Geology topics

Eric J. Sargis

Publications and source records attributed to Eric J. Sargis.

8 recordsLinked to original sources

USGS2021

Ecogeographic variation and taxonomic boundaries in Large Treeshrews (Scandentia, Tupaiidae: Tupaia tana Raffles, 1821) from Southeast Asia

The Large Treeshrew, Tupaia tana Raffles, 1821, is a small mammal (~205 g) from Southeast Asia with a complicated taxonomic history. Currently, 15 subspecies are recognized from Borneo, Sumatra, and smaller islands, and many were originally differentiated based on minor pelage differences and small sample sizes. We explored intraspecific variation in T. tana using quantitative osteological data obtained from the hands and skulls of museum specimens. Multivariate analyses reveal extensive overlap among T. tana populations in morphospace, indicating that the majority of currently recognized subspecies are not morphometrically distinct. In contrast, the separation between Bornean and Sumatran populations of T. tana is sufficient to recognize them as different subspecies. Comparisons of Bornean specimens to those on small, offshore islands reveal that the latter average smaller body size. This pattern is inconsistent with Foster’s island rule, which predicts that island populations of small mammals (< 5 kg) will average larger body size relative to mainland forms. A similar lack of support for ecogeographic rules has been noted in T. glis (Diard, 1820), suggesting that these “rules” are poor predictors of geographic variation in treeshrews.

Journal of Mammalogy↗

USGS2021

A new species of tree hyrax (Procaviidae: Dendrohyrax) from West Africa and the significance of the Niger–Volta interfluvium in mammalian biogeography

Tree hyraxes ( Dendrohyrax ) are one of only three genera currently recognized in Procaviidae, the only extant family in the mammalian order Hyracoidea. Their taxonomy and natural history have received little attention in recent decades. All tree hyrax populations of Guineo-Congolian forests of Africa are currently treated as a single species, Dendrohyrax dorsalis , the western tree hyrax, but many other groups of mammals distributed across this large biome have been shown to consist of several different species, each restricted to a distinct biogeographical region. We analysed variation in loud-call structure, pelage colour, skull morphometrics and mitochondrial genomes in populations across much of the range of D. dorsalis . This integrative approach uncovered considerable cryptic variation. The population found between the Niger and Volta Rivers in West Africa is particularly distinctive, and we describe it herein as a new species. Our study highlights the need to revise the taxonomy of the genus Dendrohyrax in light of modern systematics and current understanding of its distribution. It also adds to a growing body of evidence that the Niger–Volta interfluvium has a distinct meso-mammal fauna. Unfortunately, the fauna of this region is under major threat and warrants much greater conservation attention.

Zoological Journal of the Linnean Society↗

USGS2021

The limitations of external measurements for aging small mammals: The cautionary example of the Lesser Treeshrew (Scandentia: Tupaiidae: Tupaia minor Günther, 1876)

Age is a basic demographic characteristic vital to studies of mammalian social organization, population dynamics, and behavior. To eliminate potentially confounding ontogenetic variation, morphological comparisons among populations of mammals typically are limited to mature individuals (i.e., those assumed to have ceased most somatic growth). In our morphometric studies of treeshrews (Scandentia), adult individuals are defined by the presence of fully erupted permanent dentition, a common criterion in specimen-based mammalogy. In a number of cases, however, we have had poorly sampled populations of interest in which there were potentially useful specimens that could not be included in samples because they lacked associated skulls. Such specimens typically are associated with external body and weight measurements recorded by the original collectors, and we sought to determine whether these data could be used successfully as a proxy for age or at least to establish maturity. We analyzed four traditional external dimensions (head-and-body length, tail length, hind foot length, and ear length) and weight associated with 103 specimens from two allopatric populations of the Lesser Treeshrew ( Tupaia minor Günther, 1876) from Peninsular Malaysia and from Borneo, which we treated as separate samples (populations). Individuals were assigned to one of eight age categories based on dental eruption stage, and measurements were compared among groups. In general, mean sizes of infants and subadults were smaller than those of adults, but the majority of subadults fell within the range of variation of adults. The large overlap among infants, subadults, and adults in external measurements and weight indicates that such measures are poor proxies for age in this species, probably for treeshrews in general, and possibly for other small mammals. This has significant implications for any investigation wherein relative age of individuals in a given population is an important consideration.

