Mean Birth Date Research Articles

Coastal residence periods and reproductive timing in southern right whales, Eubalaena australis

The duration and timing of coastal residence of individually identified southern right whales at a principal aggregation area on the southern Australian coast differed markedly between females with calves and unaccompanied whales. The mean residence period of females that calved within the aggregation area was 70.9 days, with mean residence mid‐points of 20 August in 1993 and 22 August in 1994. In contrast, unaccompanied adults remained resident for an average of only 20.4 days with mean residence mid‐points of 27 July and 11 August in 1993 and 1994, respectively. Whales have been sighted at this aggregation area from mid May to late October (approx. 160 days), although the effective calving season (95‐100% of calves born) lasted only 88 days in 1993 and 96 days in 1994. The mean birth date based on first sighting with neonatal calf, and corrected for sightability bias, was 15 July in 1993 and 17 July in 1994, with 100% of calves born before 31 August 1993 and 23 September 1994.The time between birth and dispersal from the aggregation area, at or just prior to the commencement of the southward migration, was highly variable. Calves bom before the mean calving date averaged 80 days within the aggregation area, twice as long as those born after the mean (40 days). The large number of calves estimated to be less than 14 days old at first sighting, combined with the sighting of 26 pregnant females prior to parturition, suggests the majority of births occurred within, or very near to, the aggregation area.

The absence of any effect of constant long or short photoperiod on growth, sexual development and behaviour of male lambs from birth to 9 months of age

Testosterone levels are generally higher in entire male sheep when photoperiod is short as this coincides with the mating period and oestrous cycles of the female. This may increase management problems of entire male sheep through augmented agonistic and sexual behaviour. By delaying puberty in the entire male lamb, management problems may be avoided, while retaining the full benefits of some testosterone secretion to promote growth and leanness. This experiment was initiated to examine the possibility that lamb growth will be enhanced under either constant long or constant short photoperiod without increased behavioural problems.At birth (mean birth date 19 March 1994 ± 3 days), 24 twin male lambs were assigned as pairs to either long (L) or short (S) photoperiod treatments alternately. One lamb from each pair was left entire (E) and the other lamb was castrated (C) at 12 hours of age using elastrator bands.

The absence of any effect of constant long or short photoperiod on growth, sexual development and behaviour of male lambs from birth to 9 months of age

Testosterone levels are generally higher in entire male sheep when photoperiod is short as this coincides with the mating period and oestrous cycles of the female. This may increase management problems of entire male sheep through augmented agonistic and sexual behaviour. By delaying puberty in the entire male lamb, management problems may be avoided, while retaining the full benefits of some testosterone secretion to promote growth and leanness. This experiment was initiated to examine the possibility that lamb growth will be enhanced under either constant long or constant short photoperiod without increased behavioural problems.At birth (mean birth date 19 March 1994 ± 3 days), 24 twin male lambs were assigned as pairs to either long (L) or short (S) photoperiod treatments alternately. One lamb from each pair was left entire (E) and the other lamb was castrated (C) at 12 hours of age using elastrator bands.

Ungulate Grazing in Sagebrush Grassland: Effects of Resource Competition on Secondary Production

