Birth Peak (birth + peak)

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Selected Abstracts

A comparison of sex ratio, birth periods and calf survival among Serengeti wildebeest sub-populations, Tanzania

Vedasto G. NdibalemaArticle first published online: 31 AUG 200
Abstract Although adaptation and environmental conditions can easily predict demographic variation in most savannah ungulates, no study on demographic consequences arising from natural and anthropogenic factors among Serengeti wildebeest (Connochaetes taurinus) sub-populations in Tanzania has been carried out. Here, I report estimates of annual sex ratio, calf and yearling survival rate and birth seasonality between resident and migratory sub-populations to explore demographic patterns arising from the different age and sex structure. The results indicate significantly higher female-biased sex ratios in the resident and almost even sex ratios among individual migrants. The calf recruitment estimated as mother: offspring ratios indicate a more synchronous birth in the migrant than the resident sub-population. Also, birth seasonality in the migratory sub-population coincided with seasonal variability of rainfall and the timing of the birth peak was more variable in the migrants than the resident sub-population. The migratory sub-population had a higher annual proportional mean calf survival estimate (0.84) than that of the residents (0.44) probably due to higher mortality resulting from predation in the western corridor. However, the proportion of yearling survival estimates was much lower (0.31) in the migrants and relatively higher (0.39) in the residents. Different demographic outcomes resulting from environment, predation, movements and ecological factors including resource competition have conservation implications for the two sub-populations. Résumé Bien que l'adaptation et les conditions environnementales puissent facilement aider à prédire la variation démographique de la plupart des ongulés de savane, aucune étude n'a encore été réalisée sur les conséquences démographiques de facteurs naturels et anthropiques chez les sous-populations de gnous (Connochaetes taurinus) du Serengeti, en Tanzanie. Ici, je rapporte des estimations du sex-ratio annuel, du taux de survie des veaux et des jeunes d'un an et le caractère saisonnier des naissances entre des sous-populations résidentes et migratrices, pour explorer les schémas démographiques qui se dessinent à partir des différentes structures d'âge et de sexe. Les résultats indiquent un sex-ratio significativement biaisé en faveur des femelles chez les animaux résidents et un sex-ratio presque équilibré chez les individus migrateurs. Le recrutement des veaux estimé selon le rapport mère/progéniture indique des naissances plus synchrones chez la sous-population migratrice que chez la résidente. La saisonnalité des naissances chez la sous-population migratrice coïncidait avec la variabilité saisonnière des chutes de pluies, et le timing du pic des naissances était plus variable chez la sous-population migratrice que chez la résidente. La sous-population migratrice avait une espérance de vie annuelle proportionnelle des veaux plus élevée (0,84) que la résidente (0,44), probablement en raison de la plus forte mortalité causée par des prédateurs dans le corridor occidental. Cependant, les estimations de la proportion de jeunes d'un an survivants étaient beaucoup plus basses (0,31) chez les migrateurs et relativement plus hautes (0,39) chez les résidents. Les résultats démographiques différents des facteurs environnementaux, de la prédation, des déplacements et des facteurs écologiques, y compris la compétition pour les ressources, ont des implications pour la conservation des deux sous-populations. [source]

Demography of lions in relation to prey and habitat in the Maasai Mara National Reserve, Kenya

J. O. Ogutu
Abstract We studied lion demography in the Maasai Mara National Reserve between September 1990 and April 1992, with a special emphasis on the spatial and seasonal variation in demographic characteristics. Lion density (0.2,0.4 lions km,2) and pride size (range 8,48) were high because of a high resident prey biomass (10 335 kg km,2) augmented by migrant prey to 26 092 kg km,2 in the dry season. Overall, their sex ratio was almost at parity and varied neither spatially nor seasonally. Sex ratio was even among subadults but skewed toward males and females among cubs and adults, respectively. This implies an increasing differential mortality of males with age through subadulthood. The age ratio varied seasonally because of a birth peak in March,June and an influx of subadults into the reserve during July,August, coincident with increases in migrant prey. The birth peak was apparently preceded by another peak in mating activity falling between November and May. Further research should investigate the precise causes of the biased cub sex ratio, low lion density in the Mara Triangle and the higher ratio of subadults in Musiara than in the Mara Triangle or Sekenani. Résumé Nous avons étudié la démographie des lions dans la Réserve Nationale de Masai Mara entre septembre 1990 et avril 1992, en insistant particulièrement sur les variations spatiales et saisonnières des caractéristiques démographiques. La densité des lions (0.2,;0.4 lions/km2) et la taille des troupes (de 8 à 48) étaient élevées en raison de la forte biomasse des proies résidentes (10 335 kg/km2), portée à 26 092 kg/km2 en saison sèche par les proies migratrices. En général, le sex-ratio était à peu près égal et ne variait ni selon les saisons, ni selon les endroits. Les sex-ratio était égal parmi les sub-adultes, mais il déviait en faveur des mâles et des femelles chez les petits et les adultes, respectivement. Ceci implique une mortalité de plus en plus différentielle des mâles à l'âge sub-adulte. L'âge-ratio variait avec les saisons en raison d'un pic de natalité de mars à juin et d'une arrivée de sub-adultes dans la réserve en juillet-août, coïncidant avec l'augmentation des proies migratrices. Le pic des naissances était apparemment précédé par un autre pic des activités d'accouplement, entre novembre et mai. De nouvelles recherches devraient étudier les raisons précises du sex-ratio biaisé chez les lionceaux, de la faible densité des lions dans le Mara Triangle et du taux de sub-adultes plus élevéà Musiara que dans le Mara Triangle ou à Sekenani. [source]