Journal of Mammalogy↗

USGS2020

Coming of age: Morphometric variation in the hand skeletons of juvenile and adult Lesser Treeshrews (Scandentia: Tupaiidae: Tupaia minor Günther, 1876)

Morphometric analyses of the manus skeleton have proven useful in understanding species limits and morphological divergence among tupaiid treeshrews (Scandentia: Tupaiidae). Specimens in these studies are typically limited to mature individuals with fully erupted permanent dentition, which eliminates potentially confounding variation attributable to age, but can also exclude rare taxa and small island populations that are poorly represented in systematic collections. To determine the real limits associated with including immature animals in such studies, we used multivariate analyses to study sexual and age variation of the manus skeleton within two allopatric populations of the Lesser Treeshrew (Tupaia minor Günther, 1876) from the Malay Peninsula and from Borneo that we treated as separate samples. Individuals were aged using dental eruption of the permanent dentition. We also recorded the degree of epiphyseal fusion of the bones of the manus based on x-rays of study skins. We then tested our ability to distinguish the two populations using a series of discriminant function analyses of hand measurements from samples that included varying proportions of immature individuals and adults. We found no evidence of sexual dimorphism in hand proportions, permitting us to combine females and males in our samples. Epiphyseal fusion of the metacarpals and phalanges typically occurs by the time the third molars have completely erupted, and fusion of the distal epiphyses of the radius and ulna typically occurs by the time the permanent fourth premolars are in place. There is occasional asynchrony between dental age and epiphyseal fusion. In both populations, the hands of most infants and subadults provide morphometric values within the range of variation of adults, although they are typically distributed in the lower part of the adult range and have the potential to bias the sample toward lower mean size. The inclusion of infants and subadults when attempting to discriminate between two taxa generally results in lower rates of correct classifications, although the rates increase as the sample of immature individuals is limited to older subadults. As a general rule, we recommend that infants and subadults continue to be excluded from analyses when exploring taxonomic boundaries among treeshrews. In cases of extremely small sample sizes of adults, however, older subadults—in which the permanent third premolars are erupting or in place—can be used with appropriate caution.

Journal of Mammalogy↗

USGS2018

Rule reversal: Ecogeographical patterns of body size variation in the common treeshrew (Mammalia, Scandentia)

There are a number of ecogeographical “rules” that describe patterns of geographical variation among organisms. The island rule predicts that populations of larger mammals on islands evolve smaller mean body size than their mainland counterparts, whereas smaller‐bodied mammals evolve larger size. Bergmann's rule predicts that populations of a species in colder climates (generally at higher latitudes) have larger mean body sizes than conspecifics in warmer climates (at lower latitudes). These two rules are rarely tested together and neither has been rigorously tested in treeshrews, a clade of small‐bodied mammals in their own order (Scandentia) broadly distributed in mainland Southeast Asia and on islands throughout much of the Sunda Shelf. The common treeshrew, Tupaia glis , is an excellent candidate for study and was used to test these two rules simultaneously for the first time in treeshrews. This species is distributed on the Malay Peninsula and several offshore islands east, west, and south of the mainland. Using craniodental dimensions as a proxy for body size, we investigated how island size, distance from the mainland, and maximum sea depth between the mainland and the islands relate to body size of 13 insular T. glis populations while also controlling for latitude and correlation among variables. We found a strong negative effect of latitude on body size in the common treeshrew, indicating the inverse of Bergmann's rule. We did not detect any overall difference in body size between the island and mainland populations. However, there was an effect of island area and maximum sea depth on body size among island populations. Although there is a strong latitudinal effect on body size, neither Bergmann's rule nor the island rule applies to the common treeshrew. The results of our analyses demonstrate the necessity of assessing multiple variables simultaneously in studies of ecogeographical rules.