In many areas of western North America, populations of elk are believed to harm production of cattle by competing with them for limited supplies of native forage. We examined effects of variation in the population density of elk (Cervus elaphus canadensis) during winter on growth and reproduction of cattle during spring, using a randomized complete block experiment conducted in sagebrush grassland during four years. We manipulated elk numbers to achieve four levels of population density (0, 9, 15, and 31 elk/km2), replicated each level three times, and observed responses of cattle to these manipulations. Mean birth dates of calves born to cows in the intermediate (9, 15 elk/km2) treatment levels were delayed by 5 d relative to birth dates of calves born to cows in controls (0 elk/km2), but these trends were not statistically significant (quadratic effect F1,6 = 3.2, P = 0.13). Calf body mass at birth was not significantly influenced by treatment, but calf body mass at the end of spring declined linearly (F1,6 = 7.9, P = 0.03) with increasing elk population density, from a mean of 80.2 kg in the control to 73.0 kg in the 31 elk/km2 treatment. Calf body mass at weaning was weakly depressed by treatment, with the largest treatment effects occurring at the 9 elk/km2 level (F1,6 = 8.8, P = 0.02). Body mass of cows at the end of the spring grazing season tended to decline linearly with treatment, but these tendencies were not statistically significant (linear contrast F1,6 = 1.9, P = 0.22). We did not find significant effects of treatment on cow body mass at time of weaning (control vs. other contrast F1,6 = 3.3, P = 0.12) or on natality rates (control vs. other contrast F1,6 = 1.2, P = 0.31), although, in both cases, values for the control tended to exceed the treatments. Body mass of cows (t1 = ‐3.9, P = 0.0003) and calves (t1 = ‐3.9, P < 0.0001) at the end of the spring grazing season were quadratically related to the biomass of available herbaceous forage during spring. We observed a threshold in effects of forage supply on cattle production at about 45 g/m2 of live and dead herbaceous biomass. Cattle production declined with declining forage biomass when forage supply fell below this threshold. Total cattle production (kilograms per cow per year), was quadratically related to elk population density (F1,6 = 5.8, P = 0.05). Average cattle production in the control (X = 248 kg·cow‐1·yr‐1) exceeded the mean of the other treatment levels (X = 224 kg·cow‐1·yr‐1, F1,6 = 6.7, P = 0.04). Quadratic responses in cattle performance were apparently caused by compensatory growth after the spring grazing season: growth rates of cows during spring were inversely related to their subsequent growth rates during summer and fall (F1,46 = 33.5, P < 0.0001, r2 = 0.50). We conclude that elk grazing caused reductions in cattle production, but the magnitude of the effects were not proportionate to elk population density. Our studies revealed that competition between cattle and elk operates in a threshold fashion. If sufficient forage is available to cattle following elk grazing, elk populations will not harm cattle performance. In the system we studied, this threshold occurred at ≈ 45 g/m2 of live and dead herbaceous biomass.

Earlier breeding shortens life in female greater horseshoe bats

Life history theory predicts that an individual may gain in fitness by postponing reproduction if, by doing so, future reproductive capacity or longevity is enhanced. To test this theory I studied the survival and fecundity of earlier (start age 2 years) and later (start age 3 years or later) breeding female bats. Mature females produce one young annually, may miss breeding in some years and can still breed at age 29 years. Earlier breeders (e b) have similar mean skeletal size and birth date to later breeders (l b), but they have higher fat reserves late in their first winter and in their second autumn, when follicular development starts, and are probably superior foragers. eb averaged 5.6 and lb 8.1 years at death. Higher mortality in the former group was associated with parturition later in July during early breeding attempts. Lifetime reproductive success (lrs) of both groups was highly variable and strongly correlated with lifespan, which explained 99 and 96% of observed variation respectively. Differences in mean lifespan had no significant effect upon the mean lrs of eb and lb (4.4 and 5.1 births per female respectively). Although earlier breeding reduces lifespan, because it starts a year earlier and breeding rates are higher in eb than in lb (96% cf. 85% per year), overall there appear to be equal fitness benefits. During rapid population recovery after a climate-induced crash, earlier breeding was enhanced and may be advantageous until the population stabilizes. Hence studies testing life history theory should take account of population trends and climate. These seem to be crucially interconnected via food availability, the growth of individuals, and fat storage.

Effect of photoperiod before and after birth on puberty in ewe lambs.

Previous studies have shown that exposure to long days followed by short days after 12 wk of age advances the onset of puberty in ewe lambs. The aim of this experiment was to determine whether or not puberty would be advanced by exposure of lambs to such a photoperiodic regime when the long days were given before birth and the short days immediately after birth. Ewes kept in natural photoperiod (NP) were housed on January 15 and allocated at random to one of four treatments: 1) NP/NP, natural photoperiod before and after parturition (n = 26); 2) SD/SD, short days (SD; 9L:15D) before and after parturition (n = 27); 3) LD/NP, long days (LD; 18L:6D) before parturition and NP after parturition (n = 27); and 4) LD/SD, LD before parturition and SD after parturition (n = 26). Mean (+/- SEM) birth date for all lambs was March 20 +/- 1 day, and the number of days of exposure to long and short days prior to parturition was 64 +/- 2 and 67 +/- 2 days, respectively. From ewes on the four treatments, 24, 20, 14, and 16 female lambs, respectively, were reared. Lambs were weighed at birth and at weekly intervals after weaning. Reproductive activity in ewe lambs was assessed from serum progesterone concentrations in blood samples collected twice weekly from August 1 until puberty was reached or the experiment terminated on November 17 (34 wk after birth).(ABSTRACT TRUNCATED AT 250 WORDS)

