Early life experience drives short-term acclimation of metabolic and osmoregulatory traits in the leaf-eared mouse.

We studied the putative effect of early life experience on the physiological flexibility of metabolic and osmoregulatory traits in the leaf-eared mouse, Phyllotis darwini, an altricial rodent inhabiting seasonal Mediterranean environments. Adult individuals were collected in central Chile and maintained in breeding pairs. Pups were isolated after weaning and acclimated to different temperatures (cold or warm) and water availability (unrestricted and restricted) until adulthood. Subsequently, individuals were re-acclimated to the opposite treatment. Rodents reared in the warm and subjected to water restriction had lower basal metabolic rate (BMR), total evaporative water loss (TEWL) and body mass (Mb) compared with those developing in the cold treatment; nevertheless, individuals subjected to warm temperatures had greater relative medullary thickness (RMT) and urine concentrating ability (UCA). Cold-reared rodents re-acclimated to warm conditions exhibited physiological flexibility of metabolic traits; however, their osmoregulatory attributes did not vary. Conversely, warm-reared rodents re-acclimated to cold had reduced RMT and UCA, but the metabolic traits of these individuals did not change. These results suggest a trade-off between metabolic performance and renal capabilities that might hinder physiological acclimation. Our results support the hypothesis of ontogenetic dependence of short-term acclimation in osmoregulatory and metabolic traits in P. darwini.

Taxonomy of the Phyllotis osilae species group in Argentina; the status of the "Rata de los nogales" (Phyllotis nogalaris Thomas, 1921; Rodentia: Cricetidae).

The taxonomic status of populations of the genus Phyllotis from northwestern Argentina (NWA) has undergone recent changes, with the addition of two species (P. alisosiensis and P. anitae) to the traditionally recognized forms (P. caprinus, P. xanthopygus, and P. osilae). Three of these species (P. anitae, P. osilae, and P. alisosiensis) were included within the Phyllotis osilae species group. Most authors recognized three subspecies of P. osilae for NWA: P. osilae osilae, P. o. nogalaris, and P. o. tucumanus. Morphological, morphometric, and molecular studies based on recently collected specimens suggest that current classification does not reflect the diversity of this group in NWA, revealing the need of some taxonomic reallocations and new distributional delimitations. Here we propose that P. nogalaris must be recognized as a valid species and the restriction of P. osilae to southern Peru and central Bolivia. Following our results, we expect an outstanding improvement in the taxonomic knowledge of the Phyllotis osilae species group in the coming years.

Taxonomic revision of the Andean leaf-eared mouse, Phyllotis andium Thomas 1912 (Rodentia: Cricetidae), with the description of a new species.

The Andean Leaf-eared mouse, Phyllotis andium Thomas 1912, has been considered a widespread medium-size sigmodontine rodent (230 mm of total length and 35 grams approximately) that occurs from Tungurahua, Ecuador, through the Andes, to Lima, Peru. Previous studies performed on Phyllotis noted evidence of morphological geographical variation within the species, which is likely because of the several potential geographical barriers that exist within the distribution range of P. andium. We carried out a taxonomic revision of this species based on qualitative and quantitative morphological analyses of 330 specimens from 92 localities. This included appropriate comparisons with other species of the andium/amicus group and performed molecular analysis based on cytochrome b sequences. As a result, morphologic qualitative analysis suggested the recognition of three different taxa, which are supported by morphologic quantitative and molecular analyses. The three taxa here identified have allopatric distributional ranges separated by important geographic barriers. Following these identification criteria, P. andium is now recognized for the samples from Tungurahua, Ecuador to Huánuco, Peru, and includes melanius and fruticicolus as synonymous; the southern populations from the Ancash and Lima departments, in the western Peruvian Andes, are proposed to represent a new species; and we recognize P. stenops as a valid species with tamborum as a synonym. Finally, we postulate that the diversification of these three species is related to key events in the Andean orogeny.

Mesoamerican nephropathy: a neglected tropical disease with an infectious etiology?

