Seasonal Food Availability Research Articles

Changes in body condition and body size affect breeding and recruitment in fluctuating house mouse populations in south‐eastern Australia

AbstractChanges in body condition and body size in field populations of house mice, Mus domesticus, were examined to investigate why mouse populations do not increase rapidly in some years when favourable environmental and demographic conditions indicate they might. Mice had repeated seasonal patterns each year in breeding, growth rates and body condition that reflected the seasonal availability of food, but mean levels for each parameter varied among years. In most years mice lost body condition during summer, breeding declined and population growth slowed. Rapid population growth occurred when body condition was generally high and was maintained throughout summer. Female mice with large body length were more likely to breed than smaller mice, at all times, but changes in body condition accounted for most of the variability in female breeding activity between years and between habitats, and for the seasonal changes in the importance of body length. During rapid population growth, the recruitment rate of juveniles relative to the number of breeding females was 150–300% higher than in other years but adult survival rates were not higher. The data indicate that the ability of mice to maintain body condition, particularly when subject to moisture stress in summer, affects the proportion of females breeding, the number of juveniles weaned and their body condition at weaning, and is promoted by foraging conditions that favour maintenance of juvenile body condition after weaning. These factors, in turn, greatly affect juvenile recruitment rates and eventual population density of mice. Low juvenile survival is suggested as a reason that numbers of house mice in southern Australian cereal‐growing areas do not increase rapidly in some years when other parameters are favourable. Similar processes are likely to play a role in regulating other rodent populations.

Behavioral flexibility of a group of bearded capuchin monkeys (Cebus libidinosus) in the National Park of Brasília (Brazil): consequences of cohabitation with visitors

Increasing urbanization and deforestation have enhanced the opportunities of contact between humans and monkeys and the impact of human activities on primate behavior is receiving growing attention. This study explores whether activity budgets and diet of a group of capuchin monkeys (Cebus libidinosus) inhabiting the area of the swimming pools of the National Park of Brasília is affected by the presence of visitors providing food to them. During one year, both in the dry and the wet seasons, we scored capuchins' behavior via scan sampling every ten minutes. Results showed that this group spent less time foraging for wild foods than other comparable groups living in similar habitats. Moreover, capuchins relied more on human food during the dry season, when pulpy fruits were less available, than in the wet season. Our findings confirm other studies on different monkey species that have shown that access to human food decreases the time spent foraging for wild food and the home range size. They also show that capuchins are able to modify their diet, to exploit alternative food sources, and to change their activity budget in response to the availability of new food opportunities and to seasonal food availability.

Using stable isotopes to assess carbon and nitrogen turnover in the Arctic sympagic amphipod Onisimus litoralis

Food web studies based on stable C and N isotope ratios usually assume isotopic equilibrium between a consumer and its diet. In the Arctic, strong seasonality in food availability often leads to diet switching, resulting in a consumer's isotopic composition to be in flux between different food sources. Experimental work investigating the time course and dynamics of isotopic change in Arctic fauna has been lacking, although these data are crucial for accurate interpretation of food web relationships. We investigated seasonal (ice-covered spring vs. ice-free summer) and temperature (1 vs. 4 degrees C) effects on growth and stable C and N isotopic change in the common nearshore Arctic amphipod Onisimus litoralis following a diet switch and while fasting in the laboratory. In spring we found no significant temperature effect on N turnover [half-life (HL) estimates: HL-N = 20.4 at 4 degrees C, 22.4 days at 1 degrees C] and a nonsignificant trend for faster growth and C turnover at the higher temperature (HL-C = 13.9 at 4 degrees C, 18.7 days at 1 degrees C). A strong seasonal effect was found, with significantly slower growth and C and N turnover in the ice-free summer period (HL-N = 115.5 days, HL-C = 77.0 days). Contrary to previous studies, metabolic processes rather than growth accounted for most of the change in C and N isotopic composition (84-89 and 67-77%, respectively). This study provides the first isotopic change and metabolic turnover rates for an Arctic marine invertebrate and demonstrates the risk of generalizing turnover rates based on taxon, physiology, and environment. Our results highlight the importance of experimental work to determine turnover rates for species of interest.

