Seasonal change in irradiance: a zeitgeber for circannual rhythms in ground squirrels.

1. Zeitgebers for circannual rhythms have been elusive. Demonstration that an external factor is a zeitgeber requires proof of a phase-shift that endures for several years. 2. The California ground squirrel (Spermophilus beecheyi) is an ideal subject. Many features of behavior have circannual rhythms of which change in mass is the easiest to measure. The squirrels thrive in captivity for up to 10 years. The squirrels were kept in individual cages in an air conditioned room, fed lab chow ad lib, and weighed twice a month. They were exposed to a 6-month phase shift of (a) length of day (b) seasonal change in temperature, (c) both, (d) seasonal cycle of irradiance. 3. The squirrels maintained circannual rhythms for up to 9 years. Entrainment was evident only by squirrels exposed to seasonal change in irradiance.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel (Funambulus pennanti). Palm squirrels showed strongly diurnal locomotor activity rhythms (? 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel ( Funambulus pennanti ). Palm squirrels showed strongly diurnal locomotor activity rhythms (~ 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

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 individual development of circadian temperature rhythm in infants

Abstract

The aim of this study was to explore the individual development of the circadian rhythm of human body temperature. Dailyperiodicchanges of body temperature were examined longitudinally in four infants from 3 to 18 months of age. At the 3rd month of life, the day‐night rhythm of the body temperature was obscure but at the 6th month it became moreevident. From the 12th month on, the circadian temperature rhythm with phase similar to that of the adult was discerned. However, the amplitude in circadian rhythm was significantly larger in children between 6 and 18 months of age than in the adult. These findings suggest that the adult type of circadian rhythm of human body temperature is established during the first year of life with regard to phase but not to amplitude.     

The influence of temperature and photoperiod on the reproductive diapause and cold tolerance of spotted‐wing drosophila,Drosophila suzukii

Knowledge regarding the reproductive status of spotted‐wing drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), is of critical importance in predicting potential infestations of this invasive pest, as eggs are laid in ripe or ripening fruit of several commercially important small‐fruit crops. Token environmental stimuli for the induction of reproductive diapause and improved cold tolerance were identified for D. suzukii. Diapause induction was evaluated by assessing, via dissection, the number of mature eggs carried by field‐captured D. suzukii and laboratory‐reared D. suzukii held under various temperature and photoperiod regimes. Egg load decreased over time in females trapped from July to December at sites in Benton County, OR, and Ontario County, NY, both USA, and reached zero eggs by December at all sites. Photoperiods below 14 h of day length led to reduced egg maturation in laboratory‐reared flies held at moderate temperatures (15 or 20 °C). Whereas very few mature eggs were found in females held at 10 °C under short‐ or long‐day photoperiods for several weeks after eclosion, a spontaneous return to ovarian maturity was observed in short‐day‐entrained females after 7 weeks. Diapause termination was investigated by evaluating fecundity in diapausing females returned to optimal environmental conditions. Whereas long‐day‐entrained flies began producing offspring immediately upon return to optimal conditions, short‐day‐entrained flies returned after 1 and 6 weeks at 10 °C were slower to produce offspring than colony flies or short‐day‐entrained flies returned after 7 weeks. Cold tolerance was evaluated by observing chill coma recovery rates after 24 h exposure to ?1 °C. Cold‐acclimated (diapausing) females recovered from chill coma faster than cold‐hardened or unacclimated females.     

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.     

Delayed effect of pinealectomy on hibernation of the golden-mantled ground squirrel

Pinealectomy or radical sham pinealectomy were performed on adult golden-mantled ground squirrels,Spermophilus (=Citellus) lateralis, approximately 1 month prior to the date of normal winter emergence. The first hibernatory period and subsequent active season were not different in either of the operated groups from intact animals. However, although the initiation of the second hibernatory period was not affected in the pinealectomized animals, this group failed to show the progressive increase in the length of heterothermic bouts that is characteristic of normal hibernation. Also, terminal arousal occurred approximately 6 weeks earlier in the second year after pinealectomy. Male squirrels showed a corresponding time compression in their annual gonadal cycle, as was assessed by testicular state.These results suggest that the pineal gland of the golden-mantled ground squirrel is involved in the expression of the annual hibernatory cycle. In the absence of the pineal gland the adult of this species is unable to sustain the normal depth and duration of hibernation in the second over-wintering period following pinealectomy.We have carried out additional experiments with young, laboratory-bornS. lateralis and with field-caught, adultS. richardsonii. The results of these studies also are described in this paper.Presented at the Ninth International Congress of Biometeorology, 23 Sept – 1 Oct 1981, Osnabrück and Hohenheim, FRG.     