An outbreak of unexplained and severe kidney disease, "Mesoamerican Nephropathy," in mostly young, male sugar cane workers emerged in Central America in the late 1990's. As a result, an estimated 20,000 individuals have died, to date. Unfortunately, and with great consequence to human life, the etiology of the outbreak has yet to be identified. The sugarcane fields in Chichigalpa, Chinandega, Nicaragua, have been involved in the outbreak, and during our initial investigation, we interviewed case patients who experienced fever, nausea and vomiting, arthralgia, myalgia, headache, neck and back pain, weakness, and paresthesia at the onset of acute kidney disease. We also observed a heavy infestation of rodents, particularly of Sigmodon species, in the sugarcane fields. We hypothesize that infectious pathogens are being shed through the urine and feces of these rodents, and workers are exposed to these pathogens during the process of cultivating and harvesting sugarcane. In this paper, we will discuss the epidemic in the Chichigalpa area, potential pathogens responsible for Mesoamerican Nephropathy, and steps needed in order to diagnose, treat, and prevent future cases from occurring.

Estimating hantavirus risk in southern Argentina: a GIS-based approach combining human cases and host distribution.

We use a Species Distribution Modeling (SDM) approach along with Geographic Information Systems (GIS) techniques to examine the potential distribution of hantavirus pulmonary syndrome (HPS) caused by Andes virus (ANDV) in southern Argentina and, more precisely, define and estimate the area with the highest infection probability for humans, through the combination with the distribution map for the competent rodent host (Oligoryzomys longicaudatus). Sites with confirmed cases of HPS in the period 1995-2009 were mostly concentrated in a narrow strip (~90 km × 900 km) along the Andes range from northern Neuquén to central Chubut province. This area is characterized by high mean annual precipitation (~1,000 mm on average), but dry summers (less than 100 mm), very low percentages of bare soil (~10% on average) and low temperatures in the coldest month (minimum average temperature -1.5 °C), as compared to the HPS-free areas, features that coincide with sub-Antarctic forests and shrublands (especially those dominated by the invasive plant Rosa rubiginosa), where rodent host abundances and ANDV prevalences are known to be the highest. Through the combination of predictive distribution maps of the reservoir host and disease cases, we found that the area with the highest probability for HPS to occur overlaps only 28% with the most suitable habitat for O. longicaudatus. With this approach, we made a step forward in the understanding of the risk factors that need to be considered in the forecasting and mapping of risk at the regional/national scale. We propose the implementation and use of thematic maps, such as the one built here, as a basic tool allowing public health authorities to focus surveillance efforts and normally scarce resources for prevention and control actions in vast areas like southern Argentina.

Ecological segregation in a small mammal hybrid zone: habitat-specific mating opportunities and selection against hybrids restrict gene flow on a fine spatial scale.

The degree to which closely related species interbreed is determined by a complex interaction of ecological, behavioral, and genetic factors. We examine the degree of interbreeding between two woodrat species, Neotoma bryanti and N. lepida, at a sharp ecological transition. We identify the ecological association of each genotypic class, assess the opportunity for mating between these groups, and test whether they have similar patterns of year-to-year persistence on our study site. We find that 13% of individuals have a hybrid signature but that the two parental populations and backcrosses are highly segregated by habitat type and use. Also, we find that adult hybrids are comparable to parental types in terms of year-to-year persistence on our site but that, among juveniles, significantly fewer hybrids reach adulthood on site compared to their purebred counterparts. Our analyses show that this hybrid zone is maintained by occasional nonassortative mating coupled with hybrid fertility, but that these factors are balanced by lower apparent survival of juvenile hybrids and habitat-based preference or selection that limits heterospecific mating while promoting backcrossing to habitat-specific genotypes. This system presents a novel example of the role that sharp resource gradients play in reproductive isolation and the potential for genetic introgression.

Melatonin, But not auxin, affects postnatal reproductive development in the marsh rice rat (Oryzomys palustris).

Melatonin and the plant hormone auxin (indole-3-acetic acid) have some structural similarity and, may thus exert comparable physiological effects on reproduction and growth. To test this possibility, I examined the effects of melatonin and auxin administration on reproductive and non-reproductive organ development in an animal model, the marsh rice rat Oryzomys palustris. Juvenile males housed under 14L:10D conditions were injected daily for four weeks with saline, melatonin, auxin, or melatonin and auxin, and the development of the testes and other organs was assessed. Melatonin alone significantly inhibited the development of the testes, seminal vesicles, Harderian glands, and overall somatic growth, but not the spleen. Auxin did not affect any endpoint measured. When melatonin was administered simultaneously with auxin, the melatonin effects dominated in suppressing reproduction and growth. The administration of melatonin or auxin in the drinking water produced results similar to the effects of melatonin and auxin injections reported herein. Lastly, both melatonin and auxin in the drinking water failed to alter any short photoperiod-induced reproductive inhibition. These data suggest that structural similarities between melatonin and auxin do not result in similar postnatal effects on reproductive and non-reproductive organ development on a long photoperiod and further suggest that melatonin and auxin do not operate through a common physiological mechanism.

