Ammospermophilus Research Articles

Seed dispersal and seed fate in Joshua tree (Yucca brevifolia)

Joshua tree (Yucca brevifolia) is a charismatic symbol of the Mojave Desert. Despite its familiarity, we know little about the reproduction of this species, including mechanisms of seed dispersal. Here we examine mechanisms of seed dispersal and resulting seed fate. We experimentally tracked fruit and seed removal and followed the fates of Joshua tree seeds using radioactive tracers. The majority of Joshua tree fruits monitored were taken directly from the tree canopy by white-tailed antelope squirrels, and seeds and fruits on the soil surface were quickly removed by animals. Rodents given seeds labeled with scandium-46 cached them between 0.1cm and 4.1cm deep. Seedling emergence was most common for seeds planted 1cm deep, whereas seeds placed on the soil surface seldom germinated. Wind dispersal is unlikely because fruits and seeds lack adaptations for wind dispersal; wind speeds required to move Joshua tree seeds and fruits across the soil surface were higher than those typically found in the Mojave Desert. Further, rodents removed most seeds before abiotic burial was possible. We conclude that most Joshua tree seeds are dispersed by scatter hoarding by rodents, and that caches made by rodents are suitable sites for seedling emergence.

Nuevo registro de distribución de la ardilla antílope de Texas Ammospermophilus interpres para el estado de Chihuahua, México

Resumen We report the existence of the Texas antelope squirrel Ammospermophilus interpres in the Sierra Juarez, municipality of Juarez in the state of Chihuahua, Mexico. This represents the northernmost record in the state of Chihuahua, Mexico. It represents a range extension of 365 kilometers northwest from its previews records.

Effects of grazing and invasive grasses on desert vertebrates in California

AbstractMuch of California's San Joaquin Valley is a desert and, like portions of other North American deserts, is experiencing an ecological shift from being dominated by ephemeral native forbs, with widely spaced shrubs, to fire‐prone non‐native annual grasses. Small terrestrial vertebrates, many of which are adapted to open desert habitats, are declining. One hypothesis is that the invasive plants contribute to the decline by altering vegetative structure. Although cattle may have originally been a factor in the establishment of these non‐native plants, their grazing may benefit the terrestrial vertebrates by maintaining an open structure, especially during average or wet winters when the exotic grasses grow especially dense. We experimentally tested the effect of cattle grazing on invasive plants and a community of small vertebrates at a site in the southwestern San Joaquin Desert. We established and monitored 4 treatment (grazed) and 4 control (ungrazed) plots from 1997 to 2006, and assessed the abundances of blunt‐nosed leopard lizards (Gambelia sila), giant kangaroo rats (Dipodomys ingens), short‐nosed kangaroo rats (Dipodomys nitratoides nitratoides), and San Joaquin antelope squirrels (Ammospermophilus nelsoni), all of which are listed as threatened or endangered by state or federal agencies. We also recorded abundances of the non‐protected western whiptail lizards (Aspidoscelis tigris), side‐blotched lizards (Uta stansburiana), San Joaquin pocket mice (Perognathus inornatus inornatus), and Heermann's kangaroo rats (Dipdomys heermanni). Based on repeated measures analysis of variance (ANOVA) and a 0.05 alpha level, only Heermann's kangaroo rats showed a treatment effect; they were more abundant on the control plots. However, this effect could be accounted for by the natural re‐establishment of saltbush (Atriplexspp.) on part of the study site. Saltbush return also favored western whiptail lizards and San Joaquin antelope squirrels. A regression analysis indicated that populations of blunt‐nosed leopard lizard and giant kangaroo rat increased significantly faster in grazed plots than the ungrazed controls, and abundances of 6 of 8 species were negatively correlated with increased residual dry matter. With relaxed alpha values to decrease Type II error, populations of blunt‐nosed leopard lizards (500% greater), San Joaquin antelope squirrels (85% greater), and short‐nosed kangaroo rats (73% greater) increased significantly on grazed plots over the course of the study compared to ungrazed plots. We did not find grazing to negatively affect abundance of any species we studied. When herbaceous cover is low during years of below average rainfall in deserts and other arid areas, grazing may not be necessary to maintain populations of small vertebrates. However, if cattle grazing is closely monitored in space and time to minimize adverse effects on the habitat, it could be an effective tool to control dense stands of non‐native grasses and benefit native wildlife. © 2011 The Wildlife Society.