Circannual rhythms of ground squirrels: a test of the frequency demultiplication hypothesis

The frequency demultiplication hypothesis (FDH) posits that circannual rhythms are generated from circadian cycles by frequency transformation to the lower-frequency rhythm. To test the FDH, we determined the periods of the circannual body mass and estrous cycles of golden-mantled ground squirrels with circadian locomotor activity rhythms entrained to 23-, 24-, or 25-hr days (T-cycles). Circannual period length did not differ among squirrels entrained to the different T-cycles; intergroup ranges were 298-314 days and 303-312 days, respectively, for body mass and estrus. These results are not consistent with the FDH and suggest instead that separate mechanisms generate circadian and circannual rhythms. In ground squirrels the circannual system influences circadian organization, but a reciprocal influence of circadian on circannual rhythms has yet to be demonstrated.     

From the ground up: biotic and abiotic features that set the course from genes to ecosystems

Spatial variation in cone serotiny in Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) across Yellowstone National Park influences initial pine recruitment after stand‐replacing fire with tremendous population, community, and ecosystem consequences. A previous study showed that much of the spatial variation in serotiny results from the balance of selection arising from high frequencies of fire favoring serotiny countered by opposing selection exerted by American red squirrels (Tamiasciurus hudsonicus) as seed predators. This earlier study, however, assumed stable local red squirrel densities over multiple generations of pines. Here, we examine environmental properties that might contribute to long‐term stability in the densities of red squirrels among sites. We found that the amount of clay in the soil, an indicator of plant and fungal growth—the latter an important food resource for red squirrels—and the coefficient of variation (CV) in diameter at breast height (DBH) of forest trees together account for a substantial amount of variation in red squirrel density. Soil development occurs over very long time scales, and thus, intersite variation in the amount of clay is unlikely to shift across pine generations. However, CV of DBH and squirrel density increase with stand age, which acts to amplify selection against serotiny with increasing interfire interval. Regardless, much of the variation in the CV of DBH is accounted for by soil bulk density, mean annual temperature, and surface curvature, which are unlikely to vary in their relative differences among sites over time. Consequently, these soil and abiotic attributes could contribute to consistent spatial patterns of red squirrel densities from one pine generation to the next, resulting in consistent local and spatial variation in selection exerted by red squirrels against serotiny.     

Phase control of circadian activity in the antelope ground squirrel

Abstract

Wheel‐running activity of forty antelope ground squirrels, Ammospermophilus leucurus, was monitored for several months in both an outdoor cage and in the laboratory. The squirrels demonstrated a highly diurnal pattern which persisted in “constant conditions.” After removal from the field the initial free‐running period was close to 24 hrs, but typically lengthened in a nearly linear fashion at least for the first few months. There was no evidence of any difference in this trend for squirrels, in D/D, L/L 100 lx, 250 lx or 1200 lx. Eventually, about 90 percent of the squirrels had periods longer than 24 hrs.

The synchronizing capacity of the natural photoperiod was used to “catch the free‐running rhythm” and thereby demonstrate a response curve. Synchronization occurred by a shortening of the period when the time of sunrise was between 125° and 0° (subjective night) and by a lengthening of the period when the time of sunrise was between 0° and 125° (subjective day).

To more thoroughly examine the underlying mechanisms of phase control, phase‐response curves based on sixty one light‐pulse experiments were constructed. Comparisons of curves based on 6‐hr and 15‐min pulses, showed that the integral action of light is important (i.e., the entire pulse is involved in phase shifting). It was found that light pulses not only affected the phase of the rhythm but also the phase. Large phase shifts were usually associated with decreases in free‐running period. Several hypotheses on the controlling mechanisms were advanced.     

Conservation and multivariate analysis utility in characterization of ecogegraphical relationships of Trifolium and Lotus species

The maintenance of species in an environment and the durability of rangeland and fallow improvement depend on the choice of populations used. The study of abundance and the relationship between the natural distribution of spontaneous legumes species of forage and/or pastoral interest and environmental factors is an important step for an adequate utilization of local genetic resources. Collection of annual Trifolium species and the genus of Lotus were conducted at 45 sites in Morocco in May/June 2004. Ecogeographical information was recorded at each site. Twelve species of annual Trifolium and two species of Lotus were identified. Most prolific were T. tomentosum and L. corniculatus. A principal components analysis and canonical variate analysis were conducted to group the sites, using the ecogeographical variables collected at each site. The variables that were found to be most important in grouping the sites were mean coldest month temperature, mean hottest month temperature, mean annual rainfall, latitude, longitude and soil pH. None of the species identified were limited to only one ecogeographical group.     