Morphometric analysis of the placenta in the New World mouse Necromys lasiurus (Rodentia, Cricetidae): a comparison of placental development in cricetids and murids.

BACKGROUND: Stereology is an established method to extrapolate three-dimensional quantities from two-dimensional images. It was applied to placentation in the mouse, but not yet for other rodents. Herein, we provide the first study on quantitative placental development in a sigmodontine rodent species with relatively similar gestational time. Placental structure was also compared to the mouse, in order to evaluate similarities and differences in developmental patterns at the end of gestation. METHODS: Fetal and placental tissues of Necromys lasiurus were collected and weighed at 3 different stages of gestation (early, mid and late gestation) for placental stereology. The total and relative volumes of placenta and of its main layers were investigated. Volume fractions of labyrinth components were quantified by the One Stop method in 31 placentae collected from different individuals, using the Mercator software. Data generated at the end of gestation from N. lasiurus placentae were compared to those of Mus musculus domesticus obtained at the same stage. RESULTS: A significant increase in the total absolute volumes of the placenta and its main layers occurred from early to mid-gestation, followed by a reduction near term, with the labyrinth layer becoming the most prominent area. Moreover, at the end of gestation, the total volume of the mouse placenta was significantly increased compared to that of N. lasiurus although the proportions of the labyrinth layer and junctional zones were similar. Analysis of the volume fractions of the components in the labyrinth indicated a significant increase in fetal vessels and sinusoidal giant cells, a decrease in labyrinthine trophoblast whereas the proportion of maternal blood space remained stable in the course of gestation. On the other hand, in the mouse, volume fractions of fetal vessels and sinusoidal giant cells decreased whereas the volume fraction of labyrinthine trophoblast increased compared to N. lasiurus placenta. CONCLUSIONS: Placental development differed between N. lasiurus and M. musculus domesticus. In particular, the low placental efficiency in N. lasiurus seemed to induce morphological optimization of fetomaternal exchanges. In conclusion, despite similar structural aspects of placentation in these species, the quantitative dynamics showed important differences.

The taxonomic status of Oligoryzomys brendae Massoia, 1998 (Rodentia, Cricetidae), with comments on the availability of this name.

Oligoryzomys brendae was poorly described by Massoia in 1998 to include populations of the large form of Oligoryzomys that inhabits the Yungas and high altitudinal grasslands of northwestern Argentina, which were previously referred as O. longicaudatus, O. stolzmanni, O. destructor or Oligoryzomys sp. Contrary to some interpretations, we state that the name O. brendae is available, given that it fully accomplishes the requirements of the International Code of Zoological Nomenclature. We also present morphologic and genetic evidence showing that this taxon represents a distinct species and provide an emended diagnosis and re-description of it. In addition, the evidence at hand indicates that O. brendae is the only large-sized species of Oligoryzomys inhabiting northwestern Argentina.

Environmental factors affecting transmission risk for hantaviruses in forested portions of southern Brazil.

Using ecological niche modeling approaches, we assessed suites of environmental factors to identify areas of transmission risk in forested regions of southern Brazil. We studied all officially confirmed cases from the Epidemiological Surveillance System of the Brazilian Health Ministry from Atlantic Forest and Araucaria forest areas of southern Brazil (n=264), during 1993-2008. Among climate-related influences associated with hantavirus cases, winter precipitation and high photosynthetic mass (EVI) were particularly closely related to case distributions. Models identified broad risk areas for hantavirus transmission, extending along the coastal Atlantic Forest region from São Paulo to Rio Grande do Sul, eastern Paraguay, and Argentina. Host distribution maps indicate overlap of several species of Oligoryzomys in the transmission area, showing the necessity of more detailed reservoir-based studies to detect finer-scale infection foci.

Spermatocyte apoptosis, which involves both intrinsic and extrinsic pathways, explains the sterility of Graomys griseoflavus x Graomys centralis male hybrids.