Records of the Texas Antelope Squirrel (Ammospermophilus interpres) in Texas

In 1995, we caught three Texas antelope squirrels (Ammospermophilus interpres) at two sites near Brackettville, Kinney County, Texas. One was captured in a habitat atypical for the species. This is the easternmost record and first record of the species in Kinney County.

Seed dispersal by scatter‐hoarding rodents in arid environments

Summary 1. The seeds of many plants are dispersed by animals, but the nature of these plant–animal mutualisms is often moulded by the abiotic environment. Here, we show that desert peach (Prunus andersonii), with dry fruits and large seeds, relies on scatter‐hoarding rodents for dispersal and that this form of seed dispersal maintains the effectiveness of dispersal while reducing water expenditure. 2. The fruit pulp of desert peach dries and dehisces at maturity in early summer. No vertebrate frugivores consumed the fruits, but rodents quickly harvested the nuts. Nearly 75% of the nut crop was removed from plant canopies by scansorial rodents like white‐tailed antelope squirrels (Ammospermophilus leucurus), which scatter‐hoarded nuts in 1–2 seed caches 10–30 mm deep. Deer mice (Peromyscus maniculatus) also made many shallow one‐nut caches. 3. Heteromyid rodents (Great Basin pocket mouse, Perognathus parvus; Panamint kangaroo rat, Dipodomys panamintinus) primarily larder‐hoarded nuts (c. 60%) in burrows too deep for seedling emergence, but also scatter‐hoarded nuts in a few, large caches 10–40 mm deep. 4. Antelope squirrels were the most effective dispersers and deer mice and Panamint kangaroo rats were less effective. Abiotically dispersed nuts had virtually no recruitment (<1% emergence), but nuts buried in soil to simulate rodent caches had 32% emergence and 5.6% survived after 1 year. Scatter‐hoarding rodents are responsible for virtually all recruitment in desert peach. 5. Synthesis. Desert peach arose from a fleshy‐fruited ancestor that was probably dispersed by endozoochorous frugivores. About 35 species of dry‐fruited Prunus occur in arid environments across North America and Eurasia. The most parsimonious explanation for the evolution of dry‐fruited Prunus is the loss of frugivory in a diplochorous ancestor (i.e. combined frugivore and scatter‐hoarding dispersal) to maintain the effectiveness of seed dispersal while reducing the demand for water in arid environments.

Odontometric Patterns in the Radiation of Extant Ground-dwelling Squirrels within Marmotini (Sciuridae: Xerini)

I document odontometric variation across ground-dwelling squirrels of the Holarctic tribe Marmotini. Dental size, which correlates well with published average body mass values across species, accounts for most odontometric variation across the clade. Dental shape variation primarily reflects relative size of P3 (upper cheek teeth) and relative width of p4–m1 + length of m3 (lower cheek teeth). Shape variables and relative tooth crown height covary significantly across species, suggesting a common functional complex or shared genetic control. When dental morphology is mapped on published DNA-based phylogenies, Sciurotamias (Chinese rock squirrels), Ammospermophilus (antelope squirrels), and basal subgenera within Spermophilus (ground squirrels [Callospermophilus and Otospermophilus]) are shown to retain inferred primitive dental morphology—small to moderate dental size, relatively small P3, relatively narrow p4–m1 + shortened m3, and relatively low toothcrown height. Other clades depart from this morphotype in size (very small in Tamias [chipmunks] and very large in Marmota [marmots]), tooth shape (especially 2 clades representing Eurasian and North American subgenus Spermophilus), or in both attributes (notably Cynomys [prairie dogs]), with frequent homoplasy. A plot of odontometric distance against published estimates of divergence time between sister clades suggests a roughly ‘‘clocklike’’ accumulation of odontometric change through time but highlights episodes of rapid odontometric evolution during the origins of Marmota, Cynomys, and Spermophilus parryii (arctic ground squirrel).