Trade-off between resource seasonality and predation risk explains reproductive chronology in impala

S. R. Moe
Abstract We investigated the variation in birth synchrony displayed by impala Aepyceros melampus populations across their distribution from southern to eastern Africa. Our analysis was based on field data from Chobe National Park in Botswana and Mala Mala Private Game Reserve in South Africa (4 and 13 years of monitoring, respectively). We compared our results with those from other studies conducted across the impala species range. Impala lambing was highly synchronized in Chobe with 90% of lambs born within 2 weeks in mid-November. Variation in rainfall in the preceding wet season explained 74% of variation in the dates of the first lamb observation in Mala Mala. In Chobe, the earliest birth peak occurred after the highest rainfall and the body condition of lambs in that cohort was also best for both males and females. No association was found between the lunar cycle and the estimated onset of the conception period, despite previous studies having found an association between the lunar cycle and the rutting behaviour in males. On a regional scale, impalas in areas with a marked dry season (several months with no rain) tend to synchronize births with the onset of the rains, when grass quality is highest. Number of months with rain explained 78% of the regional variation in birth synchrony. Neither latitude nor total rainfall contributed significantly to a stepwise multiple regression model. These data support the theory that impalas synchronize births in areas with a highly seasonal food supply, and temporally space births in less seasonal (equatorial) areas to reduce predation risks. [source]

Reproductive parameters of wild Trachypithecus leucocephalus: seasonality, infant mortality and interbirth interval

Tong Jin
Abstract Understanding the reproductive parameters of endangered primate species is vital for evaluating the status of populations and developing adequate conservation measures. This study provides the first detailed analysis of the reproductive parameters of wild white-headed langurs (Trachypithecus leucocephalus), based on demographic data collected over an 8-year period in the Nongguan Karst Hills in Chongzuo County, Guangxi, China. From 1998 to 2002, a total of 133 live births were recorded in the population based on systematic censuses. Births occurred throughout the year, but the temporal pattern was highly correlated with seasonal variation in temperature and rainfall, with the birth peak coinciding with the dry and cold months of November,March. The average birthrate was 0.47±0.13 births per female per year and mortality for infants younger than 20 months was 15.8%. From 1998 to 2006, 14 females gave birth to 41 infants in four focal groups. The average age at first birth for female langurs was 5,6 years (n=5) and the interbirth interval (IBI) was 23.2±5.2 months (median=24.5 months, n=27). Infants are weaned at 19,21 months of age. The IBI for females with infant loss before weaning was significantly shorter than those for females whose infants survived. It appears that birth seasonality in the white-headed langurs is influenced by seasonal changes in food availability. The timing of conceptions was found to coincide with peak food availability. The reproductive parameters for white-headed langurs reported here are quite similar to those reported for other colobine species. One major difference is our observation of lower infant mortality in Trachypithecus. Am. J. Primatol. 71:558,566, 2009. © 2009 Wiley-Liss, Inc. [source]

Reproduction of the Bat Glossophaga commissarisi (Phyllostomidae: Glossophaginae) in the Costa Rican Rain Forest During Frugivorous and Nectarivorous Periods,

BIOTROPICA, Issue 3 2005
Marco Tschapka
ABSTRACT I studied the reproduction of the 9 g nectarivorous bat Glossophaga commissarisi (Phyllostomidae: Glossophaginae) in relation to its food choice in a Costa Rican lowland rain forest over more than 2 yr. Reproduction was bimodal with birth peaks in April/May and October/November. The first birth peak coincided with a period of frugivory, whereas the second occurred during a period of almost exclusive nectarivory. All adult females recaptured in consecutive reproductive periods were reproductively active on both occasions, indicating that individual G. commissarisi females regularly reproduce twice per year at the study site. Throughout the annual cycle the mean testis length of the males correlated best with the proportion of females lactating, suggesting a post partum estrous pattern. Animals became reproductively active at about 1 yr of age. Fecal samples showed that females consumed significantly more insects than males. RESUMEN Se estudió la reproducción del murciélago nectarívero Glossophaga commissarisi (Phyllostomidae: Glossophaginae) en relación a sus preferencias alimenticias en un bosque lluvioso de tierras bajas en Costa Rica por más de dos años. La reproducción fue bimodal con picos de nacimiento en abril/mayo y octubre/noviembre. El primer pico de nacimiento coincidió con un periodo de frugivoría, mientras que el segundo ocurrió durante un periodo exclusivo de nectarivoría. Todas las hembras adultas recapturadas en periodos reproductivos consecutivos estaban reproductivamente activas en ambas occasiones. Indicando así que cada hembra de G. commissarisi regularmente reproduce dos veces al año en el sitio del estudio. Durante todo el ciclo annual el promedio de la longitud de los testículos de los machos se correlacionó mejor con la proporción de hembras en lactancia, sugiriendo una norma de post partum estro. Los animales se vuelven reproductivamente activos cerca de un año de edad. Examen de muestras fecales indicaron que las hembras consumen significativamente más insectos que los machos. [source]