described.

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Ctenomys minutus, a species of tuco-tuco. Photo by Daniel Galiano

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Another species of tuco-tuco, Ctenomys ibicuiensis. Photo by Daniel Galiano

To live underground, the tuco-tucos must be able to excavate the soil. They use both claws and teeth to break up the soil, cut roots, tubers and other plant material, therefore opening their way to live underground. Adaptations to excavate with the incisors are particularly well demonstrated in this group. As soil features change, it is possible that distinct adaptations arise – the more compact the soil, the harder it is for digging. The question we asked ourselves was: does species living in more compact (harder) soils have a stronger bite force than species living in less compact (soft) soils?

In our paper published in Journal of Zoology we tried to answer this question. We used Freeman and Lemen’s bite force index (a useful formula to estimate bite force of rodents when direct measurements are not available) to estimate bite force values for 24 Ctenomys species, and we used a bulk density variable for soil compaction at each species’ distribution. Morphometric geometric techniques were applied on 1,122 specimens of the same species to investigate skull and mandible features correlated with bite force.

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Figure 1 from the article by Borges et al.: Landmarks used for measuring the shape of the skull and the mandible of the tuco-tucos.

We found that the species with strong bite force values do tend to occur in highly compact soils, while species with low bite force values tend to occur in less compacted soils. However, it turns out that species with low bite force values are also found in highly compacted soils. This makes us believe that different species developed different strategies to manage the excavation process. While some species probably rely mostly on their teeth (those species with high bite forces values occupying harder soils), others may rely on other distinct strategies to excavate, using their claws for instance, making these species able to have weaker bite force even though inhabiting highly compacted soils. We also discovered that a wider skull and a robust mandible are associated with the strongest bites, while an elongated skull and mandible are correlated with the weakest bite forces, in what seems to be a recurrent pattern for rodents and other mammal species.

Our next step is to figure out how appendages (limbs, the shoulder blade) contribute to excavation in species of tuco-tucos.

Renan Maestri

 

 





HIDDEN GEM: Contributions to a Knowledge of the Hemipterous Fauna of St. Helena, and Speculations on its Origin

12 12 2016

The Zoological Society of London has been publishing scientific papers in zoology since 1830, and our backfiles contain a wealth of ‘hidden gems‘ written by early explorers and zoologists. This article by F. Buchanan White, M.D., F.L.S., was published in 1878 in the Proceedings of the Zoological Society of London, a predecessor of Journal of Zoology. It is a fascinating early article on biogeography that reads almost like a detective story, speculating on the origin of the native fauna and flora of St Helena, a remote island in the middle of the South Atlantic Ocean. As the article describes, St Helena is situated in extreme isolation, nearly 1200 miles from the African continent and 1800 miles from South America, and has no indigenous terrestrial mammals nor any land or freshwater amphibians, reptiles or fish. However, as summarised by Buchanan White, naturalists such as T.V. Wollaston and J.C Meliss had previously reported having found native terrestrial invertebrates on the island, such as land molluscs, various Coleoptera and Hemiptera, as well as spiders and scorpions, many of which were endemic, or ‘peculiar’, to the island. Furthermore, Meliss as well as J.D. Hooker had studied the flora of St Helena and had found 77 species of plants that appeared to be ‘absolutely peculiar’.

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P.Z.S. 1878, Plate XXXI: ‘Hemiptera of St Helena’

These findings inevitably induced the question ‘Whence and by what means came this very peculiar fauna and flora?’, presenting a real puzzle for the naturalists of the day, and in this beautifully written article Buchanan White summarizes and discusses the main proposed theories, or ‘speculations’, about the geographical origins of the indigenous fauna and flora of St Helena, and the mechanisms by which they have ended up on this remote island in the middle of the ocean. The author then proposes his own theory, involving the glacial period and possible former islands acting as ‘stepping stones’ for the fauna to spread to the island, referring to ideas presented earlier by other naturalists such as Wallace and Darwin. Buchanan White concludes the paper with species descriptions of Hemiptera from St Helena, collected by Wollaston during his 6 months of exploration of the island. By the time of the writing of this article, Wollaston had sadly passed away, and before his death he had asked Buchanan White to describe all the new species in his collection ‘in a single paper and not piecemeal’, resulting in this remarkable paper from our archives which you can access and read for free.

Elina Rantanen





Author Spotlight: Comparison of Two Alternative Adhesive Pad Configurations in Geckos

4 11 2016

Left in the dust: differential effectiveness of the two alternative adhesive pad configurations in geckos (Reptilia: Gekkota)

Anthony P. Russell and Michel-Jean Delaugerre

The ability of geckos to scale smooth surfaces fascinates both scientists and the public at large – holidays taken in warm climates are often enlivened by geckos roaming in the evenings on the walls of hotel rooms, apartments and restaurants. The toe pads of these adept climbers are composed of plate-like scales, which bear microscopic hair-like outgrowths creating reversible molecular bonds with the surfaces on which they move. The form of these toe pads has long been used by scientists to distinguish between different types of geckos; the names of the genera often reflect this (for example, Hemidactylus [half-toe], Lepidodactylus [scaly-toe], Phyllodactylus [leaf-toe]).