Generic Revision in the Holarctic Ground Squirrel GenusSpermophilus

The substantial body of research on Holarctic ground squirrels amassed over the past century documents considerable variability in morphological, cytogenetic, ecological, and behavioral attributes in the genus Spermophilus F. Cuvier, 1825. Recent molecular phylogenetic studies suggest that the traditionally recognized genera Marmota Blumenbach, 1779 (marmots), Cynomys Rafinesque, 1817 (prairie dogs), and Ammospermophilus Merriam, 1892 (antelope ground squirrels) render Spermophilus paraphyletic, potentially suggesting that multiple generic-level lineages should be credited within Spermophilus. Herein, we recognize 8 genera formerly subsumed in Spermophilus, each of which is morphologically diagnosable, craniometrically distinctive, and recovered as a monophyletic clade in phylogenetic analyses utilizing the mitochondrial gene cytochrome b. Generic-level names are available for each of these ground squirrel assemblages, most of which are exclusively or predominantly North American in distribution (Notocitellus A. H. Howell, 1938; Otospermophilus Brandt, 1844; Callospermophilus Merriam, 1897; Ictidomys J. A. Allen, 1877; Poliocitellus A. H. Howell, 1938; Xerospermophilus Merriam, 1892; and Urocitellus Obolenskij, 1927). Only Spermophilus sensu stricto is restricted to Eurasia. Generic subdivision of Spermophilus more aptly illuminates the taxonomic relationships, ecomorphological disparity, and biogeographic history of Holarctic ground squirrels.

Sex Ratios of the White-tailed Antelope Ground Squirrel (Ammospermophilus leucurus), Indian Wells Valley, San Bernardino County, California

We studied the white-tailed antelope ground squirrel during spring and summer 2006 to determine young-of-the-year and adult sex ratios in the Indian Wells Valley, San Bernardino County, California. We calculated a young-of-the-year sex ratio of 1.45:1 (female to male), whereas the adult sex ratio was approximately 1:1. Our young-of-the-year sex ratio was greater than the 1.1:1 (female to male) natal sex ratio previously reported. Differences between young-of-the-year and adult sex ratios may represent low female young-of-the-year survivorship in the Indian Wells Valley.

Variation in Reproductive Patterns of Antelope Ground Squirrels, Ammospermophilus leucurus, from Oregon to Baja California

Abstract Clines of biological variation result from variation in physical environmental conditions. We report new reproductive data for a population of antelope ground squirrels, Ammospermophilus leucurus, at the northern geographic limit of this species. We then analyze reproductive patterns at 3 sites spanning the entire north–south extent of the geographic range, covering more than 2,500 km and 21 degrees of latitude. The overall breeding season of populations is relatively short in Oregon (42°N) and California (37°N), but in Baja California (24°N) it encompasses half a year. Mean litter size decreases significantly from north to south, from 9.3 (SD = 1.8) per litter in Oregon to 5.9 (SD = 1.5) in Baja California. Major shifts in seasonality of precipitation and ecosystem composition and diversity are associated with longer breeding season and lower litter size at the southern geographic extreme of A. leucurus. The environmental predictability for the northern populations is due to “constancy,” whereas...

Systematics of the Antelope Ground Squirrel (Ammospermophilus) from Islands Adjacent to the Baja California Peninsula

I assessed the systematics of Ammospermophilus leucurus from the islands of Espíritu Santo and San Marcos adjacent to the Baja California peninsula, using sequences of the mitochondrial cytochrome-b (Cytb) gene and morphological analyses from the island and the peninsular samples. For the molecular analysis, the Cytb gene (500 base pairs) was sequenced from individuals from 34 different localities. Maximum-parsimony, maximum-likelihood, and neighbor-joining analyses indicated a phylogenetic break in the central part of the Baja California peninsula. Samples from the islands of Espíritu Santo and San Marcos were grouped with the southern clade with an average of 1.2 substitutions that separated adjacent haplotypes in a network. Two haplotypes occurred on San Marcos Island; one was unique and the other also was present in peninsular samples. The only haplotype found on Espíritu Santo Island also occurred on the peninsula. Both univariate and multivariate morphometric analyses and the pattern of presence–absence of the 3rd upper premolar distinguished samples from Espíritu Santo and the Cape region of the peninsula but not between San Marcos Island and Francisco de la Sierra on the peninsula. Thus, the San Marcos Island ground squirrel could not be distinguished from those of the peninsula and should be recognized as the same subspecies that occupies the adjacent peninsular areas (A. leucurus extimus). In addition, neither molecular nor morphological data supported the recognition of A. insularis (from Espíritu Santo Island) as a distinct species from peninsular samples of A. leucurus, although this taxon showed morphological differentiation in dental characters that still support its recognition as a valid subspecies (A. leucurus insularis).

