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Smallest Mammals of the Las Vegas Wash

A list of small mammals found along of the Las Vegas Wash was originally published in the 1970s by Glen Bradley and Wesley E. Niles. Thirty years later, another small mammal survey was conducted in upland habitats by students from the University of Nevada, Las Vegas (UNLV) from July 2002 to July 2003. These are the only known surveys of small mammals focused in the history of the Wash.

Consequently, this indicates that bats and small mammals utilizing marsh habitats have not been studied in more than 30 years. Considered small mammals because they weigh less than a pound, these animals are most active at night and when people are not at the Wash, which it makes it hard to observe all of the different species unless an organized study is conducted.

House mouse found at the Las Vegas Wash
Cactus mouse (Peromyscus eremicus) found at the Las Vegas Wash.

One recommended action in the Las Vegas Wash Wildlife Management Plan calls for conducting baseline inventories to establish the abundance and diversity of wildlife that were not documented in surveys performed in the Wash between 1998 and 2007. These surveys documented the presence of birds, fish, reptiles, amphibians and small mammals in upland habitats. One of the new baseline surveys recommended was a small mammal survey focused on trapping in marsh habitats for the western harvest mouse. The trapping system used is a metal box with a door on one side that closes as soon as an animal enters.

In January 2004, the Las Vegas Wash Project Coordination Team began the first acoustic bat monitoring study in the Wash. The acoustic stations consisted of a microphone, an Anabat II bat detector and a Compact Flash Storage Zero-Crossings Analysis Interface Module (CF ZCAIM). All of these components were mounted on a three-meter metal pole secured to the ground with a concrete footing. The bats' echolocation calls were picked up by the microphone on the top of the pole and processed by the Anabat II detector. Once the calls were processed, the CF ZCAIM would record the call onto a memory card allowing staff to download and analyze the data. In the past five years of data collection, this method has detected the presence of 17 bat species.

The acoustic survey was very successful for determining the presence of different species but gave no indication of the sex, age or reproduction status of the bats. In April 2008, the Wash Team started a capture portion of the bat study. This survey used harp traps and mist nets to capture bats in revegetation sites. Once captured, biologists would record weight, forearm, ear, tragus and hind foot measurements, as well as the sex, age and reproduction status of each animal. After recording all data, the animals were released. Through this survey the Wash Team has identified eight bat species, all of which already had been detected through the acoustic survey. Staff discovered pregnant pallid bats (Antrozous pallidus), western yellow bats (Lasiurus xanthinus) and yuma myotis (Myotis yumanensis). Together, these two surveys have provided a complete study on the bats utilizing the Wash.

But bats weren't the only animals for which information was lacking for. The small mammal study conducted by UNLV revealed some of the animals, but not all because they did not have survey sites in the woodland-marsh areas of the Wash. Since Bradley and Niles listed species that were not found by UNLV, it was determined that a small mammal marsh survey was necessary to compliment the most recent inventory. The Wash small mammal study began in August 2009. In the first round of the survey, the western harvest mouse (Reithrodontomys megalotis), the deer mouse (Peromyscus maniculatus) and the white-tailed antelope squirrel (Ammospermophilus leucurus) were all identified. These three animals were listed by Bradley and Niles but were not seen by UNLV. It is exciting to realize that animals that were here in the 1970s are still using the Wash as their home today.

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Las Vegas Wash Project Coordination Team • 100 City Parkway, Suite 700 • Las Vegas, NV 89106 • (702) 822-3300