Charles H. Lowe

Critical Factors During the First Years of Life of the Saguaro (Cereus Giganteus) at Saguaro National Monument, Arizona

Germination and establishment of the saguaro giant cactus were studied by periodic observations on natural seedling populations, seedling distribution within rocky, rolling hill, and flat terrain habitats, and field—germination experiments in the Sonoran Desert. As a result of bird, mammal, and insect activity, a very small percentage of seeds (<1 x 10—3 of the seed crop) remains on the ground until suitable conditions for germination occur during the summer monsoon. Germination begins after the start of summer rains in July and continues in August and September. The principal apical stem growth of seedlings takes place during these months, with a few plants exhibiting slight growth during favorable late winter and early spring months. Establishment of seedlings is limited primarily by frost, drought, rodents, and insects, which affect the differential survival associated with seedling size, microhabitat, and season. Initial high rates of seedling mortality drop sharply after the first year and are lowest for plants associated with microenvironments among rock outcrops. The large number of seeds germinated in the alluvial soils of the flat terrain habitats is offset by a higher seedling mortality there. In the rocky habitats more seedlings survive from fewer germinations. The significant difference is attributed primarily to the effect of the microhabitat upon the operation of the critical controlling factors listed above.

AN ANALYSIS OF BACKGROUND COLOR-MATCHING IN AMPHIBIANS AND REPTILES

The colors of living amphibians and reptiles have been studied, using a General Electric recording reflectance spectrophotometer. The animals were brought to activity tempera- ture levels and the appropriate surface pressed over the reflectance port of the machine while a color record was taken. Background samples from the localities at which the animals were taken were also recorded. Reptiles and amphibians living on backgrounds of relatively uniform color tend to matcl that background through superposition with considerable fidelity. The animals color curve is superimposed over that of the background. Ventral color in most forms tested was lighter than the dorsal surfaces of the same animal. It was darker only in some forest-dwelling salamanders and in desert lava-dwelling species. The difference results primarily from the highly reflective ventral surfaces of these forms. The ventral surfaces of white-bellied amphibia show clear oxyhaemoglobin absorption peaks, as do the dorsal surfaces of some amphibia. These effects are entirely absent in curves recorded from reptiles. It is concluded that the degree of background color-matching is related to: (a) the degree of color uniformity of the animals background, (b) the degree of exposure of the color-matched species to predators, (c) the illumination level prevalent in the habitat, (d) the size range of the color-matched species, (e) the ecological restriction of the species, (f) the qualities of the visual apparatus of predators upon the species, and (g) the adaptive compromise struck by the species. The size of a color-matched animal, or the size of the part of its body that is normally exposed, is related to the point at which such color-matching breaks down. This point of just noticeable difference between animal and background is also determined by the wave-length discrimination curves of the predators, the closeness of the match involved, the uniformity of the background color and its texture, and the presence or absence of concealing patterns. Back- ground color-matching varies greatly in its degree of perfection. This variation is the result of adaptive compromise and balance between this adaptive characteristic and many others that in one way or another affect its complete expression.

Behavioral and Physiological Responses to Temperature in the Desert Pupfish Cyprinodon macularius

The ecological significance of temperature tolerance in fishes has been questioned for several years. Some authors have noted that the temperature tolerances of fishes are usually well above their habitat temperatures (Hart, 1952; Brett, 1959), that there are relatively few reports of heat death of fishes under natural conditions (reviewed in Norris, 1963), and that there are few or no changes of temperature tolerance with latitude within several species (Hart, 1952). It was concluded by both Hart (1952) and Brett (1959) that temperature tolerance is generally not of direct ecological significance. It has become apparent in our work with fishes in desert habitats that they not only undergo frequent and drastic natural heat kill, but that these fishes are also found living at temperatures that are just below their critical thermal maximum. The data presented here for the desert pupfish Cyprinodon macularius illustrate well the intense natural selection for high temperature tolerance that occurs in desert habitats, and that temperature tolerance is of primary importance ecologically to these populations.