Identification and Expression Analysis of Peroxisome Proliferator-Activated Receptors cDNA in a Reptile, the Leopard Gecko (Eublepharis macularius)

Despite the physiological and evolutionary significance of lipid metabolism in amniotes, the molecular mechanisms involved have been unclear in reptiles. To elucidate this, we investigated peroxisome proliferators-activated receptors (PPARs) in the leopard gecko (Eublepharis macularius). PPARs belong to a nuclear hormone-receptor family mainly involved in lipid metabolism. Although PPARs have been widely studied in mammals, little information about them is yet available from reptiles. We identified in the leopard gecko partial cDNA sequences of PPARalpha and beta, and full sequences of two isoforms of PPARgamma. This is the first report of reptilian PPARgamma mRNA isoforms. We also evaluated the organ distribution of expression of these genes by using RT-PCR and competitive PCR. The expression level of PPARalpha mRNA was highest in the large intestine, and moderate in the liver and kidney. The expression level of PPARbeta mRNA was highest in the kidney and large intestine, and moderate in the liver. Similarly to the expression of human PPARgamma isoforms, PPARgammaa was expressed ubiquitously, whereas the expression of PPARgammab was restricted. The highest levels of their expression, however, were observed in the large intestine, rather than in the adipose tissue as in mammals. Taken together, these results showed that the profile of PPARbeta mRNA expression in the leopard gecko is similar to that in mammals, and that those of PPAR alpha and gamma are species specific. This may reflect adaptation to annual changes in lipid storage due to seasonal food availability.

Histamine H3 Receptor and Orexin A Expression During Daily Torpor in the Djungarian Hamster (Phodopus sungorus)

Seasonal animals use different strategies to reduce energy expenditure in the face of reduced seasonal food availability. For example, the ground squirrel enters a hibernation state with reduced metabolism, hypothermia and suppressed central nervous system activity, whereas the Djungarian hamster (Phodopus sungorus) employs daily bouts of torpor associated with reduced body temperature and energy expenditure. Studies in the hibernating ground squirrel implicate an increase in histamine synthesis and histamine H(3) receptor expression in the brain as a central mechanism governing hibernation. In the present study, we demonstrate an up-regulation of H(3) receptors in several brain nuclei in the Djungarian hamster during bouts of daily torpor, a shallow form of hypothermia, suggesting that histaminergic pathways may play a general role in maintaining low body temperature and torpor state in mammals. These regions include the arcuate nucleus, dorsomedial hypothalamus, suprachiasmatic nucleus, dorsal lateral geniculate nucleus and tuberomammillary nucleus. Interestingly, expression of the mRNA for orexins, a group of neuropeptides that increase wakefulness, remains unchanged during the arousal from daily torpor, suggesting that this classic 'arousal' pathway is not involved in the transition from a hypothermic to the euthermic state.

Early is better: seasonal egg fitness and timing of reproduction in a zooplankton life‐history model

Timing of reproduction influences future prospects of offspring and therefore the reproductive value of parents. Early offspring are often more valuable than later ones when food availability and predation risk fluctuate seasonally. Marine zooplankton have evolved a diversity of life history strategies in response to seasonality. We present a state‐dependent life history model for the annual and herbivorous high‐latitude copepodCalanoides acutus. Individuals are characterised by four states; developmental stage, structural size, energy reserves and vertical location. There are two habitats, a surface habitat with seasonal predation risk and food availability, and a safer deep habitat with no food and low metabolism (diapause). Optimal life histories (diapause and energy allocation strategies) are found by dynamic programming. Seasonal egg fitness (reproductive value) emerges from the model and peak values are typically before the feeding season. Disentangling the fitness components, we conclude that seasonality in egg fitness is caused both by environmental seasonality in food and predation risk and by time‐constraints on development and diapause preparation. Realised egg production, as predicted from population simulations, does not match the seasonal peak in offspring fitness but is delayed relative to peak egg fitness. We term this an ‘internal life history mismatch’ as constraints and tradeoffs cause sub‐optimal birth dates for most eggs whereas mothers maximise their reproductive value by high number of eggs rather than few and optimally timed eggs. The earliest eggs have a disproportionately high contribution to population recruitment, emphasising the importance of early eggs and the need to understand seasonal patterns in offspring fitness.

