Calomys expulsus is endemic to the Cerrado and the Caatinga (contra Gurgel-Filho et al. 2015). All but one known records attributable to this species are located within these biomes; the exceptional record comes from a site likely harboring transitional conditions, in terms of physiognomy and climate, between those of the Caatinga and Atlantic Forest, in the Brazilian state of Pernambuco (see Gurgel-Filho et al. 2015, who referred to this species as Calomys mattevii, which we consider a junior synonym of Calomys expulsus; see “Taxonomy”, below). Specimens attributed to Calomys expulsus have been recorded in the Brazilian states of Bahia, Goiás, Minas Gerais, Pernambuco, Piauí, Sergipe, and in the Distrito Federal (Bonvicino and Almeida 2000, Bonvicino et al. 2003, Almeida et al. 2007, Haag et al. 2007, Bonvicino et al. 2012, Bezerra et al. 2014, Gurgel-Filho et al. 2015 and references therein, Nogueira et al. 2015). According to Gurgel-Filho et al. (2015), unpublished results of a phylogenetic study recovered samples of “Calomys mattevii” (=Calomys expulsus) in a clade in which samples from the states of Ceará and Tocantins – for which we presume no karyotype were available – were also included.

We provisionally consider the recently described Calomys mattevii as a junior synonym of Calomys expulsus. Gurgel-Filho et al. (2015) asserted that two specimens karyotyped by Geise et al. (1996), collected in Lagoa Santa (the type locality of Calomys expulsus), Minas Gerais, with 2n=36/FN=66, correspond to Calomys expulsus; however, Gurgel-Filho et al. (2015) did not examine these specimens. These authors alleged that the karyotype 2n=66/FN=68, widely attributed by authors to Calomys expulsus (e.g., Bonvicino and Almeida 2000, Mattevi et al. 2005, Haag et al. 2007, Bezerra et al. 2014, Salazar-Bravo 2015), would have to correspond to a different species (other than Calomys expulsus), which they described as Calomys mattevii. Although it is plausible that the specimens reported by Geise et al. (1996) were indeed Calomys expulsus, this cannot be assumed as certain because multiple species of Calomys might occur in Lagoa Santa (not only Calomys expulsus). In fact, Lagoa Santa is also the type locality of Calomys tener (see Winge 1887, Salazar-Bravo 2015), and the identity of the specimens from Lagoa Santa that were the basis of Geise et al.’s (1996) report has also been attributed to Calomys cerqueirai (Bonvicino et al. 2010; see also Colombi and Fagundes 2015, Salazar-Bravo 2015 and citations therein). Examination of the specimens from which Geise et al. (1996) obtained the reported karyotypes, and comparisons of their morphologies with respect to type material of Calomys expulsus, would be necessary to evaluate whether Gurgel-Filho et al.’s (2015) conjecture is correct. Unfortunately, it is unclear whether the specimens karyotyped by Geise et al. (1996) were ever deposited in a zoological collection, and whether their morphology matches that of Calomys expulsus – see similar concern expressed by Bonvicino and Almeida (2000, p. 347). Moreover, the karyotype reported by Geise et al. (1996) was not ever part of a formal peer-reviewed publication; it was rather reported as an abstract from a presentation at a scientific meeting – the 42th National Congress of Genetics of the Sociedade Brasileira de Genética – that have permeated into the literature. In addition, to support their conjecture and decision to describe Calomys mattevii, Gurgel-Filho et al. (2015) disregarded the use of morphological data as relevant to assess the taxonomic status of Calomys mattevii. However, we argue that it is possible to use morphometrics to discriminate among some species of Calomys; albeit not an easy task, and might not always lead to evidence that allow unambiguous discrimination among the studied species, several authors have accomplished it in the past (e.g., Bonvicino and Almeida 2000, Bonvicino et al. 2003, Cordeiro-Estrela et al. 2006).

In summary, given the aforementioned uncertainties in the description of Calomys mattevii, we provisionally consider it a junior synonym of Calomys expulsus. Collecting, karyotyping, and sequencing Calomys from the type locality of Calomys expulsus (see above), and using this material in comparative analyses that should include typical, or at least topotypical, material of other members of the large-size group of the genus (i.e., Calomys callidus, Calomys callosus, Calomys cerqueirai, and Calomys tocantinsi) is a necessary step to evaluate the taxonomic status of Calomys mattevii. On the meantime, we consider that karyotype 2n=66/FN=68 corresponds to Calomys expulsus, following Bonvicino and Almeida (2000), who asserted that the morphology of their karyotyped specimens is congruent with morphological characters and measurements of the holotype of Calomys expulsus as reported by Winge (1887) and Hershkovitz (1962), respectively.

The red list of the IUCN ver 3.1 assigned the category “Least Concern” to Calomys expulsus (see Bonvicino and Geise 2008a). The species was not included in the official list of threatened species of Brazil (ICMBIO-MMA 2016).