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Toe pad configuration of the Moorish gecko (Tarentola mauritanica). Photo by A. Leoncini

One of us (A.P.R.) has been studying gecko evolution and the form and the function of their adhesive apparatus for over 40 years, and has always been aware of two basic “designs” of this system – either a series of broad, overlapping sheets, like the blades of a Venetian blind, along almost the whole of the underside of the toe, or as one or two pairs of squarish, leaf-like plates at the very tips of the toes. The reason for the existence of these two different patterns has, however, defied explanation, although it has been proposed that the leaf-like pattern appears to be particularly associated with rocky surfaces (although the alternative pattern is also encountered among species occupying this type of habitat). What was needed to bring us closer to an answer was a more in-depth ecological study in an environment that would reveal whether there were potential functional differences between these two patterns.

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Toe pad configuration of the European leaf-toed gecko (Euleptes europaea). Photo by A. Leoncini

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The ‘dusty’ surface of the prasinite rock. Photo by Michel-Jean Delaugerre

The other one of us (M-J. D.) noted that on Giraglia, a very small, uninhabited island off the north Corsican coast, an otherwise very aggressive invasive gecko species, the Moorish gecko (Tarentola mauritanica), had colonized it in about 1950, when a concrete building was erected to house the power-plant for the newly-automated lighthouse. The Moorish gecko, which has the familiar pattern of gecko adhesive pads, with the plates extending along much of the length of each toe, likely arrived with the construction materials and established itself on the new building. In the subsequent 65 years, however, the species has failed to make inroads on the rest of the island. Already present, and native to the island, was the European leaf-toed gecko (Euleptes europaea), which elsewhere has not been able to withstand invasion by the Moorish gecko. The leaf-toed gecko has not colonized the newly-erected concrete building, but is widespread on the rocky outcrops of the island, and on the two previously-existing buildings (the 19th century lighthouse and a 16th century lookout tower), both of which are constructed from rock excavated on the island. That rock is prasinite, a metamorphic schist that is crumbly and coated with “dust” derived from the microscopic particles that weather from its surface.

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The Giraglia island. Photo by Michel-Jean Delaugerre

If the Moorish gecko is placed onto the prasinite rock, it has great difficulty moving and clinging, because its large toe pads become fouled with the released dust. By contrast, on such dusty surfaces the toe pads of the leaf-toed gecko do not become fouled because they can be curled away from the surface, leaving the claws to provide grip. The Moorish gecko, because of the highly specialized movements of its toes that bring about attachment and detachment of its large adhesive pads, cannot use its claws independently of the pads and thus cannot negotiate, and is therefore unable to colonise, the dusty prasinite surface.

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Moorish gecko (Tarentola mauritanica). Photo by Michel-Jean Delaugerre

The fortuitous combination of circumstances on Giraglia (two species of gecko with different adhesive pad structure and a dusty rocky surface) has provided a window into the functional differences between the two adhesive pad configurations. Our work has provided the foundation for more detailed observations of how geckos with these two toe pad types might subdivide surfaces on which they move in different parts of the world.





Celebrating the 50th Anniversary of Journal of Zoology

11 10 2016

cover-wallace-1865-proceedingsThe Zoological Society of London, founded by Sir Stamford Raffles in 1826, has published scientific papers in zoology since 1830, first as Proceedings and Transactions of the Zoological Society of London, and in 1966 these publications were merged to form the Journal of Zoology. We are delighted to celebrate the 50th anniversary of our journal by releasing a free Virtual Issue of 12 highly cited and influential papers published in Journal of Zoology over the past 50 years.

jzo-anniversary-issue-oct-2016-coverFurthermore, to celebrate this anniversary our October issue consists of a special selection of new research articles considered particularly interesting to our readers. Many of these articles are also featuring in the anniversary edition of the Journal of Zoology podcast. In this new episode you will hear authors summarizing the principal aims and findings of their studies, ranging from investigating the maximum length of giant squids, 3D-modeling of lion paws and tracks, and sexual dimorphism in an isopod crustacean with horns, to studying anti-predator colour adaptations in a blue-tailed lizard, anthropogenic food resource use in brown bears, tissue regeneration in unisexual salamanders and decomposition of ostrich carcasses, with implications for studying dinosaur fossils.lizard_podcast

We hope you will enjoy these special features and celebrate with us 50 years of Journal of Zoology!