Seasonal Food Use Strategies of Semnopithecus vetulus nestor, at Panadura and Piliyandala, Sri Lanka

Field studies on Semnopithecus vetulus have shown high folivory and the ability to feed heavily on mature leaves, which are constantly available. In research spanning 19 mo, I examined the feeding behavior of 2 free-ranging groups of Semnopithecus vetulus nestor in home gardens and rubber monocultures at Panadura (PT1 group) and Piliyandala (R1 group), Sri Lanka. Overall, results showed that >80% of their diet comprised seasonal plant parts, largely fruits. Despite differences in spatial and temporal food availability in their respective habitats, seasonal plant parts dominated the diets of both groups except briefly (2 mo) for R1 when mature leaf petioles were the main plant food. Both groups increased their use of seasonal foods with heightened seasonal food availability, and increased dietary diversity with declining use of their highest-ranked specific item of diet: fruits of Artocarpus heterophyllus (jakfruit, Moraceae). PT1, which was in a species-rich environment, maintained a high intake of seasonal foods year round by exploiting a large number of species, mainly for fruits. In contrast, R1, in a habitat with significantly lower tree species richness, had a less diverse diet but maintained an equally high intake of seasonal foods, primarily fruits and seeds, by exploiting a few species heavily. My study also highlights the dietary flexibility of a single colobine species in space and time. Such information is useful for conservation planning because rapidly occurring changes are taking place in natural colobine habitats.

Induction of pulsatile secretion of leptin in horses following thyroidectomy

Endocrine characteristics of Quarter Horse-type mares were determined during a 68 h feed deprivation and again in the same mares following surgical thyroidectomy (THX). A crossover experimental design was implemented, in which mares received brome hay available ad libitum (FED) or were food deprived (RES) for 68 h. Blood samples were collected every 20 min for 48 h, beginning 20 h after the onset of food deprivation. Concentrations of triiodothyronine and thyroxine were undetectable post-THX. Plasma concentrations of thyrotropin were greater post-THX versus pre-THX (P<0 x 001). Plasma concentrations of leptin were greater in the THX FED group than in the THX RES group (P<0 x 01). The existence of leptin pulse secretion was found only in post-THX compared with the same horses pre-THX (P=0 x 02). We theorize that non-pulsatile secretion of leptin may have contributed to the survival of this species, as it evolved in the regions of seasonal availability of food. Lack of pulsatile secretion of leptin may contribute to the accumulation of energy stores by modulating leptin sensitivity.

Hibernating black bears ( Ursus americanus) experience skeletal muscle protein balance during winter anorexia

Black bears spend four to seven months every winter confined to their den and anorexic. Despite potential for skeletal muscle atrophy and protein loss, bears appear to retain muscle integrity throughout winter dormancy. Other authors have suggested that bears are capable of net protein anabolism during this time. The present study was performed to test this hypothesis by directly measuring skeletal muscle protein metabolism during the summer, as well as early and late hibernation periods. Muscle biopsies were taken from the vastus lateralis of six free-ranging bears in the summer, and from six others early in hibernation and again in late winter. Protein synthesis and breakdown were measured on biopsies using 14C-phenylalanine as a tracer. Muscle protein, nitrogen, and nucleic acid content, as well as nitrogen stable isotope enrichment, were also measured. Protein synthesis was greater than breakdown in summer bears, suggesting that they accumulate muscle protein during periods of seasonal food availability. Protein synthesis and breakdown were both lower in winter compared to summer but were equal during both early and late denning, indicating that bears are in protein balance during hibernation. Protein and nitrogen content, nucleic acid, and stable isotope enrichment measurements of the biopsies support this conclusion.

