Beobachtungen zur Fortpflanzungsbiologie beim Davidshirsch.
Milu, Berlin 12: 314-325.
Reproductive biology and ecology of female polar bears (Ursus maritimus).
J. Zool. Lond., 214: 601-634.
Data on age‐specific natality rates, litter size, interbirth interval, age of first reproduction, reproductive senescence, age of weaning and cub survival were determined for a free‐ranging population of polar bears inhabiting Hudson Bay, Canada, near the southern limit of the species range. Serum progesterone levels were also determined for females at different stages of their reproductive cycle to provide corroborative support for the reproductive parameters described. Animals were live captured using immobilizing drugs and each animal uniquely marked for future identification. First parturition occurred at four or five years of age and the age‐specific natality rate increased with age until approximately 20 years, after which it dropped markedly. At least 40% of adult females displayed two‐year interbirth intervals and 55% of cubs in their second year were independent of their mother. Mean size of cub litters in spring was 1.9 and 13% of litters had three or more cubs. The natality rate for 5–20‐year‐old females was estimated as 0.9, higher than that reported for any more northerly polar bear populations where two‐year interbirth intervals are rare, fewer than 5% of yearling cubs are weaned and triplet litters occur with less than 1% frequency. Cub mortality was initially high and declined with age. Although cubs in western Hudson Bay were weaned at a younger age and a lighter weight than their counterparts in more northern populations, cub mortality rates were similar. The reason for the marked differences in reproductive parameters in the western Hudson Bay population is not known. We speculate that sea‐ice conditions may be sufficiently different to allow weaned bears at a lighter body weight to hunt seals more successfully there than further north.
Aspects of survival in juvenile polar bears.
Can. J. Zool, 74:1246-1252.
We captured, weighed, tagged, and monitored polar bear (Ursus maritimus) cubs and yearlings in western Hudson Bay to examine survival rates and correlates with survival. Cub survival between spring and autumn increased with cub mass and maternal mass, but was not related to maternal age or maternal condition. Cub survival between spring and autumn varied annually between 39.0 and 100.0% and averaged 53.2%. Whole-litter loss between spring and autumn was 30.8%, and only 38.0% of the females did not lose any cubs. Survival of spring twins was similar regardless of size, but in triplet litters, survival between spring and autumn varied according to cub size. Minimum cub survival from one autumn to the next was 34.7% and was related to cub mass, maternal mass, and maternal condition. Cub survival during autumn was estimated at 83.0%. Survival during the first year of life was no more than 44.0% but we could not estimate an annual survival rate because of the sampling regime. Possibly because harvesting was the major mortality factor for yearlings (19.4% of the yearlings were removed from the population per year), no factors examined correlated with survival of yearlings. We found no sex-related differences in survival of cubs or yearlings in any period. Relationships between survival in polar bear cubs and their condition suggest that lack of food availability, sometimes due to low maternal fat stores for lactation, leads to starvation and may be the main cause of mortality.
Aspects of survival in juvenile polar bears (PDF Download Available). Available from: https://www.researchgate.net/publication/238037133_Aspects_of_survival_in_juvenile_polar_bears [accessed Jun 21 2018].
Bei Tieren in der Keimruhe verzögert sich die Entwicklung der befruchteten Eizelle. Der Embryo (befruchtet Eizelle) nistet sich wie gewöhnlich in der Schleimhaut der Gebärmutter ein, wächst aber zunächst nicht weiter. Erst nach einigen Wochen oder sogar Monaten wird - gesteuert von Hormonen - das Wachstum wieder aufgenommen. Ohne Keimruhe würde bei einigen Tieren die Geburt der Jungen auf eine Zeit mit ungünstigen Umweltbedingungen fallen (z.B. im Winter, wo es kalt und Nahrung oft knapp ist). Durch die Keimruhe verlängert sich die Tragezeit und die Jungtiere können bei besseren Verhältnissen geboren werden. Beispiele hierzu sind das Reh, der Dachs oder der Eisbär.
Auch bei Pflanzensamen gibt es eine Keimruhe, welche die Entwicklung/Keimung verzögert. Erst bei günstigen, klimatischen Bedingungen (z.B. im Frühling) wird das Wachstum wieder aufgenommen.