Using the Lotka-Leslie Model for Sea Otters

Using the Lotka-Leslie model to assess population trend of sea otters (Enhydra lutris) by the fact that pups can be born in any month of the year. I used a Monte Carlo simulation model to devise an adjustment for this reproductive pattern. Choices of mean birth dates used in the model were tested by simulating population growth to determine λ directly, for comparison with estimates of λ from reproductive and survival rate data recorded in simulations. I used bootstrapping to obtain confidence limits on an estimate of λ from the California sea otter population and compared results with variance estimated by the delta method. The 2 approaches gave essentially the same result, but the delta method also can be used to determine sample sizes needed to obtain a given coefficient of variation on λ

Birth timing and population changes in greater horseshoe bat colonies (Rhinolophus ferrumequinum) are synchronized by climatic temperature

Abstract We counted the births of greater horseshoe bats born at three small breeding colonies in south-west Wales and south-west England at the extreme edge of the species distribution over nine summers (1984 1993). Overall birth timing was almost identical at the three sites (mean of mean birth dates = 13-14 July), but varied widely from year to year. Mean birth timing was synchronized at the three sites in a given year. Early births followed warm springs; a significant negative regression of mean birth date on mean April plus May temperature was evident. A rise of 2°C accelerated mean birth date by about 18 days. Population levels fell at all three sites following the very late mean birth date of 28 July recorded in 1986, after an extremely cold spring and summer. Recovery of populations in all sites followed a series of warm springs and early mean birth dates, but was hampered by birth sex ratios favouring males for several years. These findings confirm climate, through its effect upon birth timing, and possibly also on sex ratios, as a major factor controlling hat populations, and hence the distribution of bat species.

Birth timing and population changes in greater horseshoe bat colonies ( Rhinolophus ferrumequinum) are synchronized by climatic temperature

We counted the births of greater horseshoe bats born at three small breeding colonies in south-west Wales and south-west England at the extreme edge of the species distribution over nine summers (1984 1993). Overall birth timing was almost identical at the three sites (mean of mean birth dates = 13-14 July), but varied widely from year to year. Mean birth timing was synchronized at the three sites in a given year. Early births followed warm springs; a significant negative regression of mean birth date on mean April plus May temperature was evident. A rise of 2°C accelerated mean birth date by about 18 days. Population levels fell at all three sites following the very late mean birth date of 28 July recorded in 1986, after an extremely cold spring and summer. Recovery of populations in all sites followed a series of warm springs and early mean birth dates, but was hampered by birth sex ratios favouring males for several years. These findings confirm climate, through its effect upon birth timing, and possibly also on sex ratios, as a major factor controlling hat populations, and hence the distribution of bat species.

Effects of cohort, sex, and birth date on body development of roe deer (Capreolus capreolus) fawns.

We investigated the effects of the year of birth (cohort effect), sex and birth date on the variations observed in birth weight and postnatal growth rate in 209 newborn fawns marked during eight consecutive fawning seasons from a highly productive roe deer population under good nutritional conditions. Roe deer exhibited a fast body development with a birth weight of 1628 g and a postnatal growth rate of 139 g/day constant over the first 20 days of their life. As previously reported for dimorphic and polygynous ungulates, there was a marked cohort effect on the rate of body development. This variability was partly explained by climatic conditions during late gestation. When fewer than 5 days with rainfall over 5 mm occurred in April, fawns were heavier. High temperatures during April and during the winter could also be involved in fast body development of roe deer fawns. Sex of fawn did not affect roe deer growth pattern. This was expected on the basis of low sexual dimorphism in size and low polygyny level characteristic of roe deer. Lastly, the date of birth did not affect the body development of roe deer fawns. High constancy of mean birth dates and high synchrony of births observed in this population could account for this result.

Age and Growth of Anguilla japonica Leptocephali Collected in Western North Pacific in July 1990.

Results of age determination of Anguilla japonica leptocephali collected in the Western North Pacific, 16°29'N, 139°21'E in July 1990 are presented. The mean age of 18 leptocephali of 19.5-25.4mm TL was estimated to be 35.2 d. Backcalculated birth dates ranged from 26 May to 12 June 1990, with a mean birth date of 2 June 1990. This value was about one month earlier than the estimated birth dates of 14 larvae of 40.5-47.0mm TL collected in September 1986 in the pre-vious study. A growth rate of 0.59mm/d was calculated and was consistent with the value of 0.56mm/d in the previous study.

