F. Harvey Pough

Metabolism of Squamate Reptiles: Allometric and Ecological Relationships

We used multiple regression analysis to evaluate the relationship between metabolic rate and three independent variables-mass, temperature, and standard or resting state-for squamate reptiles. For comparisons among adults of different species, mass raised to the .80 power explains 88% of the variation in metabolic rate. (The .80 mass exponent is significantly greater than the .75 predicted by theoretical considerations.) A further 8% of the variation in metabolic rate is explained by body temperature and whether the lizard is in a standard or resting metabolic state. Residuals were used to determine whether metabolic rates varied as a function of phylogenetic relationship or ecological grouping. Familial associations explained 16% of the variation in metabolic rate for varanids, lacertids, iguanids, colubrids, scincids, xantusiids, gekkonids, and boids. More variation (45%) was explained when lizards were partitioned into four ecological categories: day-active predators, hervibores, reclusive predators, and fossorial predators. A single equation relating metabolic rate to mass is thus inappropriate to estimate the metabolism of squamates. For intraspecific comparisons, the mass exponents of the relationship between metabolic rate and mass are significantly lower than .80 for 25 of 28 data sets. Estimating the metabolic rates of juvenile squamates from equations based on comparisons among species is thus invalid. Moreover, there is significant variability among mass exponents among the 14 species that met the statistical requirements for analysis of covariance, and a common mass exponent cannot be assumed for intraspecific comparisons.

Behavioral Modification of Evaporative Water Loss by a Puerto Rican Frog

Leptodactylid frogs (Eleutherodactylus coqui Thomas) reduce rates of evaporative water loss threefold by adjusting their postures and activities in response to changing conditions of availability of water during their nocturnal activity periods. Frogs that do not make these adjustments experience a potentially lethal loss of body water on a rainless night. Dehydration of a frogs body tissues increases its resting metabolic rate and lowers its maximum rate of aerobic metabolism. Water is reabsorbed from urine in the bladder to maintain tissue water content on dry nights. Use of water—conserving postures precludes vocalization by male frogs and response to calling males by females. Frogs in water—conserving postures feed less readily than active frogs. Frogs in the forest canopy experience higher rates of evaporative water loss than those in the understory, but there are more anthropods in the canopy, and leaf surfaces are twice as likely to be wet by rain. Despite these potential benefits of activity in the forest canopy, most frogs remain in understory vegetation. In that microhabitat their behavioral and physiological adjustments permit them to occupy their normal perches despite wide fluctuations in hydric conditions.

Organismal Performance and Darwinian Fitness: Approaches and Interpretations

Organismal performance lies at the base of ecological and evolutionary processes, and the consequences of individual variation in performance have become a focus of physiological ecology. The study of the relations of performance to Darwinian fitness integrates the traditionally reductionist approach of physiological ecology with the perspectives of genetics and evolution. Four levels of biological organization have been included in this effort: the description of genetic variation, the analysis of biochemical and physiological consequences of that variation, the description of variation in the capacity of individuals to perform various activities, and the analysis of the effect of that variation in performance capacity on individual variation in Darwinian fitness. Mechanisms within each of those levels have been elaborated, and cause-and-effect links have been established among the levels. Two approaches to defining causal links are prominent. The gene-to-performance school begins with the observation of genetic variation (e.g., the presence in a species of allelic isozymes with different responses to temperature) and seeks to trace the performance of different genotypes upward through successive levels of biological organization, ultimately to variation in behavior and the relation of that variation to natural selection. A few studies have been successful in this endeavor, largely because they were based on detailed information about the ecology of the species investigated. These studies have revealed stabilizing selection maintaining genetic polymorphisms. Physiological ecologists have approached the same questions as those addressed by geneticists, but the physiologists have started with the description of individual variation in performance that is almost certainly under the control of multiple genetic loci. From this point they have moved outward in two directions. One line of investigation involves a reductionist analysis of the mechanistic basis of variation in performance, whereas the other seeks associations between performance and fitness. Substantial progress has been made in the reductionist direction, although more information is needed about the heritabilities of particular characteristics. The upward direction has been less traveled, and the chances of success will be maximized by employing the same detailed ecological perspective that has been successful for gene-to-performance studies. The activities to be studied should be selected on the basis of their demonstrable importance to the fitness of individuals under natural conditions, and the behavioral ecology literature contains many examples of activities of that sort.

