Large body size in the pygmy swordtail Xiphophorus pygmaeus

Fishes in the genus Xiphophorus (swordtails and platyfishes) are well known for the influence of the pituitary ( P ) locus on variation of male size at maturity both within and among species. We report the discovery of large male size (>29 mm SL) in several populations of the swordtail X. pygmaeus , a species previously thought to consist of only small males (<29 mm SL). Large size is geographically restricted, and average male size varies significantly by site and year sampled in a pattern suggesting a recent origin and slow spreading of the large male phenotype. However, large male size is not strongly paternally inherited in this species, as it is in its two closest relatives, X. nigrensis and X. multilineatus , showing that large size does not result from the same genetic ( P locus) mechanism. Large X. pygmaeus males do not court, can exhibit the gold morph, do not possess swords and have slender body shape. In these traits they resemble small conspecific males and small males of X. nigrensis and X. multilineatus rather than large males of these latter two species. This shows that correlations between morphological and behavioural traits that occur in X. nigrensis and X. multilineatus are absent in X. pygmaeus.     

Wood CO2 efflux in a primary tropical rain forest

The balance between photosynthesis and plant respiration in tropical forests may substantially affect the global carbon cycle. Woody tissue CO₂ efflux is a major component of total plant respiration, but estimates of ecosystem‐scale rates are uncertain because of poor sampling in the upper canopy and across landscapes. To overcome these problems, we used a portable scaffolding tower to measure woody tissue CO₂ efflux from ground level to the canopy top across a range of sites of varying slope and soil phosphorus content in a primary tropical rain forest in Costa Rica. The objectives of this study were to: (1) determine whether to use surface area, volume, or biomass for modeling and extrapolating wood CO₂ efflux, (2) determine if wood CO₂ efflux varied seasonally, (3) identify if wood CO₂ efflux varied by functional group, height in canopy, soil fertility, or slope, and (4) extrapolate wood CO₂ efflux to the forest. CO₂ efflux from small diameter woody tissue (<10 cm) was related to surface area, while CO₂ efflux from stems >10 cm was related to both surface area and volume. Wood CO₂ efflux showed no evidence of seasonality over 2 years. CO₂ efflux per unit wood surface area at 25° (F_A) was highest for the N‐fixing dominant tree species Pentaclethra macroloba, followed by other tree species, lianas, then palms. Small diameter F_A increased steeply with increasing height, and large diameter F_A increased with diameter. Soil phosphorus and slope had slight, but complex effects on F_A. Wood CO₂ efflux per unit ground area was 1.34±0.36 μmol m^?2 s^?1, or 508±135 g C m^?2 yr^?1. Small diameter wood, only 15% of total woody biomass, accounted for 70% of total woody tissue CO₂ efflux from the forest; while lianas, only 3% of total woody biomass, contributed one‐fourth of the total wood CO₂ efflux.     

Morphological Adaptations of the Ear in the Rodent Family Heteromyidae

Middle and inner ear structure and auditory sensitivity havebeen studied in all five genera of the rodent family Heteromyidae.In the most xericc genera (Dipodomys and Microdipodops) themiddle ears are greatly inflated the tympano-ossicular systemvery efficient the organ of Corti extremely modified and low-frequencysensitivity extremely acute. In the most mesic genera (Heteromysand Liomys) the middle and inner ears show few modificationsand the low frequency sensitivity is rather poor. Perognathusis intermediate in habitat structure, and low frequency sensitivity.Experimental data demonstrate that the low frequency sensitivityin Dipodomys is adaptive in predator avoidance.     

Hypotheses Derived from Morphological Data: When and How They Are Useful

SYNOPSIS. This paper discusses the validity of hypotheses basedon morphological data,and distinguishes between hypotheses,which are testable, and speculation, which is not. Specificexamples from the mammalian auditory system are examined: arecently evolved, highly derived character (enlarged middleears in desert rodents); and an older, more general character(the inner and outer hair cells of the mammalian organ of Corti).It is concluded that morphologically-based hypotheses are powerfuland important when accompanied by experimental data.     

Neuronal Repair and Recovery of Function in the Polyclad Flatworm, Notoplana acticola

SYNOPSIS. Polyclad flatworms are unable to regenerate or replaceparts of their central nervous systems but are able to repairlesions in the cerebral ganglion or peripheral nerve plexusesRepair of nerve lesions is very rapid and functional activityis reestablished within 48 hr Bilateral coordination can bereestablished in animals with either split brains or in animalswhere half of the brain, on one side has been excised Animalswith transplanted brains display normal behavior even if thebrain has been rotated and inverted Likewise, in animals whichhad the brain transplanted into the tail region, cerebral dominancewas reestablished even after the brain was rotated through 180°A time sequence of events during healing of cut nerves has beenestablished Sprouting starts approximately 4 hr after injuryContact between a filopodium from the proximal section and distalstump occurs approximately 14 hr after the injury Filopodiaare subsequently retracted and the area of contact between thetwo segments increased After 72 hr there are no filopodia andthe axon appears to be intact Healing between the two cut ends,therefore appears to involve fusion of the cut axons Indirectevidence suggests that neurones recognize their own specificsegments.