Serum concentrations of amino acids, fatty acids, lipoproteins, vitamins A and E, and minerals in apparently healthy, free‐ranging southern giraffe (Giraffa camelopardalis giraffe)

This pilot project began establishing a nutritional profile for free-ranging giraffe. The results will be used as a tool to begin assessing the nutritional status of captive giraffe. In October 2004 serum samples were collected opportunistically from seven adult and 17 sub-adult giraffe being anesthetized for different studies. Seventeen animals were from Double Drift Game Reserve and seven animals were from Kariega Private Game Reserve. The serum samples were analyzed for circulating concentrations of amino acids, fatty acids, lipoproteins, vitamins, and minerals. Information from 15 serum samples collected from anesthetized giraffe in Kruger National Park during April and August 2003 was included in the calcium and phosphorus concentration data. No significant differences were identified between genders. Significant differences between locations were identified for concentrations of certain amino acids, fatty acids, and lipoproteins. Differences between locations are likely due to different nutrient concentrations of foods and possibly the result of different animal densities forcing different food choices among locations. This pilot project may expand to include changes in circulating nutrient concentrations for free-ranging giraffe as is influenced by other locations, seasonal food availability, and different giraffe subspecies. Zoo Biol 0:1-13, 2007. (c) 2006 Wiley-Liss, Inc.

Behavioral responses of brown bears mediate nutritional effects of experimentally introduced tourism

To develop a conceptual understanding of the interaction between tourists and bears, this study examined the responses of brown bears ( Ursus arctos) to experimentally introduced tourism at Douglas River in south-central Alaska within the framework of current predator–prey theory. Factors eliciting displacement and the nutritional consequences of behavioral changes were measured. GPS collars were deployed to track temporal and spatial resource use and scan and focal observations were used to quantify foraging efficiency, vigilance behavior, and bear use of various food resources. Total food intake was quantified by combining data on food resource use based on GPS locations and foraging efficiency was measured using spotting scopes from long distances. Seasonal food availability and quality were monitored and utilized as co-variates in comparisons between pre-treatment and treatment years. Though the bear population exhibited significant behavioral differences between the presence and absence of humans, changes in behavior generally mediated effects on total food intake. Adult males at a salt marsh viewing area were the only sex/age class exhibiting reduced food intake resulting from a 15% decline in foraging time when viewers were present. Salmon intake did not differ between pre-treatment and treatment years despite a 10% decline in time spent fishing when viewers were present. The tightly controlled experimental bear-viewing introduced in this study allowed bears to evaluate predation risk associated with human activity and optimally respond to minimize costs due to changes in foraging activities.

A description of larval and early juvenile development in Paralomis spinosissima (Decapoda: Anomura: Paguroidea: Lithodidae) from South Georgia waters (Southern Ocean)

The early ontogenetic stages of Paralomis spinosissima Birstein and Vinogradow, 1972, are described in detail and illustrated, with notes on morphological variability observed. Larval and early juvenile development was described to the crab I instar reared under controlled conditions of temperature and food supply. The abbreviated larval development invariably passed through two zoeal stages and the benthic megalopa stage. The larval development was completed without food supply, and food Artemia nauplii were first given after moult to the crab-I stage. Simplification and retarded development of the mouthparts are discussed as a function of lecithotrophy of these larvae and based on morphology no facultative feeding mode is suggested. Lecithotrophy in the Southern Ocean Lithodidae is discussed to be an adaptation allowing independence from seasonal food availability at high latitudes.

