Age structure of males also can influence breeding dynamics of age-structured populations. In polygynous species, populations with greater numbers of older-age-class males have been shown to have shorter, earlier, and less socially disruptive breeding periods. Conversely, where fewer older-age-class males are present, breeding periods tend to be longer and females are frequently bred later in the season. Because later breeding may lead to later birth dates, and later birth dates to lower juvenile survival, the age structure of the male population may potentially influence both pregnancy rates and survival of juveniles, thus affecting population rate of increase. However, this cascade of effects has not been conclusively demonstrated in free-ranging populations. Few studies indicate that later-bred females have significantly later parturition dates, while much evidence indicates that female nutritional condition can override potential effects
Long-term studies or field experiments designed to test mechanisms underlying the 10-year cycle have been reported only relatively recently. Vaughan and Keith (1981) conducted such an experiment in eight enclosures (about 3-6 ha in size) of natural habitat to measure the demographic response of hares to food shortages during the winter. They used two levels of hare density, averaging about four and 13 hares ha at the start of each experiment, and two levels of food availability (high and low). They found that food shortage greatly affected the reproductive characteristics of adult hares, including the onset and termination of breeding, pregnancy rates, and ovulation and implantation rates. These changes corresponded to shorter breeding seasons and a reduction in
Laboratory experiments indicated that female voles placed in close proximity to a weasel or its odor could slow their estrous cycles, reduce pregnancy rates, increase their aggression toward males when approached even when in estrous, and slow their growth (Ronkainen & Ylonen 1994 Ylonen & Ronkainen 1994 Koskela et al. 1996). Population models suggest that reproductive delays could benefit females in fluctuating populations (Ylonen 1994 Kaitala et al. 1997). But reproductive delays have not been observed by all researchers (Norrdahl & Korpimaki 2000a) nor in all vole species, and the experimental designs of the published studies have been severely criticized (Hansson 1995 Lambin et al. 1995). More realistic enclosure experiments have failed to support the reproductive suppression hypothesis (Wolff & Davis 1997 Mappes et al. 1998).
Steroid concentrations (especially estrogen) have been used to examine pregnancy rates among free-ranging mammals (Kirkpatrick et al. 1990). This technique could be modified to distinguish male- and female-derived fecal samples. Even greater sample resolution is possible by using emerging molecular techniques. As indicated earlier, fecal samples contain epithelial cells shed from the intestine walls of the animal depositing the sample. DNA extracted from these cells has been used to identify the species that deposited the sample. Recently, several investigators have used this approach to identify sex and individual genetic markers (Kohn and Wayne 1997 Reed et al. 1997). Therefore, it is possible to substantially increase the resolution of fecal samples so that researchers can track the diet of identified free-ranging individuals. The information obtained from fecal samples could be enhanced even more by using digestibility correction factors that...
The habitats occupied by the stoats observed, their average body sizes, and the prey resources available to them were all wildly different in the two countries. The most significant contrast, though, is between kill trapping and live trapping as methods of counting the numbers of stoats present. These differences in methodology are also an advantage, because they produce complementary kinds of information. Some data (e.g., on the home ranges and behavior of individuals) can be gained only by watching undisturbed, live animals. Equally important data (e.g., on the age structure, fecundity, and pregnancy rates of
It is not correct to assume, as in some population models (Chapter 14), that common weasels can successfully produce two litters every year. On the contrary, when food is very scarce, adult females in the wild probably mate as usual but are unable to produce any surviving young even from their first litters. According to Erlinge (1974) and Tapper (1979), common weasels cannot rear young at all unless they have access to a minimum density of some 10 to 14 voles per ha, or about 400 rodents per territory (Jgdrzejwski et al. 1995). So pregnancy rates and the lengths of breeding seasons vary enormously in common weasels, from 7 to 8 months in vole peak years to total failure in crash years.
Substances transferred in the spermatophore play a role in switching off female receptivity and stimulating oviposition and egg maturation rate (Gillott, 2003). In the moth Heliothis virescens, female egg maturation is stimulated by juvenile hormone derived from the males' accessory glands (Park and Ramaswamy, 1998). In addition, it appears that male-derived factors also stimulate the female's own production of juvenile hormone (Park et al., 1998). In Helicoverpa zea and H. armigera other factors from the male accessory gland stimulate egg maturation and oviposition (Bali et al., 1996 Jin and Gong, 2001). In many moth species, receptive females attract males by releasing pheromones during a characteristic phase of 'calling' behaviour. The regulation of female pheromone production has been intensively studied in moths. A neuropeptide (PBAN) regulates pheromone production (e.g. Raina, 1993 Rafaeli, 2002), mediated by either humoral, hormonal or neural cues. There is large interspecific...
Among non-host-feeders there are species in which feeding increases the rate of ovigenesis compared with a diet of water (V. canescens, Harvey et al. 2001), but there are also species in which it appears not to do so (e.g. Phanerotoma franklini, Sisterton & Averill 2002). In M. grandii, the egg maturation rate is higher in starved, water-fed females than in sucrose-fed females (Olson et al. 2000). We discuss this somewhat surprising result below (Section 7.11).
Investment in the latter can be minimal or even non-existent fuel is not required for egg maturation, and expected life-span is short (Jervis et al. 2001, 2003). As ovigeny index decreases from 0.5, investment in initial reserves should occur at the expense of investment in eggs. When the index reaches zero, investment in eggs is either zero or (more likely) consists, at most, of allocation to immature oocytes. Allocation to initial reserves ought to be substantial in non-host-feeding, extremely synovigenic species, because their adult foods are both lipid- and nitrogen-poor (and so cannot fuel egg manufacture to a significant degree), and because, like other Hymenoptera, they are apparently unable to synthesize lipids de novo (Giron & Casas 2003).
Takahashi et al. (2005) considered only two sub-populations the winged form (mature female mosquitoes) and an aquatic population (including eggs, larvae and pupae), with mortality rates and The spatial density of the winged A. aegypti and aquatic population at point x and time t are denoted by M(x,t) and A(x,t), respectively. The specific maturation rate of the aquatic form into winged female mosquitoes is y, which is saturated by a term describing a carrying capacity K1 that is, yA(1 - M Ki). Similarly, the rate of oviposition by female mosquitoes, which is the only source of the aquatic form, is proportional to their density but is also regulated by a carrying capacity effect dependent on the occupation of the available breeders that is, rM(1 - A K2). Since the focus is on the A. aegypti dispersal as a result of a random (and local) flying movement, macroscopically represented by a diffusion process with coefficient D, coupled to a wind advection caused by a constant velocity flux...
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