Sensitivity of Syrian hamsters (Mesocricetus auratus) to amplitudes and rates of photoperiodic change typical of the tropics

Male endocrine response to seasonally varying environmental and social factors in a neotropical primate, Cebus capucinus

OBJECTIVE

Circannual variation in reproduction is pervasive in birds and mammals. In primates, breeding seasonality is variable, with seasonal birth peaks occurring even in year-round breeders. Environmental seasonality is reportedly an important contributor to the observed variation in reproductive seasonality. Given that food availability is the primary factor constraining female reproduction, predictions concerning responsiveness to environmental seasonality focus on females, with studies of males focusing primarily on social factors. We examined the influence of both environmental and social factors on male fecal testosterone (fT) and glucocorticoids (fGC) in moderately seasonally breeding white-faced capuchin monkeys (Cebus capucinus) in Costa Rica.

METHODS

Over 17 months, we collected 993 fecal samples from 14 males in three groups. We used LMM to simultaneously examine the relative effects of photoperiod, fruit biomass, rainfall, temperature, female reproductive status (i.e., number of periovulatory periods, POPs), and male age and dominance rank on monthly fT and fGC levels.

RESULTS

Male age and rank had large effects on fT and fGC. Additionally, some hormone variation was explained by environmental factors: photoperiod in the previous month (i.e., lagged photoperiod) was the best environmental predictor of monthly fT levels, whereas fGC levels were best explained by lagged photoperiod, fruit biomass, and rainfall. POPs predicted monthly fT and fGC, but this effect was reduced when all variables were considered simultaneously, possibly because lagged photoperiod and POP were highly correlated.

CONCLUSIONS

Males may use photoperiod as a cue predicting circannual trends in the temporal distribution of fertile females, while also fine-tuning short-term hormone increases to the actual presence of ovulatory females, which may occur at any time during the year.

Photoperiod modulation of aggressive behavior is independent of androgens in a tropical cichlid fish

Photoperiod is a major environmental cue that signals breeding conditions in animals living in temperate climates. Therefore, the activity of the reproductive (i.e. hypothalamic-pituitary-gonadal, HPG) axis and of the expression of reproductive behaviors, including territoriality, is responsive to changes in day length. However, at low latitudes the seasonal variation in day length decreases dramatically and photoperiod becomes less reliable as a breeding entraining cue in tropical species. In spite of this, some tropical mammals and birds have been found to still respond to small amplitude changes in photoperiod (e.g. 17min). Here we tested the effect of 2 photoperiod regimes, referred to as long-day (LD: 16L:08D) and short-day (SD: 08L:16D), on the activity of the HPG axis, on aggressive behavior and in the androgen response to social challenges in males of the tropical cichlid fish Tilapia rendalli. For each treatment, fish were transferred from a pre-treatment photoperiod of 12L:12D to their treatment photoperiod (either LD or SD) in which they were kept for 20days on stock tanks. Afterwards, males were isolated for 4days in glass aquaria in order to establish territories and initial androgen levels (testosterone, T; 11-ketotestosterone, KT) were assessed. On the 4th day, territorial intrusions were promoted such that 1/3 of the isolated males acted as residents and another 1/3 as intruders. Territorial intrusions lasted for 1h to test the effects of a social challenge under different photoperiod regimes. Photoperiod treatment (either SD or LD) failed to induce significant changes in the HPG activity, as measured by androgen levels and gonadosomatic index. However, SD increased the intensity of aggressive behaviors and shortened the time to settle a dominance hierarchy in an androgen-independent manner. The androgen responsiveness to the simulated territorial intrusion was only present in KT but not for T. The percent change in KT levels in response to the social challenge was different between treatments (SD>LD) and between male types (resident>intruder). The higher androgen response to a social challenge in residents under SD may be explained by the time course of the androgen response that due to the long time it takes to fight resolution under LD, might have been delayed. This result illustrates the importance of incorporating time response data in social endocrinology studies.