Mountain reedbuck Redunca fulorufula growth and age determination using dentition

Lower jaws and measurements were obtained from 473 mountain reedbuck Redunca fulvorufula culled on two reserves in the north‐eastern karoo of South Africa. All mandibles were allocated to subjective age classes that were based on tooth‐eruption and tooth‐wear. In a sample of 69 jaws. the molar and premolar cementum annuli were counted by examining bisected whole teeth under reflected light. One light line appeared to have been deposited per year, and a usable relationship between tooth‐wear category and cementum line count was obtained. although indistinct and paired lines often confused the picture. Good correlations were also obtained between the number of cementum lines and the crown height of the first molar (r2= 0.68) and the second molar (r2= 0.77). Actual ages of immatures were estimated by comparing peaks in the distribution in the different age classes with the age since expected mean birth date in different culls. The age of transition to adult dentition (pL 30 months) was determined from two females that were earmarked when young and shot two years later. Von Bertalanffy (1957) growth curves were fitted to measurements of body mass, jaw length and horn length in males to aid field identification of immatures. Asymptotic body mass was reached at 30 months in females and 40 months in males, and asymptotic jaw length was reached at 30 months in both sexes. Horns in males only reached full length at about five years.

Clines revisited: The timing of pupping in the harbour seal (Phoca vitulina)

The regional variation in the pupping season of the harbour seal (Phoca vitulina) was reviewed using the birth periods reported for 65 colony sites distributed over a range from 30.4d̀ to 78.5d̀ North latitude. The birth timing of P. v. vitulina was not related to latitude, but birthing in P. v. concolor along eastern North America exhibited a latitudinal cline. The timing of birth in P. v. richardsi varied in three distinct patterns: (1) a significant unidirectional latitudinal cline extending between Baja California and the west coast of Washington; (2) a cluster comprised of Puget Sound, Washington and Vancouver Island, British Columbia colonies in which birthing occurred an average of 65 days later than on the Washington coast; and (3) a cluster from northern British Columbia and Alaska which did not demonstrate a latitudinal cline. Insufficient data were available for the analysis of P. v. mellonae or P. v. stejnegeri.We found great regional variation in the timing of birth among all colonies, with mean birth dates occurring as early as 15 March and as late as 3 September. Little variation existed north of 50d̀. To the south of 50d̀, however, most of the variation could be attributed to correlation with latitude or to affiliation with the Puget Sound, Washington‐Vancouver Island, British Columbia geographic area.Clinal variation in pupping could result from: (1) geographic variations in a selective factor with perhaps gene exchange between contiguous populations playing a role in smoothing the variation; or (2) for populations between Mexico and the west coast of Washington, regional variation in a non‐selective environmental variable, such as photoperiod.

PRECISE BIRTH TIMING IN CAPTIVE HARBOR SEALS (PHOCA VITULINA) AND CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS)

AbstractThe absolute and relative precision of birth timing was assessed in 27 captive harbor seals and 30 captive California sea lions. Data were obtained from zoos and aquaria and only adult females with at least three live births were selected for analysis. Individuals of both species demonstrated very high absolute precision with narrow ranges of birth dates and small standard deviations of the individual mean birth dates. In addition, with the exception of one sea lion, no temporal trends for earlier or later birthing were found. Whereas marked differences in birth timing were found among individual harbor seals, these relative differences tended to disappear when subspecies affiliation and location were taken into account. Moderate inter‐individual differences were also found among the California sea lions, but these differences persisted following the partitioning of adult females by location.The absolute precision in birth timing strongly suggests that environmental regulation of birth timing occurs via a highly precise cue, such as photoperiod. The relative differences among individuals may indicate that birth timing in pinnipeds has a heritable component, and hence may evolve in response to natural selection, thus allowing adaptation of the pupping season to specific seasonal environments.

Timing of Parturition and Lactation in Southern Mule Deer

Factors related to timing of fawning in southern mule deer (Odocoileus hemionus fuliginatus) were studied in montane areas of San Diego Co., California, from June 1977 to January 1979. Mean date of birth was 27 June (SD = 7.4 days). Rates of nursing were highest in June and declined thereafter. Forage available to females following fawning was dry and of low quality; deer increased their consumption of browse during this period. A resultant shortened period of maternal care may have contributed to mortality (50–74%) of fawns. Timing of parturition may be related to the requirements of the maternal female during the last third of gestation when favorable temperatures and adequate precipitation combine to produce abundant forage. Sex ratios (males:females) changed markedly from yearlings (3.1:1) to adults (0.2:1), presumably because rutting activities followed harsh summer conditions.