Seasonal changes in carbohydrate metabolism and its relationship with summer mortality of Pacific oyster Crassostrea gigas (Thunberg) in Marennes–Oléron bay (France)

This paper investigates the biological responses of Crassostrea gigas under traditional culture conditions on a mudflat in Marennes–Oléron bay. Summer mortality has been regularly observed in recent years in oysters reared using “on bottom” culture conditions. The present study attempts to provide a better understanding of the mortality phenomenon through biological parameters. Classical ‘field parameters’ such as mortality and growth rates, and quality index (dry meat weight / dry shell weight × 1000) were monitored. Additional parameters, as biochemical composition of oyster meat and glucose incorporation capacity, were included as potential new bioindicators. The work highlighted a critical timing (May–June) preceding the summer mortality and characterised by an arrest in lipid synthesis and a decrease in carbohydrate content. During this period, growth (especially gonad maturation) either slowed down significantly or even stopped. The first mortality event occurred during a growth renewal period at the end of June. Short-term analysis (15 days) provided information to identify such responses which may indicate a physiological stress and demonstrating the need for further investigation. The seasonal food availability (estimated from chlorophyll a levels) did not facilitate the mortality understanding which occurred after water temperature went above 18–19 °C. Nevertheless, this study shows carbohydrate anabolism contributed in the physiological stress leading to mortality events.

CHALLENGING THE COLD: CRABS RECONQUER THE ANTARCTIC

Recent records of lithodid crabs in deeper waters off the Antarctic continental slope raised the question of the return of crabs to Antarctic waters, following their extinction in the lower Miocene ;15 million years ago. Antarctic cooling may be responsible for the impoverishment of the marine high Antarctic decapod fauna, presently comprising only five benthic shrimp species. Effects of polar conditions on marine life, including lowered metabolic rates and short seasonal food availability, are discussed as main evolutionary driving forces shaping Antarctic diversity. In particular, planktotrophic larval stages should be vulnerable to the mismatch of prolonged development and short periods of food avail- ability, selecting against complex life cycles. We hypothesize that larval lecithotrophy and cold tolerance, as recently observed in Subantarctic lithodids, represent, together with other adaptations in the adults, key features among the life-history adaptations of lithodids, potentially enabling them to conquer polar ecosystems. The return of benthic top predators to high Antarctic waters under conditions of climate change would considerably alter the benthic communities.

Potential causes and life‐history consequences of sexual size dimorphism in mammals

ABSTRACT1. Male‐biased sexual size dimorphism (SSD) in mammals has been explained by sexual selection favouring large, competitive males. However, new research has identified other potential factors leading to SSD. The aim of this review is to evaluate current research on the causes of SSD in mammals and to investigate some consequences of SSD, including costs to the larger sex and sexual segregation.2. While larger males appear to gain reproductive benefits from their size, studies have also identified alternative mating strategies, unexpected variance in mating success and found no clear relationship between degree of polygyny and dimorphism. This implies that sexual selection is unlikely to be the single selective force directing SSD.3. Latitude seems to influence SSD primarily through variation in overall body size and seasonal food availability, which affect potential for polygyny. Likewise, population density influences resource availability and evidence suggests that food scarcity differentially constrains the growth of the sexes. Diverging growth patterns between the sexes appear to be the primary physiological mechanism leading to SSD.4. Female‐biased dimorphism is most adequately explained by reduced male–male competition resulting in a decrease in male size. Female–female competition for dominance and resources, including mates, may also select for increased female size.5. Most studies found that sexual segregation arises through asynchrony of activity budgets between the sexes. The larger sex can suffer sex‐biased mortality through increased parasite load, selective predation and the difficulty associated with sustaining a larger body size under conditions of resource scarcity.6. None of the variables considered here appears to contribute a disproportionate amount to SSD in mammals. Several promising avenues of research are currently overlooked and long‐term studies, which have previously been biased toward ungulates, should be carried out on a variety of taxa.