Predictable timing of oestrus in the tropical bat Saccopteryx bilineata living in a Costa Rican rain forest

Many tropical mammals reproduce seasonally, although the circum-equatorial climate is more stable and less seasonal than that of temperate zones. The mechanisms underlying seasonal reproduction in the tropics remain enigmatic. Female reproduction and its relation to environmental factors were investigated in the Neotropical bat species Saccopteryx bilineata. Colonies consist of year-round stable groups of several females that are defended each by an adult male. Females give birth to a single offspring each year and it is suggested that mating is restricted to November and December. In this study, it was asked whether females of a Costa Rican colony come into oestrus around the same time each year and whether oestrus times are synchronized. Oestrogen and progesterone metabolites were monitored from faeces between October and January in four years. Oestrus was identified in 32 females. In addition, climatic factors such as rainfall and temperature were monitored at the study site. Results indicate that (1) females exhibit monoestry, (2) oestrus dates cluster around the first half of December, (3) reproduction is strongly seasonal and highly predictable and (4) oestrus times are possibly influenced by long-term cues like photoperiod and short-term cues like sudden changes in rainfall and temperature.

Climate change and seasonal reproduction in mammals

Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors.

Effects of prolonged acclimation to intermediate photoperiod and photo-schedule reversal in photosensitive golden hamsters

We investigated the effect of prolonged acclimation to 12 hr of light and photo-schedule reversal during the time of photosensitivity in golden hamsters (Mesocricetus auratus). Before the experiments, animals were housed under natural photoperiod and then transferred to 12L:12D (light 12 hr:dark 12 hr) in autumn for 12 weeks. After 4 weeks of acclimation, photo-schedule was reversed (12D:12L). First experiments were done after 4 weeks of acclimation to an ambient temperature (T(a)) of 23 degrees C and a 12L:12D photo-schedule. We examined the daily variations in brown adipose tissue (BAT) capacity for nonshivering thermogenesis (NST). Noradrenaline (NA) injections were given every 4 hr while BAT temperature (T(BAT)) and preferred ambient temperature (PT(a)) were monitored continuously and simultaneously in a thermal gradient system. Then, we investigated the effect of light-dark cycle reversal on a daily rhythm of NST. The hamsters were acclimated to the photo-schedule reversed by 12 hr and the same T(a). After 4 and 8 weeks of acclimation to a reversed photo-schedule, the experiments were repeated. We found that the daily rhythm of the response to NA was entrained to the new light-dark cycle after 4 weeks of acclimation to a reversed photo-schedule. Maximum effect of NA was always recorded during the light phase and in the latter part of the dark phase of the day. NA-induced increase in T(BAT) was correlated with the decrease in PT(a), and was also inversely correlated with pre-injection T(BAT). These data imply that the daily rhythm of the capacity for NST opposes the daily rhythm of body temperature (T(b)). After 8 weeks of acclimation to the reversed photo-schedule, the rhythmicity of the response to NA disappeared, and the daily fluctuations in T(BAT) were the smallest. This lack of rhythm may be a physiological adaptation to winter conditions when the daily amplitude of T(b) rhythm is markedly reduced and, as a consequence, NST capacity does not vary within the day. Moreover, after 8 weeks of acclimation to reversed photo-schedule, NST capacity decreased while response to saline increased. During the experiments, hamsters were photosensitive and were changing to their winter status. However, because of the lack of cold during acclimation, the capacity for NST did not increase. Increased responsiveness to saline, indicating an increase in stress-induced thermogenesis, might be advantageous for "fight or flight" reaction.

Genetic variation in photoperiodism among naturally photoperiodic rat strains

Rattus norvegicus has been considered nonphotoperiodic, but Fischer 344 (F344) rats are inhibited in growth and reproductive development by short photoperiod (SD). We tested photoresponsiveness of the genetically divergent Brown Norway (BN) strain of rats. Peripubertal males were tested in long photoperiod or SD, with or without 30% food reduction. Young males were photoresponsive, with reductions in testis size, body mass, and food intake in SD and with enhanced responses to SD when food restricted. Photoperiods < or =11 h of light inhibited reproductive maturation and somatic growth, whereas photoperiods of 12 h or more produced little or no response. F344/BN hybrids differ from both parent strains in the timing, amplitude, and critical photoperiod of photoperiodic responses, indicating genetic differences in photoperiodism between these strains. This is consistent with the hypothesis that ancestors of laboratory rats were genetically variable for photoperiodism and that different combinations of alleles for photoperiodism have been fixed in different strains of rats.

