Modeling Orthokinetic Coagulation in Spatially Varying Laminar Flow.

An orthokinetic coagulation model including the effects of agglomeration and local stress-induced aggregate breakup was developed. This model was used to simulate coagulation in the flow between two eccentrically located and rotating cylinders. Four methods of modeling coagulation in the flow system were examined. The first technique used a volume-weighted average of the local strain rates, while a second method used an equivalent volume-weighted power (G). A third method treated each volume element as a separate batch reactor and determined a final volume-averaged floc population. The final modeling technique applied mass transfer between each of the elements. Results indicated that substantial differences in average particle diameters and populations were generated with each of the methods, especially where mass transfer between the elements was considered. It was concluded that mass transfer between regions of varying flow strain rate and/or velocity gradient should be included in accurate coagulation modeling. Copyright 2000 Academic Press.

Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting.

In previous studies we have shown that those male Mongolian gerbils, Meriones unguiculatus, that as fetuses resided in intrauterine positions (IUPs) located between two female fetuses (2F males) have lower circulating levels of testosterone, less well-developed genital musculature, and lower reproductive success than males gestated in IUPs between two male fetuses (2M males). We have also found that such 2F males spend more time caring for conspecific young than do 2M males, and that presence of a 2F male, but not of a 2M male, with a lactating female and her litter decreases the cost to a dam of suckling one litter while gestating a second litter conceived in postpartum oestrus. Here we show that some 2F males, those with circulating levels of testosterone similar to those seen in females of their species, show no interest in females in oestrus and fail to impregnate females with which they are paired. Such 'asexual' 2F males spend 30-50% more time caring for nestlings than do sexually active 2F males. We suggest that such asexual, highly parental 2F males are incapable of direct reproduction and are obligate helpers at the nest that can contribute to their own fitness only by assisting to rear collateral kin. Copyright 2000 The Association for the Study of Animal Behaviour.

Incorporation of Aggregate Breakup in the Simulation of Orthokinetic Coagulation.

The agglomeration and breakup of floc aggregates formed in orthokinetic coagulation is examined. By considering local flow strain-rate, a breakup rate kernel is derived based on flow-induced normal stresses. The new breakup kernel is included in a population size class balance for floc aggregates. The resulting population balance was solved numerically over a wide range of parameters to obtain a variety of floc size distributions. Results indicate that the inclusion of a breakup kernel in orthokinetic coagulation modeling eliminates the computational growth to a maximum size class, producing more realistic distributions. The breakup kernel was rigorously compared to prior research and found to be consistent with the earlier theories of coagulation agglomeration and breakup. Copyright 1999 Academic Press.

Parenting and potency: alternative routes to reproductive success in male Mongolian gerbils

Adult male Mongolian gerbils, Meriones unguiculatusgestated in intrauterine positions between two female fetuses (2F males) are less likely than are adult males gestated between two male fetuses (2M males) to impregnate strange female gerbils with whom they are paired. The reduced copulatory success of 2F males is correlated with both lower circulating levels of and reduced sensitivity to testosterone. We asked whether 2F male gerbils compensated for their reduced copulatory success by increasing their parental effort. 2F male gerbils engaged in less sexual activity with their mates, but were more frequently in contact with pups than were 2M males, huddling over the young when their mates were absent from the nest. Although there were no differences in rates of survival or growth of pups reared by pairs consisting of a female and either a 2M or 2F male, mates of 2F males delivered significantly more pups as a consequence of copulations occurring during postpartum oestrus than did either mates of 2M males or females rearing young alone. We interpreted these results as consistent with Ketterson & Nolan's (1992, Am. Nat. (Supplement)140, 533-562) hypothesis of a testosterone-mediated trade-off between investment in sexual and parental behaviours.1997The Association for the Study of Animal Behaviour