Diversity of Function of SCN Pacemakers in Behavior and Ecology of Three Species of Sciurid Rodents

Information on the widespread function of the suprachiasmatic (SCN) pacemaker has increased dramatically in the past decade for laboratory rodents and for humans. Interest is now also growing in the adaptive value of SCN pacemakers in wild species of free-living mammals in natural pristine habitat. The squirrel family, Sciuridae, is highly diverse in North America. Radiation into many specialized groups has evolved in response to the temporal and spatial characteristics of specific habitats. The sciurids thus offer possibilities for comparison of SCN function for closely related species from very different habitats. Results from field and laboratory investigations concerning the ecological significance of the SCN are reported here for three ground squirrel species. The semi-fossorial antelope squirrels of the American southwest deserts were arrhythmic in above ground activities after deletion of the SCN pacemaker. In a desert enclosure, predation rates rose dramatically for lesioned animals in comparison to intact controls. The semi-arborial chipmunks were prone to capture by mustelid burrow predators such as weasels after SCN-deletion, even though the chipmunks did not exhibit any above ground activity at night. The telltale cue for the predator was apparently arrhythmic restlessness within the complex home burrow. The semi-fossorial golden-mantled squirrels of the Cascade Mountains of western United States rely on profound, long-lasting hibernation for over-wintering. Periods of torpor alternate with brief euthermic arousal episodes lasting less than 24 h throughout the 5- to 8-month winter phase. Deletion of the SCN in golden-mantled squirrels resulted in aberrations of hibernation arousal patterns that were metabolically expensive and put individuals at risk of starvation immediately after emergence in spring.

Effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla).

The effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla) was investigated in field enclosures. The rodent species were Panamint kangaroo rat (Dipodomys panamintinus), Great Basin pocket mouse (Perognathus parvus), pinyon mouse (Peromyscus truei), deer mouse (Peromyscus maniculatus), least chipmunk (Tamias minimus), and white-tailed antelope ground squirrel (Ammospermophilus leucurus). Five measures of seed harvesting and caching behavior were assessed and used to calculate the effect of species on the probability of effective dispersal (i.e., movement of seeds to potential establishment sites) of piñon pine seeds. These measures were proportion of seeds harvested, proportion of harvested seeds that were scatter hoarded, relative dispersion of scatter-hoarded seeds, seedling emergence and seedling establishment. To evaluate the effects of the last two variables, a 2-year study of seed germination and seedling establishment was conducted in the field. Seedling survival was 72-90% for seedlings emerging from 1-4 cm deep but was markedly lower from surface caches and for depths >4 cm. All seedlings in the open microsite perished before the end of the first year, whereas 38-97% of seedlings under shrubs survived. Cache size did not affect seedling survival over the 2 years, but large caches reduced the total number of potential establishment sites. All six species harvested over 90% of the piñon seeds presented and scatter hoarded from 31% (pocket mouse) to 66% (chipmunk) of the seeds they harvested. Piñon mice and deer mice distributed seeds to more cache sites (i.e., put fewer seeds in caches) than the other species and, consequently, created more potential establishment sites. All species scatter hoarded seeds at depths suitable for seedling emergence, but pocket mice and piñon mice cached under shrubs more than the other species, the microhabitat where seedlings had the greatest probability of establishing. Factors that had the greatest effect on the probability of effective dispersal were microhabitat selection and cache size.

Carnivory on Desert Cottontails by Texas Antelope Ground Squirrels

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Temperature Cycles as Zeitgeber for the Circadian Clock of Two Burrowing Rodents, the Normothermic Antelope Ground Squirrel and the Heterothermic Syrian Hamster

The circadian systems of two burrowing rodents, the normothermic diurnal antelope ground squirrel (Ammospermophilus leucurus) and the heterothermic nocturnal Syrian hamster (Mesocricetus auratus) were compared with respect to entrainment by temperature cycles. Both species were subjected to the same ambient temperature (Ta) cycles with amplitudes between 4 and 12ºC at constant illuminations (100 and 0.05 lux in squirrels; 1.0 lux in hamsters). Wheel running activity was continuously measured. There was considerable interindividual variation in the daily pattern of wheel-running activity and in the ability to entrain to Ta cycles of the same amplitude in both species. The activity rhythms of about 33 to 67% of the animals of the two species entrained to Ta cycles with amplitudes of 6 to 12ºC. One of six squirrels and one of nine hamsters even entrained to Ta cycles of 4ºC. In the antelope ground squirrels, activity occurred predominantly in the cooler phase of the Ta cycle, whereas hamsters were mainly active during the warmer phase. In some squirrels, the activity rhythms were split in two main components which were both entrained to the cooler fraction of the Ta cycle, sometimes with additional (masking) activity during the warmer fraction (above 30ºC). The results do not support the earlier view that temperature cycles affect the circadian systems of heterothermic mammals, including hibernators, more strongly than those of normothermic species. It is suggested that behavioral and physiological adjustments to the environmental conditions play an important role for mammalian circadian systems to respond to temperature changes as a zeitgeber.