Photoperiodism in humans and other primates: evidence and implications

Most of the anatomical and molecular substrates of the system that encodes changes in photoperiod in the duration of melatonin secretion, and the receptor molecules that read this signal, have been shown to be conserved in monkeys and humans, and the functions of this system appear to be intact from the level of the retina to the level of the melatonin-duration signal of change of season. While photoperiodic seasonal breeding has been shown to occur in monkeys, it remains unclear whether photoperiod and mediation of photoperiod's effects by melatonin influence human reproduction. Epidemiological evidence suggests that inhibition of fertility by heat in men in summer contributes to seasonal variation in human reproduction at lower latitudes and that stimulation of fertility by lengthening of the photoperiod in spring contributes to the variation at higher latitudes. Parallels between the seasonality of human reproduction and seasonal affective disorder suggest that they may be governed by common biological processes. Historical and experimental evidence indicates that human responses to seasonal changes in the natural photoperiod may have been more robust prior to the Industrial Revolution and that subsequently they have been increasingly suppressed by alterations of the physical environment.

Inhibition of reproductive maturation and somatic growth of Fischer 344 rats by photoperiods shorter than L14:D10 and by gradually decreasing photoperiod

Photoperiod is the major regulator of reproduction in temperate-zone mammals. Laboratory rats are generally considered to be nonphotoresponsive, but young male Fischer 344 (F344) rats have a uniquely robust response to short photoperiods of 8 h of light. Rats transferred at weaning from a photoperiod of 16 h to photoperiods of < 14 h of light slowed in both reproductive development and somatic growth rate. Those in photoperiods < 13 h of light underwent the strongest responses. The critical photoperiod of F344 rats can be defined as 13.5 h of light, but photoperiods of </= 12.5 h are required to fully suppress reproduction and somatic growth. This demonstrates that the 12-h photoperiod that is standard in some laboratory colonies would have significant effects on reproductive maturation and growth rate of this common rat strain. Young F344 rats in decreasing photoperiods that mimic natural change experienced delayed reproductive development and decreased growth rate to a greater extent and for a longer duration than those transferred at birth to a short photoperiod. The effects of gradual changes in photoperiod persisted for at least 12 wk after weaning. This indicates that young male F344 rats possess responses to photoperiod that would result in functional photoperiodism in a wild mammal.

Change in photoperiodic cycle affects life span in a prosimian primate (Microcebus murinus)

The lesser mouse lemur, a small prosimian primate, exhibits seasonal rhythms strictly controlled by photoperiodic variations. Exposure to day lengths shorter than 12 h results in complete sexual rest, fattening, lethargy, and reduced behavioral activities; whereas exposure to day lengths greater than 12 h induces sexual activity, an increase in behavioral activities, and high hormonal levels. The objective of this study was to test whether long-term acceleration of seasonal rhythms may affect survival and longevity of this primate. In captivity, acceleration of seasonal rhythms was obtained by exposing the animals to an accelerated photoperiodic regimen consisting of 5 months of long photoperiod followed by 3 months of short photoperiod. The age-specific survival rate in animals exposed from birth to accelerated photoperiodic conditions (n = 89) was compared to the age-specific survival rate of animals maintained under a natural photoperiod (n = 68). Independent of sexes, the mean life span (45.5 +/- 2.1 months) and maximal survival (79.3 +/- 3.3 months) were significantly (p < .01) shortened in mouse lemurs exposed to the accelerated photoperiodic cycle compared to those in animals living under annual photoperiod (63.2 +/- 2.5 and 98 +/- 3.9 months for mean life span and maximal survival, respectively). This reduction of about 30% of life span was not accompanied by a desynchronization of biological rhythms under photoperiodic control and was not related to an increase in reproduction or in duration of time spent in active conditions. However, when the number of seasonal cycles experienced by 1 individual is considered rather than chronological age, the mean life span was 5 seasonal cycles and maximum survival reached 9-10 cycles, independent of sex or of photoperiodic regimen. These results suggest that in mouse lemurs, as in other seasonal mammals, longevity may depend on the expression of a fixed number of seasonal cycles rather than on a fixed biological age.