Tooth microwear pattern in roe deer (Capreolus capreolus L.) from Chiz� (Western France) and relation to food composition

This study was designed to correlate the dental microwear pattern in a population of roe deer ( Capreolus capreolus ) with seasonal dietary data. Fifty roe deer specimens from the Chizé Forest in western France provides the material of this study. The shearing facet of the second molars is digitized using a 256 level gray scale light stereomicroscope equipped with a CCD camera. Numbers and dimensions of scratches and pits are quantified using a semi-automatic protocol. Considering the population as a whole, the results are consistent with previous dental microwear analysis and suggest a browsing diet (sensu lato). In fact, the percentage of pits exceeds 50%, which is correlated with a browsing and not grazing. Furthermore, the dental microwear excludes foraging on tubers or roots. Although no significant differences are found between sexes, males from the autumn sample have a higher number of scratches on the shearing molar facets than the females. This may be correlated with the more important intake of fruits by males than females during the rutting period. The number of scratches also suggests a dietary difference between two seasonal clusters. Summer and autumnal samples have a higher frequency of highly scratched dental facets that correlate with the fruit consumption by the roe deer during these periods. Fewer dental pits were found in the roe deer sampled during summer and spring than during the other seasons. This suggests seasonal diet shifts among plant life forms with consumption of soft green leaves during the spring and summer. In winter and autumn, the roe deer feeds often on dead leaves, acorns and ivy. These food items are often consumed with sand or bark particles during the prehension of the food, this would increase the likelihood of more pits than would occur with soft semi-ligneous leaves. This seasonal dental microwear pattern has revealed a relationship between seasonal food availability and deer diets. The dental microwear analysis on this population of roe deer will help determine the paleodietary patterns of extinct ungulates.

Growth Pattern of Children of Two Social Groups: A One-Year Longitudinal Study

Physical growth assessment best demonstrates the health and nutritional status of children. Populations having low dietary intakes experienced a pattern of growth characterised by slow growth rate. However slow growth is also an indicator of poor environmental conditions affecting past present and future health. Again a well marked seasonal effect on growth velocity has been observed in the industrialised countries of the temperate areas. Growth in height and weight are on average are fastest in spring and autumn respectively. The average velocity of height from March to May is about twice that from September to October in most of the older western European data (Tanner 1999). Panter-Brick (1997) has shown that growth velocity among children is affected by seasonal changes and to previous growth status. Cole (1993) is of the opinion that in addition to seasonal changes food availability parasitic load infection etc. affects growth rates of children. (excerpt)

Tooth microwear pattern in roe deer ( Capreolus capreolus L.) from Chizé (Western France) and relation to food composition

This study was designed to correlate the dental microwear pattern in a population of roe deer ( Capreolus capreolus) with seasonal dietary data. Fifty roe deer specimens from the Chizé Forest in western France provides the material of this study. The shearing facet of the second molars is digitized using a 256 level gray scale light stereomicroscope equipped with a CCD camera. Numbers and dimensions of scratches and pits are quantified using a semi-automatic protocol. Considering the population as a whole, the results are consistent with previous dental microwear analysis and suggest a browsing diet (sensu lato). In fact, the percentage of pits exceeds 50%, which is correlated with a browsing and not grazing. Furthermore, the dental microwear excludes foraging on tubers or roots. Although no significant differences are found between sexes, males from the autumn sample have a higher number of scratches on the shearing molar facets than the females. This may be correlated with the more important intake of fruits by males than females during the rutting period. The number of scratches also suggests a dietary difference between two seasonal clusters. Summer and autumnal samples have a higher frequency of highly scratched dental facets that correlate with the fruit consumption by the roe deer during these periods. Fewer dental pits were found in the roe deer sampled during summer and spring than during the other seasons. This suggests seasonal diet shifts among plant life forms with consumption of soft green leaves during the spring and summer. In winter and autumn, the roe deer feeds often on dead leaves, acorns and ivy. These food items are often consumed with sand or bark particles during the prehension of the food, this would increase the likelihood of more pits than would occur with soft semi-ligneous leaves. This seasonal dental microwear pattern has revealed a relationship between seasonal food availability and deer diets. The dental microwear analysis on this population of roe deer will help determine the paleodietary patterns of extinct ungulates.