Effect of Paloverde (Cercidium) Trees on the Radiation Flux at Ground Level in the Sonoran Desert in Winter

The radiation flux at winter solstice is reported for desert microenvironments under Cercidium crowns and in open desert. In addition to the net radiation flux and that of the upper and lower hemispheres, hourly radiation temperatures (equivalent black—body temperatures) were measured for soil surfaces, sky, and tree crowns. Radiation temperature—time as a limiting factor for organisms in the sense of resistance time was investigated. Winter environmental modification provided by Cercidium of 4—5 m height is large, and greater than expected. The effective (net) incoming radiation at ground level at midday in the open is more than twice that under the paloverde, and the effective (net) outgoing radiation in the open at night is more than twice as great as that under the tree. For these data, a computer model of the energy budget of a 1—kg hypothetical homoiotherm predicts (i) on the ground surface under the paloverde at night, an effective body—surface temperature of 7 degrees C with an increse of metaboli...

Metabolism of the Iguanid Lizard Uta Stansburiana in the Supercooled State

I NVESTIGATIONS of the biological effects of low temperatures have been conducted on a variety of organisms, with attention centered primarily on insects, intertidal and marine invertebrates, fishes, and mammals. Much of this is reviewed in the recent monograph by Smith (1961), an excellent review of past and present low-temperature research. In the area of low-temperature tolerances of amphibians and reptiles, little of significance has been reported since the pioneering investigations of Weigmann (1929), Kalabukhov (1934), and Rodionov (1938). The present investigation on respiratory metabolism of the lizard Uta stansburiana Baird and Girard in the supercooled state followed the recent discovery that Uta and other species of reptiles are capable of significant supercooling of the body fluids below the freezing point of blood (Lowe, Lardner, and Halpern, in press). An experimental design was constructed to test winter versus summer

The Salamanders of Arizona

The populations of the single species of salamander known to occur in Arizona, Ambystoma tigrinum (Tiger Salamander), have been essentially in uncertain status notwithstanding the recent works of Dunn (1940), Bishop (1943), Stebbins (1951), and Reed (1951). The present confused taxonomic, ecologic, and geographic picture for this area is illustrated by the pecularity that neither Bishop nor Stebbins, the two authors who have ably attempted to map the distribution of the western subspecies of A. tigrinum, has included the geographic position of the type locality of A. t. nebulosum within the mapped range presented by each of them for that subspecies. Field work by the author during the past few years in Arizona and neighboring states has enabled some clarification of the general problem and the populations of Arizona may now be profitably treated. The populations are considered as comprising three subspecies. Transformed individuals of each subspecies have been collected with some individuals from new localities. These, in addition to those at which A. tigrinum has been previously collected, make a total of approximately 30 verifiable localities for this species in Arizona; they are distributed approximately as follows: nebulosum 21, utahense 8, and stebbinsi 1. With the exception of what may be morphologic intermediates (= intergrades), the subspecies are distinctive in 100%o of the individuals known for each. Dunn (1940) has correctly emphasized the earlier conclusion of Cope (1867) that in final analysis an intelligible -picture of the geographic variation of Ambystoma tigrinum must be primarily based upon color pattern (rather than structural) characteristics of the transformed individuals.

The Systematic Status of Plethopsis with a Discussion of Speciation in the Genus Batrachoseps

This genus is related to Plethodon from which it differs in having only four toes, vomerine and parasphenoid teeth often forming a continuous series, incompletely ossified parietals and frontals and multiple testes in large males. From Hemidactylium it differs in lacking the basal constriction of the tail, in the condition of the parietals and frontals, in the number of costal grooves, the multiple testes and the lack of a marked sexual dimorphism. It resembles Batrachoseps in the incompletely ossified parietals and frontals but differs in the possession of two premaxillae, larger hind limbs, in the number and disposition of the teeth, multiple testes and lack of worm-like body.

Supercooling of the Saguaro Species Drosophila Nigrospiracula in the Sonoran Desert

Winter—active February adults (vicinity Tuscon, Arizona) were observed to be supercooled during freezing nights in their desert habitat, with 24—hr minimum extremes reaching —5.1°C, and to become active (flying) a few minutes after the warming following sunrise. The LD50 for experimental supercooling of the population sampled was —7.73°C, the LD100 between —9° and —10°C, and the LD0 between —5° and —6°C. The ability to survive winter nocturnal temperature extremes in the supercooled state makes it possible for the species to be surface active and breeding throughout the winter over the geographical distribution of its primary host plant, the saguaro (Cereus giganteus), in Arizona and Sonora, Mexico. See full-text article at JSTOR