Median Birth Date Research Articles

Stone’s sheep (Ovis dalli stonei) lambing and nursery habitat selection

Wildlife conservation often focuses on mitigating disturbance in critical habitats like reproductive ranges. We studied lambing and nursery habitat selection by Stone’s sheep ( Ovis dalli stonei (J.A. Allen, 1897)) in the Cassiar Mountains in British Columbia. We estimated the timing of parturition and delineated lambing and nursery habitats based on movement behaviours of collared females ( n = 18) and vaginal implant transmitters. We identified 23 lambing events in 2018 ( n = 4), 2019 ( n = 13), and 2020 ( n = 6). The median birth date was 17 May and ranged from 3 May to 14 June. Females remained in lambing habitats from 1.5 to 11.3 days with a median of 5.5 days. We examined habitat selection during the lambing and nursery periods at the home-range scale using resource selection functions, and at a finer scale using integrated step-selection analyses. Females selected southwest slopes in rugged terrain at mid-elevations, suggesting selection for warmer micro-climates and features that facilitate predator avoidance. Females avoided habitats near roads during the lambing period but showed selection for habitats near roads during the nursing period. We developed predictive maps of suitable lambing and nursery ranges to inform land planning to help reduce overlap of anthropogenic disturbance with potential reproductive habitats.

Size at Birth, Postnatal Growth, and Reproductive Timing in an Australian Microbat.

SynopsisReproductive phenology, size at birth, and postnatal growth are important life history traits that reflect parental investment. The ability to document detailed changes in these traits can be a valuable tool in the identification and management of at-risk wildlife populations. We examined reproductive traits in a common, widespread Australian microbat, Chalinolobus gouldii, at two sites over two years and derived growth curves and age estimation equations which will be useful in the study of how intrinsic and extrinsic factors alter parental investment strategies. We found that male and female offspring did not differ significantly in their size at birth or their postnatal growth rates. Bats born in 2018 were smaller at birth but grew at a faster rate than those born in 2017. When date of birth was compared across sites and years, we found bats born in 2018 had a later median birthdate (by 18 days) and births were more widespread than those born in 2017. Cooler and wetter weather during late gestation (Nov) in 2018 may have prolonged gestation and delayed births. With many bats facing threatening processes it is important to study reproductive plasticity in common and widespread “model” species, which may assist in the conservation and management of threatened microbats with similar reproductive traits.

Early pupping of a sub-Antarctic fur seal at Inaccessible Island, Tristan da Cunha

A sub-Antarctic fur seal, Arctocephalus tropicalis, pup was located on Inaccessible Island in the Tristan da Cunha archipelago (TdC), well before the start of the annual breeding (pupping and mating) season in 2018. It was born a few days before 27 September 2018, at least 59 days before the next pup was observed on the island in 2018 and 75 days before the median birthdate for the species at the TdC. A malfunctioning of the obligate delay of implantation of the blastocyst in its mother was likely the cause of the early birth.

Discovery of a secular trend in Cayo Santiago macaque reproduction.

Reproductive synchrony and the consequent clustering of births are hypothesized to be regulated by seasonal changes in rainfall and food availability. Such climate-related seasonality is, however, questionable in tropical populations occupying temporally invariant habitats year round. Using the long-term data of the Cayo Santiago rhesus macaques from 1973 to 2013, this study distinguishes synchrony (a greater than chance clustering of births) from seasonality (a cluster of births during a period of the year when abiotic conditions are favorable) and shows that females are highly synchronized (>72% of births in a 3-month period) but the effects of environmental zeitgebers on reproduction are overridden by biological factors. Specifically, biotic and abiotic factors including (i) loss of immature offspring; (ii) population density; (iii) age at delivery; (iv) rainfall; and (v) changes in colony management were modeled in relation to the annual onset of births and the median birth date. Females experiencing loss of immature offspring had an interbirth interval of <365 days in average and the proportion of these females increased up to 48% due to changes in colony management overtime, although reproductive synchrony increased with increasing population density. A secular trend in both the onset of births and the median date of birth is documented and the model predicts that the median birth date will advance across all calendar-based seasons by 2050. The secular trend in reproduction appears to be triggered by changes in the age at delivery of females, the absence of physiological constraints from maternal investment due to offspring loss, shorter interbirth interval, and a higher degree of coordination due to increasing population density. This study challenges the reproductive phenology previously described for rhesus macaques highlighting the importance of long-term studies in addressing the ultimate causes of reproductive synchrony.