Tooth microwear pattern in roe deer (Capreolus capreolus L.) from Chizé (Western France) and relation to food composition

This study was designed to correlate the dental microwear pattern in a population of roe deer (Capreolus capreolus) with seasonal dietary data. Fifty roe deer specimens from the Chizé Forest in western France provides the material of this study. The shearing facet of the second molars is digitized using a 256 level gray scale light stereomicroscope equipped with a CCD camera. Numbers and dimensions of scratches and pits are quantified using a semi-automatic protocol. Considering the population as a whole, the results are consistent with previous dental microwear analysis and suggest a browsing diet (sensu lato). In fact, the percentage of pits exceeds 50%, which is correlated with a browsing and not grazing. Furthermore, the dental microwear excludes foraging on tubers or roots. Although no significant differences are found between sexes, males from the autumn sample have a higher number of scratches on the shearing molar facets than the females. This may be correlated with the more important intake of fruits by males than females during the rutting period. The number of scratches also suggests a dietary difference between two seasonal clusters. Summer and autumnal samples have a higher frequency of highly scratched dental facets that correlate with the fruit consumption by the roe deer during these periods. Fewer dental pits were found in the roe deer sampled during summer and spring than during the other seasons. This suggests seasonal diet shifts among plant life forms with consumption of soft green leaves during the spring and summer. In winter and autumn, the roe deer feeds often on dead leaves, acorns and ivy. These food items are often consumed with sand or bark particles during the prehension of the food, this would increase the likelihood of more pits than would occur with soft semi-ligneous leaves. This seasonal dental microwear pattern has revealed a relationship between seasonal food availability and deer diets. The dental microwear analysis on this population of roe deer will help determine the paleodietary patterns of extinct ungulates.

Circadian rhythms in diet and habitat use in red ruffed lemurs (Varecia rubra) and white-fronted brown lemurs (Eulemur fulvus albifrons).

Daily variation in niche use among vertebrates is attributed to a variety of factors, including thermoregulatory, reproductive, and nutritional requirements. Lemuriform primates exhibit many behavioral and physiological adaptations related to thermoregulation and sharp, seasonal reproduction, yet they have rarely been subjects of a quantitative analysis of circadian (or daily) rhythms in niche use. In this study, I document daily rhythms in diet and microhabitat use over an annual cycle in two sympatric, frugivorous lemurs, Varecia rubra and Eulemur fulvus albifrons. Data on diet, forest site, and forest height were recorded at 5-min time points on focal animals and divided into three time blocks for analysis (06:00-10:00 hr, 10:00-14:00 hr, and 14:00-18:00 hr). I employed multivariate tests of independence to examine daily rhythms in diet and microhabitat use according to sex, season, and reproductive stage. Throughout the day, V. rubra is frugivorous and dwells in the upper canopy, with notable departures (especially for females) during the hot seasons, gestation, and lactation. E. f. albifrons has heterogeneous daily rhythms of food choice and microhabitat use, particularly across seasons, and both sexes are equally variable. These daily rhythms in diet and microhabitat use appear related to thermoregulatory and nutritional requirements, seasonal food availability and circadian rhythms of plant (and possibly insect) palatability, predator avoidance tactics, and in the case of Varecia, to reproduction. Daily rhythms of food choice in V. rubra support two previously suggested hypotheses explaining why primates consume more nonfruit items late in the day, whereas those of E. f. albifrons are too variable to lend support to these hypotheses.