Spermatogenic impairment and the apoptotic pathways involved in establishing sterility of male hybrids obtained from crossing Graomys griseoflavus females with Graomys centralis males were studied. Testes from G. centralis, G. griseoflavus and hybrids were compared at different ages. Terminal transferase-mediated dUTP nick-end labelling assay (TUNEL), Fas, Bax and cytochrome c labelling were used for apoptosis evaluation, and calbindin D(28k) staining as an anti-apoptotic molecule. In 1-month-old animals, spermatocytes were positive for all apoptotic markers, but moderate TUNEL (+) spermatocyte frequency was only found in G. centralis. At subsequent ages, the apoptotic markers were downregulated in testes from parental cytotypes, but not in hybrid testes. TUNEL (+) spermatocytes were present at 78% and 44% per tubule cross-section in 2- and 3-month-old hybrid animals, respectively. Pachytene spermatocyte death in adult hybrids occurs via apoptosis, as revealed by high caspase-3 expression. Calbindin was highly expressed in spermatocytes of adult hybrids, in which massive cell death occurs via apoptosis. Calbindin co-localisation with TUNEL or Fas, Bax and cytochrome c was very limited, suggesting an inverse regulation of calbindin and apoptotic markers. Hybrid sterility is due to breakdown of spermatogenesis at the pachytene spermatocyte stage. Both extrinsic and intrinsic pathways are involved in apoptosis of spermatocytes, which are the most sensitive cell type to apoptotic stimuli.

Reproduction, development and growth of Akodon lindberghi (Hershkovitz, 1990) (Rodentia, Muridae, Sigmodontinae) raised in captivity

The reproduction, development and growth of Akodon lindberghi were studied in captivity. The colony was derived from animals captured in Simão Pereira, Minas Gerais state, which represents a new area of geographical distribution known for this species. Twelve males and twelve females were crossed, producing 144 young in 53 litters. Post-partum oestrus was observed and gestation length was estimated in 23 days. Litter size ranged from 1 to 4 with a mean of 2.72 (SD = 0.97, n = 53) and modal size of 3. Sexual dimorphism was neither present in body mass at birth nor at weaning. There was a significant negative correlation between litter size and mass at birth or weaning. Permanent emergence of adult external appearance occurred at 15 days. Puberty for males and females was 43 and 42 days, respectively, and the first fecundation event for two females was recorded at 47 and 54 days of age. The weight growth was described by fitting a Gompertz model. No significant difference was found in any parameter of growth curves for males and females. Measurements (head-body, tail, hind foot and internal and external ear lengths) obtained for adult individuals also did not reveal the presence of sexual dimorphism.

A reprodução, o crescimento e o desenvolvimento de Akodon lindberghi foram estudados em cativeiro. A colônia foi derivada de animais capturados em Simão Pereira, Minas Gerais, Brasil, localidade que representa uma nova área de distribuição geográfica conhecida para esta espécie. Doze machos e doze fêmeas foram acasalados, produzindo 144 filhotes em 53 ninhadas. Observou-se estro pós-parto e o período de gestação foi estimado em 23 dias. O tamanho médio da ninhada foi de 2,72 (SD = 0,97, n = 53), com amplitude de 1 a 4 e moda de 3 filhotes. Não houve dimorfismo sexual do peso dos filhotes ao nascimento e ao desmame. O tamanho da ninhada foi negativamente correlacionado com o peso ao nascimento e ao desmame. A emergência da aparência externa adulta (pêlos, dentes e olhos e orelhas abertos) ocorreu aos 15 dias. A puberdade em machos e fêmeas ocorreu aos 43 e 42 dias, respectivamente, e a primeira fecundação para duas fêmeas, aos 47 e 54 dias. O crescimento do peso foi descrito através do modelo de Gompertz. Não houve diferença significativa entre machos e fêmeas em nenhum parâmetro das curvas de crescimento, bem como nas medidas corporais (cabeça-corpo, cauda, pé posterior, e orelhas interna e externa) obtidas para animais adultos.

Reproduction, development and growth of Akodon lindberghi (Hershkovitz, 1990) (Rodentia, Muridae, Sigmodontinae) raised in captivity.

The reproduction, development and growth of Akodon lindberghi were studied in captivity. The colony was derived from animals captured in Simão Pereira, Minas Gerais state, which represents a new area of geographical distribution known for this species. Twelve males and twelve females were crossed, producing 144 young in 53 litters. Post-partum oestrus was observed and gestation length was estimated in 23 days. Litter size ranged from 1 to 4 with a mean of 2.72 (SD = 0.97, n = 53) and modal size of 3. Sexual dimorphism was neither present in body mass at birth nor at weaning. There was a significant negative correlation between litter size and mass at birth or weaning. Permanent emergence of adult external appearance occurred at 15 days. Puberty for males and females was 43 and 42 days, respectively, and the first fecundation event for two females was recorded at 47 and 54 days of age. The weight growth was described by fitting a Gompertz model. No significant difference was found in any parameter of growth curves for males and females. Measurements (head-body, tail, hind foot and internal and external ear lengths) obtained for adult individuals also did not reveal the presence of sexual dimorphism.

