Thursday, 23 October 2014

Origins of the laboratory guinea pig

Newborn guinea pigs (Cavia porcellus)
(c) Per Svendsen
The domestication of guinea pigs dates back hundreds of years. Animals raised for food or the laboratory are of a species (Cavia porcellus) that is not found in the wild. Its closest relatives are the Brazilian guinea pig (C. aperea) and the montane guinea pig (C. tschudii) from western South America including Peru.

Analysis of mitochondrial DNA (here and here) clearly shows that the domestic guinea pig groups with the montane species. Furthermore, analysis of the skeletons of mummified guinea pigs from archaeological sites in Southern Peru and Northern Chile (here) indicates that domestication started in the Andes, the natural habitat of C. tschudii.

Karyotypes of Cavia tschudii male (left) and C. porcellus female (right)
From Walker et al. 2014 (here) (c) Laura I. Walker et al.
A recent paper (here) compared the chromosomes of the wild and domestic species and confirmed that differences arose in the course of this first phase of domestication.

Guinea pigs were imported by European traders in the 16th Century and underwent a second phase of domestication that resulted in the laboratory guinea pig. There was even a further phase of domestication in South America with two lineages of guinea pigs selected for improved meat quality.

Thursday, 9 October 2014

Plesiorycteropus was a giant tenrec

Innomate (pelvic bone) of Plesiorycteropus an extinct mammal
from Madagascar (Drawing by Forsyth Major 1908)
The recent fauna of Madagascar included species that were much larger than those found today an example being the gorilla-sized lemur Megaladapis. One of these fossils Plesiorycteropus has defied interpretation. It has been compared to the aardvark and other myrmecophagous (termite-eating) mammals such as pangolins and anteaters.

The most thorough study of the subfossil material, by McPhee (here), defined two species, P.  madagascariensis and P. germainepetterae, and erected a new Order Bibymalagasia to accomodate them. More recently Asher (here) assigned Plesiorycteropus to the superordinal clade Afrotheria.

Phylogenetic analysis based on collagen sequencing has
Plesiorycteropus as sister to the tenrecs from Buckley 2013 (here)
A recent study (here) relied on molecular sequencing of collagen from a fossil Plesiorycteropus and a range of living mammals. The technique, which uses soft-ionization mass spectroscopy, is a promising alternative to sequencing of ancient DNA. The resultant tree confirmed that Plesiorycteropus belonged to Afrotheria but related to tenrecs rather than the aardvark. Therefore it is part of the radiation from a single colonisation event (Sweepstakes Distribution).

Tailless tenrec (Tenrec ecaudatus) the largest Malagasy tenrec
weighs about 1 kg. Photo by Markus Fink (GNU Free Documentation Licence)
Estimated size of Plesiorycteropus was 6-18 kg so it certainly was a giant compared with living Malagasy tenrecs and the giant otter shrew which weigh about 1 kg.

Wednesday, 1 October 2014

Picturing Madagascar's past

Cover features elephant birds Aepyornis hildebrandti 
and the giant lemur Archaeoindris fontoynontii
Extinct Madagascar by Steven M. Goodman and William L. Jungers has just been published by  The University of Chicago Press (ISBN-13 978-0-226-14397-2). As implied by the subtitle Picturing the Island's Past the text centres around 20 superb colour plates by the Bulgarian artist Velizar Simeonovski each depicting the fauna found at a particular fossil site.

The emphasis is on the larger species of reptiles, birds and mammals that became extinct and the authors explore possible causes ranging from climate change to hunting. This bias means that smaller mammals, such as tenrecs and endemic rodents, are scarcely mentioned except in the synoptic tables.

Aldabra giant tortoises - relatives of the extinct Malagasy species
Muhammad Mahdi Karim (Gnu Free Documentation Licence)
However, I found it fascinating to learn that the extinct giant tortoises (Aldabrachelys spp.) are likely to have been important grazers and that the extant species on the Aldabra Atoll represent a greater biomass per square km than the mammals on the Serengeti grasslands.

Malagasy crowned eagle piercing the shoulder blades of a sloth lemur
The chapter on an extinct raptor, the Malagasy crowned eagle Stephanoaetus mahery, was exciting; by analogy with the extant African species S. coronatus, it likely was a predator of large lemurs including the likewise extinct sloth lemurs (Palaeopropithecus spp.). But the book's plan makes it harder to find a coherent account of, for example, the "Madagascar aardvark" Plesiorycteropus madagascariensis. No reference is made to a study showing that this enigmatic species belongs to the Afrosoricida and thus is related to the tenrecs and golden moles (here).

Apart from the plates, figures are in grey tone. They include historical photos such as Grandidier's 1898-99 expedition. However, even more recent photos lack sharpness and contrast, perhaps due to the quality of paper chosen.

The introductory chapters give useful information on the geology and history of the island including the origins of the Malagasy people from Austronesians and Africans. In general the impact of humans in past centuries may have been overstated. The chapter on elephant birds concludes there is no evidence they were hunted to extinction and some evidence they may have been victims of climate change. The real and frightening aspect is the enormous habitat loss that has occurred in the last 60 years and that is still ongoing.