Ecology and Evolution↗

USGS2016

Skeletal variation and taxonomic boundaries among mainland and island populations of the common treeshrew (Mammalia: Scandentia: Tupaiidae)

Treeshrews (order Scandentia) include 23 currently recognized species of small-bodied mammals from South and Southeast Asia. The taxonomy of the common treeshrew, Tupaia glis , which inhabits the Malay Peninsula south of the Isthmus of Kra, as well as a variety of offshore islands, has an extremely complicated history resulting from its wide distribution and subtly variable pelage. In our ongoing investigation of species boundaries in Tupaia , we compared island and mainland populations of T. glis using multivariate analyses. Specifically, we compared the skull and hand morphology of 13 island populations, most of which have been recognized as separate species or subspecies, to that of the mainland population. Island populations generally average smaller body size than those on the mainland, but none of the island populations are sufficiently distinct from the mainland population to warrant species recognition. This has important conservation implications for this widespread and morphologically variable species. It also highlights the potential role that ecogeographic explanations can play in understanding intraspecific variation, a role that should be considered in taxonomic studies and investigated further in T. glis and other treeshrews.

Malay Peninsula↗

USGS2014

Island history affects faunal composition: the treeshrews (Mammalia: Scandentia: Tupaiidae) from the Mentawai and Batu Islands, Indonesia

The Mentawai and Batu Island groups off the west coast of Sumatra have a complicated geological and biogeographical history. The Batu Islands have shared a connection with the Sumatran ‘mainland’ during periods of lowered sea level, whereas the Mentawai Islands, despite being a similar distance from Sumatra, have remained isolated from Sumatra, and probably from the Batu Islands as well. These contrasting historical relationships to Sumatra have influenced the compositions of the respective mammalian faunas of these island groups. Treeshrews (Scandentia, Tupaiidae) from these islands have, at various times in their history, been recognized as geographically circumscribed populations of a broadly distributed Tupaia glis, subspecies, or distinct species. We used multivariate analyses of measurements from the skull and hands to compare the island populations from Siberut (Mentawai Islands) and Tanahbala (Batu Islands) with the geographically adjacent species from the southern Mentawai Islands (T. chrysogaster) and Sumatra (T. ferruginea). Results from both the skull and manus of the Siberut population show that it is most similar to T. chrysogaster, whereas the Tanahbala population is more similar to T. ferruginea, confirming predictions based on island history. These results are further corroborated by mammae counts. Based on these lines of evidence, we include the Siberut population in T. chrysogaster and the Tanahbala population in T. ferruginea. Our conclusions expand the known distributions of both the Mentawai and Sumatran species. The larger geographical range of the endangered T. chrysogaster has conservation implications for this Mentawai endemic, so populations and habitat should be re-evaluated on each of the islands it inhabits. However, until such a re-evaluation is conducted, we recommend that the IUCN Red List status of this species be changed from ‘Endangered’ to ‘Data Deficient’.

Mentawai;Batu Islands↗

USGS2013

Morphological distinctiveness of Javan Tupaia hypochrysa (Scandentia, Tupaiidae)

The common treeshrew, Tupaia glis, represents a species complex with a complicated taxonomic history. It is distributed mostly south of the Isthmus of Kra on the Malay Peninsula and surrounding islands. In our recent revision of a portion of this species complex, we did not fully assess the population from Java (T. “glis” hypochrysa) because of our limited sample. Herein, we revisit this taxon using multivariate analyses in comparisons with T. glis, T. chrysogaster of the Mentawai Islands, and T. ferruginea from Sumatra. Analyses of both the manus and skull of Javan T. “glis” hypochrysa show it to be most similar to T. chrysogaster and distinct from both T. glis and T. ferruginea. Yet, the Javan population and T. chrysogaster have different mammae counts, supporting recognition of T. hypochrysa as a distinct species. The change in taxonomic status of T. hypochrysa has conservation implications for both T. glis and this Javan endemic.

Journal of Mammalogy↗