The Response of Antelope Ground Squirrels ( Ammospermophilus leucurus ) to Human Impacts in the Western Mojave Desert

Many areas of the western Mojave Desert have been severely impacted during the past 30 years because of human activity on lands adjacent to rapidly developing desert cities. Antelope ground squirrels were studied in order to determine the degree and scope of these borderland utilization zones. Three suburban-to-rural transects, plus one control site, were located near the city of Ridgecrest. Each transect was surveyed and rated for 10 types of human impact, and live-trapping for ground squirrel abundance was conducted along the entire length. Both the cumulative human impact ratings and the number of ground squirrel captures tended to increase with increasing proximity to the edge of development. This suggests that these ground squirrels are an edge-enhanced species and may in turn be an indicator of an overall reduction in habitat quality.

Effects of extremely low frequency (ELF) magnetic fields on circadian rhythm generated in suprachiasmatic nucleus

Circadian activity parameters of 53 white-tailed antelope ground squirrels, Ammospermophilus leucurus, were measured to determine the role of the suprachiasmatic nuclei (SCN) pacemaker in their health and survival. Wheel-running activity was monitored in the laboratory with 44 individuals to document the presence of free-running circadian rhythms and ability to entrain to light-dark cycles. Twenty-two individuals were returned to the desert site of origin, including 12 intact control animals and 10 animals whose circadian timing had been disrupted by SCN-lesioning. Time of activity was recorded continuously for 15 days in a large outdoor enclosure by a motion detector, a microchip transponder detector, and video surveillance. An unplanned nighttime attack by a feral cat resulted in the death of 60% of the SCN-lesioned animals and 29% of the control animals in the enclosure. Surface activity of SCN-lesioned animals at the food cache occurred both in daytime and at night, ranging from 16.0% nighttime activity for one partially lesioned individual to 52.1% for one completely lesioned animal. Controls were strongly day-active, with nighttime surface trips constituting only 0–1.3% of all excursions to the cache. Nine wild free-ranging individuals, including one with a radiotransmitter collar, were exclusively day-active. One of the functions of the SCN in mammals may be to reduce activity of animals during times that are unfavorable for activity.

Circadian Performance of Suprachiasmatic Nuclei (SCN)-Lesioned Antelope Ground Squirrels in a Desert Enclosure

Circadian activity parameters of 53 white-tailed antelope ground squirrels, Ammospermophilus leucurus, were measured to determine the role of the suprachiasmatic nuclei (SCN) pacemaker in their health and survival. Wheel-running activity was monitored in the laboratory with 44 individuals to document the presence of free-running circadian rhythms and ability to entrain to light-dark cycles. Twenty-two individuals were returned to the desert site of origin, including 12 intact control animals and 10 animals whose circadian timing had been disrupted by SCN-lesioning. Time of activity was recorded continuously for 15 days in a large outdoor enclosure by a motion detector, a microchip transponder detector, and video surveillance. An unplanned nighttime attack by a feral cat resulted in the death of 60% of the SCN-lesioned animals and 29% of the control animals in the enclosure. Surface activity of SCN-lesioned animals at the food cache occurred both in daytime and at night, ranging from 16.0% nighttime activity for one partially lesioned individual to 52.1% for one completely lesioned animal. Controls were strongly day-active, with nighttime surface trips constituting only 0–1.3% of all excursions to the cache. Nine wild free-ranging individuals, including one with a radiotransmitter collar, were exclusively day-active. One of the functions of the SCN in mammals may be to reduce activity of animals during times that are unfavorable for activity.