Data logging of body temperatures provides precise information on phenology of reproductive events in a free-living arctic hibernator

Precise measures of phenology are critical to understanding how animals organize their annual cycles and how individuals and populations respond to climate-induced changes in physical and ecological stressors. We show that patterns of core body temperature (T _b) can be used to precisely determine the timing of key seasonal events including hibernation, mating and parturition, and immergence and emergence from the hibernacula in free-living arctic ground squirrels (Urocitellus parryii). Using temperature loggers that recorded T _b every 20 min for up to 18 months, we monitored core T _b from three females that subsequently gave birth in captivity and from 66 female and 57 male ground squirrels free-living in the northern foothills of the Brooks Range Alaska. In addition, dates of emergence from hibernation were visually confirmed for four free-living male squirrels. Average T _b in captive females decreased by 0.5–1.0°C during gestation and abruptly increased by 1–1.5°C on the day of parturition. In free-living females, similar shifts in T _b were observed in 78% (n = 9) of yearlings and 94% (n = 31) of adults; females without the shift are assumed not to have given birth. Three of four ground squirrels for which dates of emergence from hibernation were visually confirmed did not exhibit obvious diurnal rhythms in T _b until they first emerged onto the surface when T _b patterns became diurnal. In free-living males undergoing reproductive maturation, this pre-emergence euthermic interval averaged 20.4 days (n = 56). T _b-loggers represent a cost-effective and logistically feasible method to precisely investigate the phenology of reproduction and hibernation in ground squirrels.     

Circannual and seasonal variations in plasma luteinizing hormone levels of ovariectomized ground squirrels (Spermophilus lateralis)

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and ovariectomized squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. LH was undetectable (less than 0.9 ng/ml) in plasma of intact females at all times of year. Females ovariectomized (OVX) at 9.5 months of age in March showed substantial increases in plasma LH in May and June but LH was undetectable between July and November. Females ovariectomized at 13 months of age in July first manifested detectable LH levels the following January and February (6-7 months post-ovariectomy). Very few adult females trapped in May and ovariectomized in August had detectable LH levels within 2 months of ovariectomy; however, females ovariectomized the following February had detectable LH titers 1 month later. Long-term studies of individual OVX squirrels indicated peak LH levels between March and June, 1980, relatively low or undetectable titers between August and December and elevated LH levels between January and March, 1981. The results are suggestive of a circannual rhythm of LH secretion which appears restricted to one season of the year and occurs independently of steroid feedback from the ovaries; ovarian steroids only modulate the levels of plasma LH during the brief annual period of hypothalamo-hypophysial activity. We suggest that onset and termination of LH release are mediated by central nervous system circannual clocks.     

Photosynthetic Acclimation to Temperature in the Desert Shrub, Larrea divaricata: I. Carbon Dioxide Exchange Characteristics of Intact Leaves

Larrea divaricata, a desert evergreen shrub, has a remarkable ability to adjust its photosynthetic temperature response characteristics to changing temperature conditions. In its native habitat on the floor of Death Valley, California, plants of this C₃ species when provided with adequate water are able to maintain a relatively high and constant photosynthetic activity throughout the year even though the mean daily maximum temperature varies by nearly 30 C from winter to summer. The temperature dependence of light-saturated net photosynthesis varies in concert with these seasonal temperature changes whereas the photosynthetic rate at the respective optimum temperatures shows little change.

Experiments on plants of the same age, grown at day/night temperatures of 20/15, 35/25, and 45/33 C with the same conditions of day length and other environmental factors, showed a similar photosynthetic acclimation response as observed in nature. An analysis was made of a number of factors that potentially can contribute to the observed changes in the temperature dependence of net CO₂ uptake at normal CO₂ and O₂ levels. These included stomatal conductance, respiration, O₂ inhibition of photosynthesis, and nonstomatal limitations of CO₂ diffusive transport. None of these factors, separately or taken together, can account for the observed acclimation responses. Measurements under high saturating CO₂ concentrations provide additional evidence that the observed adaptive responses are primarily the result of changes in intrinsic characteristics of the photosynthetic machinery at the cellular or subcellular levels. Two apparently separate effects of the growth temperature regime can be distinguished: one involves an increased capacity for photosynthesis at low, rate-limiting temperatures with decreased growth temperature, and the other an increased thermal stability of key components of the photosynthetic apparatus with increased growth temperature.

     

Seasonal cycle in vitamin A1/A2-based visual pigment composition during the life history of coho salmon (Oncorhynchus kisutch)

Microspectrophotometry of rod photoreceptors was used to follow variations in visual pigment vitamin A₁/A₂ ratio at various life history stages in coho salmon. Coho parr shifted their A₁/A₂ ratio seasonally with A₂ increasing during winter and decreasing in summer. The cyclical pattern was statistically examined by a least-squares cosine model, fit to the 12-month data sets collected from different populations. A₁/A₂ ratio varied with temperature and day length. In 1+ (>12 month old) parr the A₂ to A₁ shift in spring coincided with smoltification, a metamorphic transition preceding seaward migration in salmonids. The coincidence of the shift from A₂ to A₁ with both the spring increase in temperature and day length, and with the timing of seaward migration presented a challenge for interpretation. Our data show a shift in A₁/A₂ ratio correlated with season, in both 0+ (<12 months old) coho parr that remained in fresh water for another year and in oceanic juvenile coho. These findings support the hypothesis that the A₁/A₂ pigment pair system in coho is an adaptation to seasonal variations in environmental variables rather than to a change associated with migration or metamorphosis.     