Reproductive performance of farmed wapiti ( Cervus elaphus nelsoni)

Calving and weaning rates following 128 exposures of hinds to stags were recorded over a 11 year period for a research wapiti herd. Slightly less than 50% of hinds bred as yearlings and calved on their second birthday. About 90% of adults (3–22 years of age) calved. Only one set of twins occurred in 103 births. The proportion of female calves increased with age of the hind. Calf mortality was 15%, most occurring within the first several days after birth. Male calves were heavier than female calves ((mean ± SE) 19.2 ± 0.4 vs 16.8 ± 0.3 kg) and 2-year-old hinds had lighter calves than older hinds (13 vs 18 kg). Two-year olds calved later than older hinds and male calves were born later than female calves (mean birth date 1 June vs 26 May). Male calves weighed 125 ± 4 kg and female calves weighed 116 ± 3 kg at 120 days, the usual weaning age. Birth weight accounted largely ( r 2 = 0.57) for the difference in weaning weights between genders. Overall, the reproductive performance of wapiti compared well with that of the more widely-farmed red deer.

Reproductive seasonality in harbour porpoises, Phocoena phocoena, from the Bay of Fundy

I examined the reproductive tracts from 244 harbour porpoises, Phocoena phocoena, captured incidentally in commercial fishing operations in the Bay of Fundy during June to September, 1985–1988. Evidence from follicular growth and the development of corpora lutea suggests that ovulation and conception occur in late June. This interpretation is supported by subtracting the duration of gestation from estimated mean birth date, and also by seasonal changes in testicular mass. Six or 7 weeks of preimplantation pregnancy follows conception and foetuses are first detectable in early August. Gestation lasts for approximately 10.6 months and parturition occurs during mid-May. Females that fail to conceive during the breeding season must wait 12 months for another opportunity to mate. Estimates of pregnancy rate must account for the seasonal nature of reproduction. The observed pattern of reproductive seasonality may reflect seasonal variation in prey quality, and perhaps availability.

Age and birth date of the glass eel, Anguilla japonica, collected in Taiwan.

Forty glass eels, Anguilla japonica, collected at the Giese shore and the Tanshui River mouth in Taiwan were aged with the otolith daily increments, and their birth dates were analyzed. The mean radius of sagitta was 153μm, and the mean width of increments was 1.3μm until about 50 days and 1.7μm during 50-90days. The mean age of the fish collected on January 3 was estimat-ed as 122d while that of March 9 sample was 120d. There were no significant differences in age and body length between the two samples The mean birth date of the January 3 samples was September 2, while that of the March 9 samples was November 9 of the previous year. They were significantly different, suggesting that fish born earlier in the spawning place migrated earlier to Taiwan with constant age and size. Spawning season of the Japanese eel might be summer-autumn contrary to the previous understanding (winter), and the time required for oceanic migra-tion would be only 4 months.

Growth and Carcass Composition of Implanted and Non-Implanted Heifers and Steers Reared Together

The national cow herd consists of 1.64m dairy and 0.43m beef cows. They produce a total of 1.76m reared calves annually. Of these 0.42m heifers go as herd replacements, leaving 0.46m heifers and 0.88m males available for beef production. There is a 2:l ratio of males to femaies in the population of calves available for beef production. Also, there are few straightbred dairy heifer calves available and the mean birth date of heifer calves for beef production is later than that cf males. The objectives of the present experiment were (i) to compare the performance and carcass composition of non-implanted and implanted heifers and steers and (ii) to compare the carcass composition of serially slaughtered non-implanted and implanted heifers.Sixty tour (48 female and 16 male) Spring born Hereford x Friesian calves (initial live-weight 45 kg) were purchased and reared on milk replacer and concentrates. After 81 days they were blocked on weight and assigned from within sex type to 8 treatment groups.

Effect of pinealectomy on plasma melatonin, prolactin and progesterone concentrations during seasonal reproductive quiescence in the tammar, Macropus eugenii.

Non-lactating female tammars were pinealectomized (N = 5) or sham-operated (N = 6) in October, during the austral period of increasing daylength. After autopsy in February the completeness of pinealectomy was assessed by histological examination of the epithalamic region. Pinealectomy, but not the sham-operation, abolished the nocturnal rise in plasma melatonin but weekly plasma prolactin concentrations were similar in both groups from October to February. Plasma progesterone concentrations remained low (less than 160 pg/ml) in both groups until after the summer solstice in December, when there was a 2-4-fold increase (greater than 300 pg/ml) by late January, indicating reactivation of the quiescent corpus luteum. The mean date of birth or oestrus for the pinealectomized tammars was January 11 +/- 4.8 days (s.e.m.), and for the sham-operated tammars January 24 +/- 5.1 days. These results demonstrate that the pineal is not necessary for the maintenance of seasonal quiescence or to induce reactivation of the corpus luteum at the start of the normal breeding season after the summer solstice.