Trypanosoma cruzi (Kinetoplastida Trypanosomatidae): ecology of the transmission cycle in the wild environment of the Andean valley of Cochabamba, Bolivia.

An active Trypanosoma cruzi transmission cycle maintained by wild rodents in the Andean valleys of Cochabamba Bolivia is described. Wild and domestic Triatoma infestans with 60% infection with T. cruzi were found and was evidenced in 47.5% (rodents) and 26.7% (marsupial) by parasitological and/or serologycal methods. Phyllotis ocilae and the marsupial species Thylamys elegans, are the most important reservoirs followed by Bolomys lactens and Akodon boliviensis. In spite of both genotypes (TCI and TCII) being prevalent in Bolivia, in our study area only T. cruzi I is being transmitted. Our data suggest that wild T. infestans and wild small mammals play an important role in the maintenance of the transmission cycle of T. cruzi. Furthermore, the finding of high prevalence of T. cruzi infection in wild T. infestans point to the risk of the dispersion of Chagas' disease.

Developmental regulation of skull morphology II: ontogenetic dynamics of covariance.

Canalization may play a critical role in molding patterns of integration when variability is regulated by the balance between processes that generate and remove variation. Under these conditions, the interaction among those processes may produce a dynamic structure of integration even when the level of variability is constant. To determine whether the constancy of variance in skull shape throughout most of postnatal growth results from a balance between processes generating and removing variation, we compare covariance structures from age to age in two rodent species, cotton rats (Sigmodon fulviventer) and house mice (Mus musculus domesticus). We assess the overall similarity of covariance matrices by the matrix correlation, and compare the structures of covariance matrices using common subspace analysis, a method related to common principal components (PCs) analysis but suited to cases in which variation is so nearly spherical that PCs are ambiguous. We find significant differences from age to age in covariance structure and the more effectively canalized ones tend to be least stable in covariance structure. We find no evidence that canalization gradually and preferentially removes deviations arising early in development as we might expect if canalization results from compensatory differential growth. Our results suggest that (co)variation patterns are continually restructured by processes that equilibrate variance, and thus that canalization plays a critical role in molding patterns of integration.

Effects of photoperiod, melatonin, and the pineal gland on compensatory gonadal hypertrophy during postnatal development in the marsh rice rat (Oryzomys palustris).

The roles of photoperiod, melatonin, and the pineal gland in regulating the magnitude of compensatory gonadal hypertrophy (CGH) and other reproductive and non-reproductive organ growth during post-weaning development were examined in the marsh rice rat Oryzomys palustris. Juvenile rice rats of both sexes were left gonadally intact (control group) or unilaterally castrated (ULC) and housed on 12L:12D, 14L:10D, or 16L:8D. Within a photoperiod (14L:10D and 16L:8D, but not 12L:12D), growth of the remaining testis, but not the remaining ovary, as well as several additional organs in both sexes were significantly affected, suggesting that the compensatory hypertrophy of the testis is photoperiod-dependent. There was no effect of testis asymmetry on CGH as ULC of either testis in rice rats housed on 14L:10D resulted in a comparable increase of CGH. Melatonin implants in rice rats maintained on 16L:8D had little to no effect (CGH included) on most parameters examined. Both melatonin implants and pinealectomy (separate experiments) in rice rats transferred to 12L:12D prevented short photoperiod-induced effects on CGH, the growth of the reproductive organs and the Harderian glands. Evening melatonin injections had a significant inhibitory effect on the growth of the remaining testis (no CGH was observed) and all other parameters measured. Lastly, ULC did not alter the percentage of males which successfully mated compared to intact animals. Taken together, these data suggest that photoperiod, melatonin, and the pineal gland can affect and regulate reproductive (e.g., CGH in some cases) and non-reproductive growth during postnatal development in the marsh rice rat.

Developmental regulation of skull morphology. I. Ontogenetic dynamics of variance.