Effects of the Reversal of Day and Night Temperatures on Tillering and on the Elongation of Stems and Leaf Blades of Wheat

Different cultivars of wheat (Triticum aestivum L.) were grownin cabinets, under a 12 h photoperiod, at constant temperatures,and high day/low night and low day/high night temperatures.Plants were also transferred at different ages, between 18/10°C and 10/18 °C regimes. Application of the growth regulatorsCCC and TIBA was tested at 18/10 °C and GA₃ and IAA at 10/18°C. The reversal of day and night temperatures did not affect spikedifferentiation or the numbers of leaves and elongating internodes.However, tillering and tiller development were markedly promotedby the low day/high night temperature regimes whereas the elongationof leaf blades and stem internodes were suppressed under theseregimes. These effects were attributed to the effects of thetemperature regimes on the endogenous hormone balance of theplants. Considering the results of the transfer and growth regulatortreatments it was concluded that there were no obligatory associationsamong the number of tillers appearing, their subsequent development,leaf blade length, and stem elongation. It is suggested thatthe study of the physiological mechanisms controlling thesecharacters may benefit from experimentation under reciprocallydiffering day night temperature regimes.     

Nonphotic entrainment in a diurnal mammal, the European ground squirrel (Spermophilus citellus).

Entrainment by nonphotic, activity-inducing stimuli has been investigated in detail in nocturnal rodents, but little is known about nonphotic entrainment in diurnal animals. Comparative studies would offer the opportunity to distinguish between two possibilities. (1) If nonphotic phase shifts depend on the phase of the activity cycle, the phase response curve (PRC) should be about 180 degrees out of phase in nocturnal and diurnal mammals. (2) If nonphotic phase shifts depend on the phase of the pacemaker, the two PRCs should be in phase. We used the diurnal European ground squirrel (Spermophilus citellus) in a nonphotic entrainment experiment to distinguish between the two possibilities. Ten European ground squirrels were kept under dim red light (<1 lux) and 20 +/- 1 degrees C. During the entrainment phase of the experiment, the animals were confined every 23.5 h (T) to a running wheel for 3 h. The circadian rhythms of 6 squirrels entrained, 2 continued to free run, and 2 possibly entrained but displayed arrhythmicity during the experiment. In a second experiment, a photic pulse was used in a similar protocol. Five out of 9 squirrels entrained, 1 did not entrain, and 3 yielded ambiguous results. During stable entrainment, the phase-advancing nonphotic pulses coincided with the end of the subjective day, while phase-advancing light pulses coincided with the start of the subjective day: mean psi(nonphotic) = 11.4 h; mean psi(photic) = 0.9 h (psi defined as the difference between the onset of activity and the start of the pulse). The data for nonphotic entrainment correspond well with those from similar experiments with nocturnal Syrian hamsters where psi(nonphotic) varied from 8.09 to 11.34 h. This indicates that the circadian phase response to a nonphotic activity-inducing stimulus depends on the phase of the pacemaker rather than on the phase of the activity cycle.     

Growth and development of infant giant pandas (Ailuropoda melanoleuca) at the Wolong Reserve,China

From 1991 to 1993 inclusive, seven infant giant pandas (Ailuropoda melanoleuca) were born at the China Conservation and Research Center for the Giant Panda. Average daily weight gain in the first 6 months for mother-reared infants (n = 5) was 71.3 g/day; for one partially mother-reared and partially hand-reared infant, 41.5 g/day; and for one completely hand-reared infant, 50.3 g/day. There was a significant difference in growth rates across the first 6 months in all methods of rearing. In addition, a comparison of growth rates across the three rearing methods showed significant differences in the first, second, third, fifth, and sixth months. Average daily body length increase for mother-reared infants was 4.1 mm/day; for partially mother-reared/partially hand-reared infants, 4.0 mm/day; and for the completely hand-reared infant, 2.8 mm/day. In mother-reared infants, body length increase during the first month was significantly greater than during the following months, and was slowest during the sixth month. At birth, infants were all pink in color with a light white coat of lanugo. Black pigmentation was first noted at 7–10 days of age, which was also the time that initial hair coat growth was seen. Eyes opened at 35–48 days of age. Ears opened at 31–50 days of age. Deciduous dentition was first seen at 82–121 days of age, while permanent dentition began to erupt at 350 days of age. © 1996 Wiley-Liss, Inc.