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Shared phylogenetic breaks often are associated with clear geographic barriers but some common phylogeographic breaks may lack obvious underlying mechanisms. A phylogenetic break involving multiple taxa was found in the Baja California Peninsula that was associated with a past sea barrier. However, geological evidence is lacking for this barrier's past existence, and despite its current absence, the genetic breaks have persisted. This work explores the relationships between the current climatic niches for matrilineages of 11 vertebrate species as a possible explanation for the current geographic partitioning of matrilineages. We evaluated the climatic occupancy of each matrilineage through ecological niche models, background similarity, niche overlap, niche divergence, and Mantel tests. We found disparities in the climatic occupancy between north and south matrilineage of each taxon. Northern matrilineages are associated with lower temperatures and winter rains, while southern matrilineages reside in areas with higher temperatures and summer rains.

Desert shrews of the genus Notiosorex comprise four species with morphological characteristics that are difficult to distinguish among the species. Indeed, N. cockrumi was described using only genetic markers. Based on molecular divergence documented in N. crawfordi, we hypothesize that a fifth species is present in the Baja California peninsula. Genetic variation at the species level was analyzed using individuals from locations west of the Colorado River in the Baja California peninsula, Mexico, and California, United States. Molecular markers of mitochondrial origin (cytochrome b, 1,140 bp; cytochrome c oxidase subunit I, 542 bp; and cytochrome c oxidase subunit III, 672 bp), as well as the nuclear intron 7 of the beta fibrinogen gene (385 bp) were used to construct a phylogeny for species of the genus Notiosorex. Genetic distances of 12.46–15.58% between west and east of the Colorado River were obtained using p-distance models. Our phylogenetic analyses showed almost identical topologies, placing populations from west of the Colorado River in three monophyletic clades with high bootstrap support values. Results of molecular phylogenetic identity among shrews of the genus Notiosorex support the existence of an undescribed, polytypic species of Notiosorex west of the Colorado River.

Adaptation and evolution of terrestrial vertebrates inhabiting islands have been the topic of many studies, particularly those seeking to identify trends or patterns in body size in mammals, albeit not necessarily in shape, in relation to mainland populations. The spiny pocket mouse, Chaetodipus spinatus, is distributed in the Baja California peninsula and its surrounding islands. Insular populations became isolated ∼12,000 due to changes in sea level; these populations' matrilinear (mitochondrial) DNA shows minor interpopulation variation. We tested the hypothesis that adaptation and evolution in these island populations involve variation in both skull size and skull shape (using geometric morphometrics) relative to mainland populations, rather than only in size as previously assumed. A total of 363 specimens from 15 insular and peninsular populations were used in analysis of the skull length and geometric morphometric analyses. Our findings revealed significant differences related to skull size among population. The skull shape analyses showed two significantly different morphotypes: one for all island specimens and one for all mainland samples. Our analyses support the hypothesis that insular populations may not only vary in size relative to mainland populations, but may also show variations in shape, regardless of differing conditions across islands.

Leucism is a partial hypopigmentary congenital disorder previously recorded in several species of mammals. This abnormal coloration is unusual in the wild. In August 2013, in Baja California, México, we collected 2 Peromyscus fraterculus (one female and one male) exhibiting leucism. Leucism has not been previously reported in the genus Peromyscus; therefore, we documented the first known record. Leucism is expressed in natural populations having low genetic diversity and may increase selective pressure on affected individuals.

Arthropods are the primary dietary constituents of species of the genus Myotis. The genus comprises 3 ecomorphotypes of polyphyletic origin, each associated with a different foraging strategy: aerial, trawling, and gleaning, related to the subgenera Selysius, Leuconoe, and Myotis, respectively. We explored the extent to which differences in diet characterized these ecomorphotypes. Based on a broad review of the literature, we classified the diet of species of Myotis based on hardness and vagility of consumed prey. A significant negative relationship was found between the percent volume consumption of hard and soft arthropods and between the aerodynamic characteristics of fast- and slow-flying prey. A cluster analysis yielded 3 groups of Myotis species based on their diet: 1) those for which hard prey represent more than 80% of the volume of excreta; 2) those for which hard prey represent 45% to 80% of the volume of excreta; and 3) those for which hard prey represent less than 45% of the volume of excreta. These 3 groups are related to bat size and consistent with the recognition of 3 ecomorphotypes. Nonetheless, some species may display flexibility in diet composition depending on food availability. More specifically, larger species that consume hard prey also consume soft prey, whereas smaller bats may be unable to consume hard prey because of biomechanical limitations. Regardless of their phylogenetic lineages, latitudinal distribution, or biogeographic affinity, species of Myotis of similar size and morphology consume arthropods with similar characteristics.

Santa Catalina Island has an endemic mouse, Peromyscus slevini, which is the only native rodent species on the island. However, specimens of P. fraterculus have been recorded on the island. P. fraterculus is the most common species of Peromyscus off Santa Catalina Island in the Baja California peninsula. The records show the absence of P. slevini in the 1990s and an increasing number of P. fraterculus during the 2000s. P. slevini has recently been collected in 2007. The current situation in Santa Catalina Island shows a strong expansion of P. fraterculus and the restriction of P. slevini to the canyons. This study confirms for the first time the recent invasion of a peninsular native species in one island on the Gulf of California. Peromyscus fraterculus is a better desert adapted species in contrast to P. slevini. Under those conditions, in the near future, P. slevini could be extinct.

Peromyscus fraterculus (Miller, 1892) is a small rodent commonly called the northern Baja deermouse. Its pelage is dark, the tail is considerably longer than the body, and the number of caudal vertebrae ranges from 30 to 34. The geographic distribution of P. fraterculus includes the area west of the Colorado River, from southern California in the United States of America southward to the Baja California Peninsula, Mexico, south of La Paz Isthmus. It is found in a broad variety of habitats from sea level to 1,175 m. P. fraterculus does not have special conservation status, although the island populations have conservation issues, mainly due to the introduction of non-native species. It is listed as “Least Concern” (LC) by the International Union for Conservation of Nature and Natural Resources.

The pocket mouse from the southern Baja California peninsula, Chaetodipus dalquesti, is synonymized to Chaetodipus ammophilus, following the Principle of Priority of the Code of Zoological Nomenclature, because C. ammophilus was described before C. dalquesti. The subspecies are reassigned.