Author Spotlight: The Need for New Categorizations of Dietary Specialism in Animals

13 09 2016

Emilio Pagani-Núñez, Craig A. Barnett, Hao Gu & Eben Goodale

Guangxi University, Nanning, Guangxi Zhuang Autonomous Region, People’s Republic of China

 

Moreover, it is not understood when a species will evolve a particular niche width through the device of being polymorphic with phenotypes which are resource specialists, monomorphic with one generalist phenotype, or some intermediate condition.                                                 J. Rouhgarden (1974) The American Naturalist, 108 (962): 429-442

 

Certain ideas are rapidly and broadly accepted by the scientific community, sometimes even by a whole society.  This is the case of the concepts of ‘generalism’ and ‘specialism’, which have improved our understanding of ecological and evolutionary processes in nature.  However, to some extent, many authors have stressed the need to review and reformulate the existing theory about diet specialization.  This motivated us to write our review article “The need for new categorizations of dietary specialism incorporating spatio-temporal variability of individual diet specialization”, published in the September 2016 issue of the Journal of Zoology.

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Great tit nestling (Parus major), photo by Emilio Pagani-Nunez

At present, there is no consensus among researchers on a definition of diet specialization. We searched papers defining great and blue tits as ‘generalists’ or ‘specialists’ and we observed a great inconsistency among authors in how they classified the two species.  Traditionally, diet specialization was conceptualized as a continuous gradient from totally generalist species to completely specialized species that were behaviourally and physically adapted to eat specific food types. However, the discovery of individual diet specialization (IDS) challenges the traditional gradient of specialization within and among species. IDS means that, within species and populations, different individuals can specialize in the exploitation of different subsets of the available resources.

Species with broad diets and widespread distributions can provide useful examples of how species theoretically considered as generalists can show a surprising level of specialization at both the population and the individual level. Conversely, irrespective of distribution range, species inhabiting variable environments can evolve strikingly variable levels of specialization among individuals from both spatial and temporal perspective. In this paper, we reviewed the literature on specialization in great tits and blue tits.  Both species are considered caterpillar specialists at the centre of their ranges, but this reliance on caterpillars declines at the periphery of their ranges (e.g. the Mediterranean).  Specifically, we found that great tits show a higher level of IDS than blue tits do, developing new dietary specializations when the most common prey is not available.

Another issue that has not been well explored in the literature is the role of foraging innovations in promoting novel dietary specializations. Research in foraging innovations also has a long history of research (e.g. the opening of milk bottle tops by great and blue tits in United Kingdom). There have been few attempts to link foraging innovations to the capacity to evolve contrasting levels of specialization from the species to the individual level. However, the capacity for foraging innovations may explain why certain ‘generalist’ species show such contrasting levels of specialization between populations and individuals.

Here, we introduce a new categorization of specialization that takes into account all these factors.  We define obligate specialists as species composed of individuals that are primarily adapted to exploiting a single resource (e.g. hummingbirds). Facultative specialists are species in which individuals are adapted to exploit a single food type or niche, but occasionally they can exploit alternative food resources (e.g. birds of prey). Facultative generalists are species composed of individuals that are able to develop novel specializations to preferably exploit alternative resources, (e.g. great tits and sea otters).  Finally, obligate generalists are able to exploit a wide variety of resources (or one if it is very abundant), with a limited capacity to incorporate foraging innovations (e.g. many passerine species).

By publishing our new classification, our aim is to provide clearer and more workable definitions of diet specialization within and among populations and species. We also are interested in initiating a debate among ecologists about the necessity of incorporating IDS as well as spatial and temporal variation into studies of foraging ecology.  This general framework is also attractive because it is applicable to a wide variety of taxa and makes comparison among taxa easier.

 





New Journal of Zoology Podcast

10 08 2016

A new episode of the Journal of Zoology podcast is now available and you can listen to it here.

JZO_butterfly_podcast_imageIn this episode, Tim Thurman and Brett Seymoure talk to us about their study on two mimetic butterflies and how similar they appear to the eyes of their predators, we will learn from Lucy Lush how biologging technology can improve our ability to record and analyse wild animal behaviour, and we are told by Niklas Björklund how ants can help protect conifer seedlings from pests in forest plantations via non-consumptive interactions with pine weevils.

You may subscribe to iTunes to receive the latest Journal of Zoology podcasts.





Winner of the 2015 Journal of Zoology ‘Paper of the Year’ award

2 08 2016

JoZ Blog

Biochemical correlates of aggressive behavior in the Siamese fighting fish

M. D. Regan, R. S. Dhillon, D. P. L. Toews, B. Speers-Roesch, M. A. Sackville, S. Pinto, J. S. Bystriansky and G. R. Scott

 

Aggressive interactions between individuals of the same species can result in the evolution of exaggerated body traits that improve success in these interactions, and subsequently, access to resources such as food and mates. Although conspicuous morphological adaptations such as antlers are usually what come to mind, metabolic processes that occur hidden within cells are required to sustain aggressive behaviour, so their enhancement may also be important for a successful outcome.

Siamese fighting fish, Betta splendens Siamese fighting fish, Betta splendens; photo by Dave Toews

With this in mind, we designed a study to examine the intersection of aggressive behaviour and metabolic biochemistry using the Siamese fighting fish, a staple of the world’s pet shops. Male Siamese fighting fish are notoriously…

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