Birth seasonality and pattern in black-and-white snub-nosed monkeys (Rhinopithecus bieti) at Mt. Lasha, Yunnan.

Seasonal variation in environmental factors is vital to the regulation of seasonal reproduction in primates. Consequently, long-term systematic data is necessary to clarify the birth seasonality and pattern of primates in highly seasonal environments. This study indicated that black-and-white snub-nosed monkeys (Rhinopithecus bieti) at Mt. Lasha exhibited strict birth seasonality with a pulse model. Infants were born with a certain degree of synchronization. Birth distribution showed three birth peaks, and the birth pattern showed a "V" style in even-numbered years and a gradual increase in odd-numbered years. The beginning date, end date and median birth date were earlier in even-numbered years than those in odd-numbered years. The higher latitude of their habitats, earlier birth date, shorter birth period, fewer birth peaks and stronger birth synchrony might be adaptations for strongly seasonal variation in climate and food resources. After the summer solstice when daylight length began to gradually shorten, R. bieti at Mt. Lasha started to breed during the period with the highest environmental temperature and food availability, which implied that photoperiod may be the proximate factor triggering the onset of estrus and mating. It appears that R. bieti coincided conception and mid-lactation with the peak in staple foods, and weaning with the peak in high quality of foods. Thus, food availability was the ultimate factor regulating reproductive seasonality, and photoperiod was the proximate factor fine-turning the coordination between seasonal breeding and food availability.

Variable Variation: Annual and Seasonal Changes in Offspring Sex Ratio in a Bat

Many organisms produce offspring with sex-ratios that deviate from equal numbers of males and females, and numerous adaptive explanations have been proposed. In some species, offspring sex-ratio varies across the reproductive season, again with several explanations as to why this might be adaptive. However, patterns for birds and mammals are inconsistent, and multiple factors are likely involved. Long-term studies on a variety of species may help untangle the complexity. I analyzed a long-term data set on the variation in offspring sex-ratio of the big brown bat, Eptesicus fuscus, a temperate-zone, insectivorous species. Sex ratio varied seasonally, but only in some years. Births early in the season were significantly female biased in years in which parturition occurred relatively early, but not in years with late parturition. Survival of female pups increased with earlier median birth date for the colony, and early-born females were more likely to survive and reproduce as one-year olds, compared to later-born pups. I argue that, due to the unusual timing of reproductive activities in male and female bats that hibernate, producing female offspring early in the year increases their probability of reproducing as one year olds, but this is not the case for male offspring. Thus, mothers that can give birth early in the year, benefit most by producing a female pup. The relative benefit of producing female or male offspring varies depending on the length of the growing season and thus the time available for female pups to reach sexual maturity. This suggests that not only does sex-ratio vary seasonally and among years, depending on the condition of the mother and the environment, but also likely varies geographically due to differences in season length.

Demographic consequences of increased winter births in a large aseasonally breeding mammal (Bos taurus) in response to climate change

1. Studies examining changes in the scheduling of breeding in response to climate change have focused on species with well-defined breeding seasons. Species exhibiting year-round breeding have received little attention and the magnitudes of any responses are unknown. 2. We investigated phenological data for an enclosed feral population of cattle (Bos taurus L.) in northern England exhibiting year-round breeding. This population is relatively free of human interference. 3. We assessed whether the timing of births had changed over the last 60 years, in response to increasing winter and spring temperatures, changes in herd density, and a regime of lime fertilisation. 4. Median birth date became earlier by 1·0 days per year. Analyses of the seasonal distribution of calving dates showed that significantly fewer calves were born in summer (decline from 44% of total births to 20%) and significantly more in winter (increase from 12% to 30%) over the study period. The most pronounced changes occurred in winter, with significant increases in both the proportion and number of births. Winter births arise from conceptions in the previous spring, and we considered models that investigated climate and weather variables associated with the winter preceding and the spring of conceptions. 5. The proportion of winter births was higher when the onset of the plant growing season was earlier during the spring of conceptions. This relationship was much weaker during years when the site had been fertilised with lime, suggesting that increased forage biomass was over-riding the impacts of changing plant phenology. When the onset of the growing season was late, winter births increased with female density. 6. Recruitment estimates from a stage-structured state-space population model were significantly negatively correlated with the proportion of births in the preceding winter, suggesting that calves born in winter are less likely to survive than those born in other seasons. 7. This is one of the first studies to document changes in the phenology of a year-round breeder, suggesting that the impact of climate on the scheduling of biological events may be more extensive than previously thought and that impacts may be negative, even for species with relatively flexible breeding strategies.