Male parental care and its adaptive significance in a neotropical frog

Abstract In the Puerto Rican frog Eleutherodactylus coqui, parental care is performed exclusively by males, and consists of attending the eggs and hatchlings at a terrestrial oviposition site. The two major behavioural components of parental care are egg brooding and nest defence against conspecific egg cannibals. Defence behaviour includes aggressive calling, biting, sustained biting, wrestling, and blocking directed against nest intruders. Parental care lasts from oviposition to hatching (17–26 days) and often for several days after hatching. During pre-hatching development, males are present in their nests 97.4% of the time during the day and 75.8% of the time at night. A large portion of this time is spent brooding eggs. In a field experiment, males were removed from their nests and the fate of clutches was monitored. Compared to control clutches (males not removed), experimental clutches had significantly lower hatching success and suffered significantly greater mortality from desiccation and cannibalism. Hence, parental care yields significant benefits to male fitness via increased offspring survival.

Population density of tropical forest frogs: relation to retreat sites

The forest frog Eleutherodactylus coqui defends specific sites for retreats and nests in the Luquillo Forest, Puerto Rico. The hypothesis that shortages of nest and retreat sites limit population size was tested by placing 100 bamboo frog houses in plots measuring 100 square meters in areas of high frog density. These new sites were readily adopted by adult frogs. After one year, experimental plots had significantly more nests and frogs of all sizes than did control plots.

Post-metamorphic change in activity metabolism of anurans in relation to life history

SummaryNewly-metamorphosed individuals of some species of frogs and toads differ from adults in behavior, ecology, and physiology. These differences may be related to broader patterns of the life histories of different species of frogs. In particular, the length of larval life and the size of a frog at metamorphosis appear to be significant factors in post-metamorphic ontogenetic change. These changes in performance are associated with rapid post-metamorphic increases in oxygen transport capacity. Bufo americanus (American toads) and Rana sylvatica (wood frogs) spend only 2–3 months as tadpoles and metamorphose at body masses of 0.25 g or less. Individuals of these species improve endurance and aerobic capacity rapidly during the predispersal period immediately following metamorphosis. Increases in hematocrit, hemoglobin concentration, and heart mass relative to body mass are associated with this improvement in organismal performance. Rana clamitans (green frogs) spend from 3 to 10 months as larvae and weigh 3 g at metamorphosis. Green frogs did not show immediate post-metamorphic increases in performance. Rana palustris (pickerel frogs) are intermediate to wood frogs and green frogs in length of larval life and in size at metamorphosis, and they are intermediate also in their post-metamorphic physiological changes.American toads and wood frogs appear to delay dispersal from their natal ponds while they undergo rapid post-metamorphic growth and development, whereas green frogs disperse as soon as they leave the water, even before they have fully absorbed their tails. The very small body sizes of newly metamorphosed toads and wood frogs appear to limit the scope of their behaviors. The brief larval periods of these species permit them to exploit transient aquatic habitats, but impose costs in the form of a period of post-metamorphic life in which their activities are restricted in time and space compared to those of adults.

Ontogenetic change in blood oxygen capacity and maximum activity in garter snakes (Thamnophis sirtalis)

SummaryThere is an ontogenetic increase in the time that garter snakes (Thamnophis s. sirtalis) can maintain maximum activity at 25°C. Newborn snakes are exhausted by 3–5 min of activity while adults can be active for 20–25 min. The increased endurance of adult snakes results from ontogenetic increases in both aerobic and anaerobic energy generation. At rest juvenile and adult snakes have the same whole-body lactic acid concentrations, but at exhaustion adult lactic acid concentrations are 1.5 times those of juveniles. This increase in anaerobic energy production accounts for part of the endurance of adult snakes, but increased aerobic metabolism appears to be more important. Among the mechanisms increasing aerobic metabolism are more effective pulmonary ventilation and a 3-fold ontogenetic increase in blood oxygen capacity.The rapid exhaustion of small garter snakes probably limits the microhabitats they can occupy and the sorts of hunting methods they can employ. Small garter snakes feed only on small prey that are easily subdued. There is an ontogenetic increase in the relative size of prey eaten by garter snakes that parallels the ontogenetic increase in endurance. Adult feeding habits are adopted at the same body size at which adult blood oxygen capacity and endurance are attained.

