Hymenolepis nana: intestinal tissue phase in actively immunized mice.

We investigated the fate of the intestinal cestode Hymenolepis nana in immunized mice. Immunity was induced by infection with the parasite eggs. These immunized animals and unimmunized controls were then challenged with 50,000 H. nana eggs. The mice were killed 4 to 90 hr after challenge, and H. nana in the intestinal tissue were counted. At 4 hr after challenge the unimmunized and immunized animals had approximately equal numbers of oncospheres. By 12 hr there were fewer parasites in the immunized than in the unimmunized animals. At 90 hr, no H. nana were seen in the immunized mice, whereas in the unimmunized animals the median number of cysticercoids was more than 1,000. It appears, therefore, that in mice well immunized to H. nana by infection, challenge oncospheres can burrow into the intestinal tissue before they are killed. The reduced number of oncospheres in the immunized mice 12 hr after challenge, and the accumulation of eosinophils near individual oncospheres still present, indicate that an immune response to the parasite was taking place. Absence of a lymphocyte infiltration near any of the oncospheres suggests that the mechanism of immunity was not lymphocyte mediated; thus, the histopathology of the reaction is consistent with that of humoral immunity.     

The relationship of body size to survivorship of hatchling snapping turtles (Chelydra serpentina): an evaluation of the “bigger is better” hypothesis

In many organisms, body size is positively correlated with traits that are presumably related to fitness. If directional selection frequently favors larger offspring (the “bigger is better” hypothesis), the results of such selection should be detectable with field experiments. We tested the “bigger is better” hypothesis in hatchling snapping turtles (Chelydra serpentina) by conducting one long-term and three short-term experiments on the University of Michigan E.S. George Reserve in southeastern Michigan. In the fall of 1995 and 1996, we released hatchlings at artificial nests separated from the nearest wetland by fences. We recorded the proportion of hatchlings recaptured, the time it took hatchlings to move to fences from artificial nests 45, 55, and 80 m away, and dispersion along the fence. We determined whether the response variables and probability of recapture at fences were associated with hatchling body size. During 1995, average travel times of hatchlings from the experimental nests were not related to distance from the fence; however, time to recapture was positively correlated with dispersion from the zero point on the fence, and the maximum time to reach the fence was almost twice as long for hatchlings from the 80-m nest compared to those from the 45-m nest. Sixty-seven percent of the hatchlings reached the fence and the proportions doing so from each nest were not different. Body size was not significantly related to probability of recapture in either of the 1995 experiments. In 1996, 59% of released hatchlings were recaptured. Time to recapture was not related to dispersion from the zero point or to body size. Cubic spline analysis suggested stabilizing selection on body size. We also conducted a set of long-term hatchling release experiments between 1980–1993 to compare the survival of hatchlings released at nest sites to that of hatchlings released directly into marshes, and we looked for relationships between survivorship and hatchling body size. During 7 years in which more than 30 hatchlings were released, 413 hatchlings were released directly into the marsh and 262 were released at nests: their probability of survival did not differ. Over all years, for both release groups combined and for each group separately, survival was not related to body size. In 1983 alone, survival was also not related to body size for either group or for both groups combined. In our three short-term experiments and one long-term experiment, we found no evidence to support the “bigger is better” hypothesis. When selection on body size did occur, selection was stabilizing, not directional for larger size. Received: 4 June 1998 / Accepted: 24 June 1999