Interannual variation in pup production and the timing of breeding in benthic foraging Australian fur seals

AbstractLarge interannual variations in reproductive success caused by fluctuations in oceanography and prey availability are common to many species of air breathing epipelagic predators. In contrast, little is known about variation in benthic foragers such as Australian fur seals (Arctocephalus pusillus doriferus). Between 1997 and 2007, pup production was assessed in 9 yr, while the timing of breeding and adult female condition was assessed in 5 yr at Kanowna Island in Bass Strait, southeastern Australia. Pup production was variable (= 1,726 ± 42, range = 1,386–2,301), but without temporal trend, as was median birth date (= 23 November ± 1, range = 21–25 November) and pupping synchrony (period of 90% births: = 28 ± 2 d, range = 23–31 d). Pup production was negatively correlated with median birth date and positively correlated with female condition, winter sea‐surface temperature (SST) and zonal wind strength within Bass Strait. Pup production was also negatively correlated with SST in the previous summer within Bass Strait and in the eastern Great Australian Bight upwelling region. The results suggest that the reproductive success of Australian fur seals is influenced by oceanography but less so than in otariids foraging epipelagically in major upwellings. Despite spanning several El Niño events, no correlation between pup production and the Southern Oscillation Index was observed.

Birth seasonality and interbirth interval of captive Rhinopithecus bieti

This study, which is based on 10 years of birth records, shows that black-and-white snub-nosed monkeys (Rhinopithecus bieti) in captivity display marked birth seasonality. The birth season starts in December and ends in June, with a peak from March to May, and a median birth date of April 10. More male infants than female ones are born in captivity. More males were born at the Kunming Institute of Zoology (KIZ) than at the Kunming Zoo (KZ). Of 17 interbirth intervals (IBIs), 29% were from females that had lost an infant at <1 year of age or experienced stillbirth, and 71% were from females whose infant survived more than 1 year. The mean IBI for the former group (428+/-SD 87 days) was significantly shorter than that for the latter group (706+/-71 days), in agreement with reports of other Colobine species. Infant mortality was lower in captivity than in the field, which may reflect the relatively stable food availability and climate in captivity compared to the harsh conditions in the wild.

Postnatal growth and reproductive biology of Rhinolophus hipposideros (Chiroptera: Rhinolophidae)

AbstractThe timing of birth and postnatal growth in lesser horseshoe bats Rhinolophus hipposideros were studied during 2000 and 2001 at three maternity roosts located in the provinces of Carinthia and Salzburg, Austria. Postnatal growth curves for length of forearm and body mass were determined separately for the different colonies and years. Cooler weather during June 2001 coincided with a median birth date that was 8 days later than in 2000, whilst the difference between colonies was 4 and 5 days, respectively, for the pooled data of both years. Cool temperatures in June also coincided with a longer duration of the parturition period. Postnatal growth rates were higher than in most other bat species but differences between years and colonies were considerable. These differences may be attributed to the ambient roost temperatures, whereby the young of two cooler roosts showed significantly reduced growth rate during 8 days of cool weather in 2000. Moreover, later born and therefore smaller pups were more strongly affected by low temperatures than earlier born and larger ones. The mean length of forearm of young lesser horseshoe bats at the beginning of September equalled that of adult bats, whilst their body mass was still lower. Non‐volant juveniles did not cluster following the nightly emergence of adult females, and a preferred location within the roosts could not be identified. High and very low roost temperatures, however, did have an influence on the location of non‐volant young in the colonies.