Energy costs of subduing and swallowing prey for a lizard

We measured the oxygen consumption (aerobic energy cost) and lactic acid production (anaerobic energy cost) of scincid lizards, Chalcides ocellatus, eating domestic crickets. Aerobic metabolism accounted for 90% or more of the total energy cost of subduing and swallowing prey. The time required to subdue and swallow a cricket was linearly correlated with oxygen consumption. Oxygen consumption increased as a power function of cricket mass, but the maximum size of crickets swallowed by the lizards was set by morphological rather than by energetic constraints. The energy cost of subduing and swallowing was 0.2—0.4% of the utilizable energy of the cricket eaten. Net energy gain per unit time spent subduing and swallowing prey (e/t) declined monotonically with increasing cricket mass. Because the energy cost of eating is trivial, the shape of the e/t curve is determined by the function relating prey mass to the time required for subduing and swallowing; the energy value of prey was proportional to prey mass, whereas the time required for subduing and swallowing increased faster than prey mass. The energy value of anthropods is so high, relative to the costs for a lizard of pursuring, subduing, and swallowing, that these costs can be ignored for most ecological purposes.

Mimicry of vertebrates: are the rules different?

Examples of mimicry among vertebrates are numerically fewer than examples involving insects. The relatively small number of species of vertebrates, compared with the number of species of insects, probably explains some of the apparent scarcity of mimicry. Possibly more important is a mismatch between the primarily visual sensory world of humans and the predominantly chemosensory, auditory, and tactile worlds of most other vertebrates, which has probably concealed many manifestations of mimicry. Systematic investigation of the information that vertebrates convey through these sensory modalities will probably reveal many additional examples of mimicry. Concrete homotypies-those cases in which the model can be identified as a particular species of animal-are widespread among fishes and amphibians and have been suggested for birds and mammals. Both Batesian and Mullerian protective mimicry systems have been described. Because vertebrates grow during their lifetimes without conspicuous changes in morphology, size limitation is manifested in some mimetic systems. Non-protective mimicry also occurs among vertebrates: mimicry of females is an alternative reproductive strategy for males, and many nest parasites mimic the eggs and young of their hosts. Abstract homotypies-cases in which the model cannot be identified as a particular species or group of species-are characteristic of mimicry of venomous snakes. Large body sizes, ontogenetic change in body size, and potentially lethal deterrents to attack by predators are the special characteristics of venomous snakes as models in mimicry systems. The risk for a dupe that mistakes a venomous snake for its mimic makes mimicry of venomous snakes a high-stakes game. The special characteristics of these mimicry systems (broad generalization of the characteristics of models, mimics that incorporate features of two or more models, and innate avoidance of models by predators) probably reflect that risk for a dupe. As such, these mimicry systems provide a new perspective on the mechanics of mimicry that may have parallels in some examples of mimicry of invertebrates.

Water Balance of Terrestrial Anuran (Eleutherodactylus Coqui) Eggs: Importance of Parental Care

The terrestrial eggs of the coqui of Puerto Rico are brooded almost continuously by the male parent from the time of oviposition until the fully metamorphosed hatchlings emerge from the eggs 15-20 d later. The gelatinous layer surrounding each egg offers no resistance to the exchange of water by the egg, and rates of exchange are determined by microclimatic conditions, structural characteristics of the nest, and the behavior of the male frog. During development in natural nests, the eggs experience a three- to fourfold increase in mass. Laboratory experiments coupled with field observations indicate that this increase is the result of the transfer of liquid water from the incubating male to his eggs. The transfer is driven by a difference in water potential between the eggs and the body fluids of the male frog that is nearly constant throughout incubation, despite an increase in egg mass. Eggs must take up water during incubation. Eggs that do not experience an increase in mass during development either die or produce small hatchlings. A water uptake that doubles the initial mass of the egg is necessary to produce a full-size hatchling with normal tissue water content.