In the absence of processes regulating morphogenesis and growth, phenotypic variance of a population experiencing no selective mortality should increase throughout ontogeny. To determine whether it does, we measure variance of skull shape using geometric morphometrics and examine its ontogenetic dynamics in the precocial cotton rat (Sigmodon fulviventer) and the altricial house mouse (Mus musculus domesticus). In both species, variance of shape halves between the two youngest samples measured (between 1 and 10 days postnatal and 10 and 15 days postnatal, respectively) and thereafter is nearly constant. The reduction in variance did not appear to result from a general regulation of skull size or developmental timing, although skull size may also be regulated and developmental timing is an important component of the variation in skull shape of young house mice. The ontogenetic dynamics of variance suggest two possible scenarios. First, variation generated during fetal or early postnatal growth is not immediately compensated and therefore accumulates, whereas later in growth, variation is continually generated and rapidly compensated. Second, variation generated during fetal and early postnatal growth is rapidly compensated, after which no new variance is produced. Based on a general model for bone growth, we hypothesize that variance is generated when bone grows under the direction of disorganized muscular movements and decreases with increasing neuromuscular control. Additionally, increasing coherence of signals transmitted by the growing brain and sensory organs, which exert tensile forces on bone, may also canalize skull shape.

Do precocial mammals develop at a faster rate? A comparison of rates of skull development in Sigmodon fulviventer and Mus musculus domesticus.

Variation in neonatal maturity among mammals is often explained by variation in gestation length, but species may also differ in developmental rate, a quantity that is difficult to measure because the conventional formalism makes two important and potentially unrealistic assumptions: (1) ontogeny of form can be described by a single line, and (2) species have the same ontogeny of form. We examine two species, one precocial (Sigmodon fulviventer), the other altricial (Mus musculus domesticus), and find that neither assumption is met. Therefore, we introduce an alternative metric, the rate of shape differentiation away from the average neonate. We find that S. fulviventer has a lower developmental rate than M. m. domesticus; consequently, while more mature at birth, S. fulviventer loses ground to M. m. domesticus over time. Surprisingly, despite differences in gestation length and developmental rate, these species reach developmental and life-history milestones at nearly identical degrees of skull shape maturity.

Effects of prenatal and postnatal photoperiods and of the pineal gland on early testicular development in the marsh rice rat (Oryzomys palustris).

Prepubertal male rice rats are responsive reproductively to photoperiod early in postnatal development. Testicular growth was inhibited in animals gestated and reared to 28 days of age on photoperiods ranging from 8 to 13 h of light per day, and stimulated in animals reared on 14 h or more of light per day, suggesting a critical photoperiod between 13 and 14 h for testicular growth. In addition, postnatal photoperiod markedly affected testicular development in animals exposed to various prenatal and postnatal photoperiods. Postnatal exposure to constant light unmasked effects of the prenatal photoperiod on testicular development. Exposure of rice rats to short or long photoperiods during the period of lactation (Days 1-14 of life) affected testicular maturation. In addition, alteration of the photoperiod experienced after Day 14 of life (post-lactational) markedly affected testicular development and was the primary factor determining whether maturation would occur. Finally, pinealectomy had little to no effect on the magnitude of testicular development in animals maintained on the presurgical photoperiod, but did prevent animals from demonstrating the appropriate testicular response to a new photoperiod after surgery. These data suggest that early testicular development in rice rats is affected by the prenatal and postnatal photoperiod and that the pineal gland is involved in the transduction of this environmental information.

Effect of photoperiod on gonadal maintenance and development in the marsh rice rat (Oryzomys palustris).

Reproductive responsiveness to photoperiod was examined in adult and prepubertal rice rats. In Experiment 1, adult males on 16L:8D and 14L:10D maintained testicular size, while exposure to 12L:12D and 10L:14D resulted in testicular regression, followed by spontaneous testicular recrudescence. Adult females (Experiment 2) on long photoperiods had larger uterine and ovarian weights at most times examined and maintained vaginal patency compared to females on short photoperiods. Reproductive development of male rice rats (Experiment 3) was stimulated on 16L:8D and 14L:10D with rapid testicular development occurring by 12 weeks of age, while in males reared on 12L:12D and 10L:14D reproductive development proceeded slowly and was completed at approximately 26 weeks of age. Females on 16L:8D and 14L:10D experienced rapid uterine development compared to those on 12L:12D and 10L:14D. Ovarian weights on 14L:10D were significantly greater than ovarian weights on 12L:12D. Finally, vaginal patency was attained more rapidly on 16L:8D than on 14L:10D and was markedly inhibited on 12L:12D and 10L:14D. These data show that reproduction in adult male and female rice rats is regulated, at least in part, by photoperiod and that reproductive development in juveniles is also under photoperiodic control.