POPULATION SIZE AND BREEDING SEASON OF THE ANTARCTIC FUR SEAL ARCTOCEPHALUS GAZELLA AT HEARD ISLAND—1987/88

AbstractBreeding colonies of the antarctic fur seal Arctocephalus gazella on Heard Island (53°10′S, 73°30′E) are situated on the sheltered northern and eastern coasts on flat vegetated terrain near streams and pools. Pupping in the 1987/88 summer began on 21 November, with 90% of births in 26 d. The median birth date was 11 December. Pup counts at Heard Island made in seven breeding seasons from 1962/63 to 1987/88 show an exponential rate of increase of 21%, which may be inflated due to undercounting in early years. The total of 248 births in 1987/88 represents an exponential increase of 37% since the previous year, but pups may have been undercounted then. Based on the number of pups born, the breeding population is estimated at 870–1,120. During the breeding season, the largest number of animals ashore was 835. Many non‐breeding fur seals began hauling out from early January and 15,000 animals were estimated to be ashore by late February, a fat larger number than expected from the size of the breeding population. Both the breeding and non‐breeding components of the population may be augmented by immigration. The source of immigrants may be undiscovered breeding colonies of this species in the northwestern sector of the Kerguelen Archipelago or the concentration at South Georgia. Further censuses are required at Heard Island to monitor the population growth.

The reproductive biology of female Père David's deer (Elaphurus davidianus)

AbstractData for this study came from breeding records of 27 Père David's (Elaphurus davidianus) hinds maintained in large pastures and from estrous records of four hand‐reared nulliparous hinds. The mean estrous cycle length ranged from 17.5 to 19.6 days. Standing estrus resembled that of other cervids, except that a low, moaning vocalization was given in response to contact, and activity (as measured by pedometers) did not increase. Mean gestation length was 183.38 ± SD 6.11 days (n = 21), and nearly all females conceived in the second and third years. The median interbirth interval was 362 days. The median birth date was April 8, and 80% of the births occurred over a 9.5‐week period. Multiparous hinds gave birth an average of 20.5 days earlier in the season than primiparous hinds. There was no dimorphism in birth weight. The results are discussed in light of comparative data for other species.

Climate and seasonal reproduction in the Cayo Santiago macaques.

This paper reports the results of an eight-year study of seasonal reproduction in the free-ranging colony of rhesus monkeys on Cayo Santiago, Puerto Rico. There was a significant correlation between the start of the annual spring rainy season and the estimated median conception date (r = .94, p < .001) and median birth date (r = .94, p < .001). Conceptions followed the commencement of spring rains by 70 days. Significant and negative correlations were found between the number of days from the start of the rains to the first conception (r = -.93, p < 0.01) and median conceptions (r = -.91, p < .01). When spring rains were delayed, the interval between the onset of rains and conceptions decreased linearly. The median birth date for the Cayo Santiago population was accurately predicted for 1984 using two methods. When results from Cayo Santiago were applied retrospectively to data from the colony of rhesus monkeys near La Parguera, Puerto Rico, it explained the three-month difference in seasonality of reproduction between the two populations located at the same latitude (18°N). Photoperiod, as a function of latitude, appears to set the temporal limits of seasonal reproduction in rhesus macaques, while the onset of spring rains regulates reproductive activity within that range, at least in the Cayo Santiago population. The biological channel through which the effect is mediated has not been identified.

A Ten-Year Summary of Reproductive Data for Free-Ranging &lt;i&gt;Macaca mulatta&lt;/i&gt;

Analyses were made of population statistics and reproduction parameters covering a ten-year period at the free-ranging rhesus monkey colony at La Parguera, Puerto Rico. The mean annual rate of population increase was 13 %. Young females gave birth to their first infant at four years of age on the average. More than 60 % of the births occurred during May and June with an annual median birth date (1966–1972) ranging from May 30 to June 8. Females not having an infant one year gave birth earlier than the median date the next year, while a large percentage of females giving birth late in a birth season did not produce an infant the next year. Between 40 and 50 % of the infants born first or second to a female did not survive their first year, but by the fourth infant born to the same female only 9 % died during the first 12 months. The social rank of adult females affected several parameters: infants born to high-ranking females had a higher rate of survival than young of low-ranking females; a larger percentage of high-ranking females gave birth each year than low-ranking females and daughters of high-ranking females produced their first infants at an earlier age than daughters of low-ranking females. Both males and females survived in equal proportions until four years of age, after which males had higher mortality rates and the death rate for females remained very low and constant.