Optimal body temperatures with shuttling: desert antelope ground squirrels

A variety of animals periodically shuttle between two sites, such as one containing food and another containing oxygen, water, or conditions for heat exchange. The amount of a non-food resource obtained, together with its rate of use, influences travelling and foraging time. Three criteria are evaluated to examine how control of body temperature may be optimal with shuttling: (1) maximizing proportion of time foraging, (2) maximizing rate of net energy gain for a shuttle cycle and (3) maximizing time travelling and foraging relative to time spent at a heat-exchange site. Detailed information on heat exchange by desert antelope ground squirrels, Ammospermophilus leucurus, cooling in their burrows and heating while travelling and foraging on the hot desert surface is used to calculate body temperatures that maximize each criterion. Results also are discussed for analogous models for intermittent bird incubation and ectotherm basking. The third criterion is rejected based on published body temperature patterns. The other criteria are plausible but assume temperature control in anticipation of environmental conditions. Body temperatures that maximize the first criterion maximize rates of net energy gain when the rate of gross energy gain is constant. The optimal body temperatures of ground squirrels exiting a burrow should decrease with: (1) increasing travel time, (2) higher heating or cooling rates, (3) decreased body temperature upon return to a burrow and (4) higher rates of gross energy gain.

Food-Item Use by Coyotes at the Naval Petroleum Reserves in California

Coyote (Canis latrans) food habits have been extensively documented (Bekoff, 1977). Use of food items by coyotes varies with location and time in response to spatial and temporal variation in food availability. Thus, siteand time-specific investigations are necessary to accurately describe local coyote foraging patterns. Relatively few investigations of coyote food habits have been conducted in California (e.g., Ferrel et al., 1953), and none were specific to the San Joaquin Valley. Additionally, coyotes in the southern San Joaquin Valley are sympatric with the endangered San Joaquin kit fox (Vulpes macrotis mutica = V. velox macrotis, cf. Dragoo et al., 1990) and potentially compete with foxes for food resources. Also, several other federally listed and candidate species occur in the southern San Joaquin Valley and may be preyed upon by coyotes. These species include the endangered giant kangaroo rat (Dipodomys ingens) and the blunt-nosed leopard lizard (Gambelia silus), and candidate species shortnosed kangaroo rat (Dipodomys nitratoides) and San Joaquin antelope squirrel (Ammospermophilus nelsoni). We investigated food-item use by coyotes at the Naval Petroleum Reserves in California (NPRC) to (1) determine annual and seasonal item use, and in particular to evaluate the dietary importance of leporids which are important prey for kit foxes (O'Farrell et al., 1987; J. H. Scrivner et al., in litt.), (2) assess the effect of annual leporid abundance on coyote food habits, and (3) determine whether coyotes prey on giant kangaroo rats, blunt-nosed leopard lizards, shortnosed kangaroo rats, and San Joaquin antelope squirrels. NPRC is an area of active petroleum production located 42 km SW of Bakersfield in western

Seasonal Water, Energy and Food Use by Free-Living, Arid-Habitat Mammals

In arid ecosystems, the dry season is probably an especially challenging time for animals to find adequate food and water resources. Both stressful and benevolent times of the year can be identified by measuring rates of water and energy use (with doubly labelled water), diet and behaviour throughout a year in free-ranging animals. Such field studies have been completed on four species of arid-land mammals. Jackrabbits (Lepus californicus) are strict herbivores, and most Mojave Desert individuals have no access to drinking water, and depend on their food for their water. When vegetation dries up during the summer drought, most jackrabbits apparently die of dehydration and malnutrition, but populations are replenished by bountiful reproduction after winter rains. However, springbok antelope (Antidorcas marsupialis), also strict herbivores, can maintain water and energy balance throughout the year in the Kalahari in southern Africa by drinking water if available, but, if not, they obtain enough water from dietary plants, probably by feeding before dawn, when food items have taken up water from the humid air. Antelope ground squirrels (Ammospermophilus leucurus) are omnivorous in the Mojave Desert, and are active diurnally. They have lower water and energy requirements than non-desert mammals of the same body mass, like many desert mammals, but their ability to find succulent plant or animal foods in all seasons appears to be their primary survival tool. On the other hand, Merriam's kangaroo rats (Dipodomys merriami) are essentially nocturnal, and were thought to be strict granivores that never drank. A recent study, done throughout a year in the Mojave Desert, confirms their lack of drinking, but reveals that they eat much green vegetation in late winter and spring, probably in connection with reproduction. They obtain additional water throughout the year by caching dry seeds in humid burrows, where seeds take up water hygroscopically before being eaten. Although springbok, antelope ground squirrels and Merriam's kangaroo rats were in negative energy and water balance at times during the year, only jackrabbits were severely stressed by the dry season. Each species has a unique suite of desert survival mechanisms. Future research should include carnivores as well as arid-habitat marsupials.