David E. Davis

A Model For The Ecology Of Avian Malaria

Although studies on Plasmodium infections of wild birds have been reported frequently in the literature, our knowledge of the ecology of these parasites remains incomplete. A synthesis of data and ideas from these field studies, and recent experimental work led to the construction of the following hypothetical model for the ecology of avian malaria: During the late spring, summer, and early fall susceptible birds (young of the year or previously uninfected adults) contract the infection from the bite of an infected mosquito on the breeding ground. The birds migrate or remain in the area and infections become latent over the winter. In the spring, migratory birds return to the breeding area and all birds commence reproductive activity. With the onset of migration and breeding activity, parasite populations become elevated in the birds. This relapse of malarial infections coincides with emergence of vectors. The mosquitoes obtain the parasite, passing it on to susceptibles in the population (whose numbers are simultaneously increasing as the result of reproduction), and the cycle continues. Under favorable conditions, transmission rates equal or exceed a level needed to replace mortality of infected birds. Under unfavorable conditions the parasite is maintained by the bird reservoir, the population of susceptibles increases, and transmission is postponed until favorable conditions return and transmission to the expanded population of susceptibles replenishes the supply of infected adults. Such a cycle, in which the parasite, vector, and susceptible host populations reach a maximum in an apparently favorable sequence, with provision for occasional failure of transmission, has obvious survival value. A more complete understanding of the ecology of avian malaria will be achieved with the investigation of specific problem areas defined in this model. The model may prove of additional value in suggesting an ecological approach to our understanding of the epidemiology of human malarias. It may also have applicability in other disease systems where bird-mosquito relationships are similar, such as certain of the arboviruses.

Entrainment of Circannual Rhythm in Weight of Woodchucks

For the first time the entrainment of an endogenous annual rhythm has been demonstrated. Woodchucks ( Marmota monax ) sent to Australia from Pennsylvania shifted the annual phase of maximum and of minimum weight by six months.

Antibodies to larval Taenia crassiceps in hibernating woodchucks, Marmota monax

Abstract An experiment was designed to study the effects of hibernation on the production and induction of antibody response to cysticerci of Taenia crassiceps in woodchucks ( Marmota monax ). Eight woodchucks were placed at 6 °C (hibernators) and eight at 20 °C (nonhibernators). The food was removed from all woodchucks to induce hibernation or lethargy. Four of each group were infected before hibernation and four after. Cysticerci were injected intraperitoneally. The response was judged by hemagglutination titration. The results showed no difference in antibody level among the four groups. Thus, it is concluded that antibodies to at least some helminths are produced during the period of hibernation. Whether or not production occurs only during the brief periods of arousal was not determined.

An empirical formula for the growth of some vertebrate populations

Abstract Analysis of data obtained on house mice and white footed mice in the laboratory and on trumpeter swans in the field shows that the population growth can be described empirically in terms of power functions of time much better than by the usual logistic formula. The power function formula permits estimates of ultimate population and of the point of maximum rate of increase of population.

Circannual rhythm of testicular size in California ground squirrels

Abstract A circannual rhythm of testicular size existed in California ground squirrels (Spermophilus beecheyi) kept at 20°C under 3 regimes of daylength: Santa Barbara (SB), LD 12:12, and southern hemisphere (SH). Seasonal descent of testes was measured on a scale of 0–4. The squirrels on SB daylength maintained a cycle of descent of 11.13 months; those on LD 12:12 of 11.61 and those on SH daylength 10.65. The latter 2 groups showed great variation. The persistence of the rhythm under constant daylength (LD 12:12) and also on a 6‐months phase shift indicates that the rhythm is circannual.

Evaluation of technics for measuring mortality.

An evaluation is a statement of the accuracy of some measure. Thus one should evaluate a measure of mortality by determining how close the measure comes to the true mortality. The obvious consequence of this statement is that some measure of true mortality must be available, a problem that may be insurmountable. The purpose of the present review of techniques of measuring mortality is to present some basic definitions that should clarify the use of terms and also to indicate some possibilities for use of some measures in the field. This presentation will omit mathematical details that are best given in other places. There will be no discussion of the problem of diagnosis of the cause of death. Fortunately, it is relatively easy to decide whether the animal is dead or alive. Lastly, this summary will not include lists of data about mortality rates which are available in the literature.

Failure of temperature to entrain a circannual rhythm of mass in California ground squirrels (Spermophilus beecheyi)

Abstract To determine what environmental factor is the Zeitgeber for the annual change in mass, California ground squirrels (Spermophilus beecheyi) were exposed to several regimes of temperature and length of day. The squirrels were captured when age 4–5 months, kept individually in cages with excess food and nesting material. The temperature was varied with a 6‐month phase shift (SH). The length of day was the same as Santa Barbara (SB) or 6‐month phase shift (SH). The light was provided by cool white bulbs at 10–600 lux. The first experiment consisted of keeping squirrels at either SB or SH length of day and at constant temperature (20°) for three years and then changing to SH temperature for 3 years. None entrained although at age 6, 2 might be entrained. The second experiment repeated the first except that the SH temperature started at age zero. None has entrained by age 5. None of a third group, placed at SH temperature and SB length of day, has entrained by age 3. The conclusion is that these regime...

Comments on rabies control.

The possible effect of a change in habitat on the number of foxes and hence on rabies has been examined by comparing the areas of abandoned farms and the number of cases of rabies in foxes for Georgia from 1930-1969. The relation was postulated from the observed abundance of foxes in recently abandoned farms in Georgia. Abandoned farmland in Georgia increased greatly around 1945-50, and an epidemic of rabies occurred in foxes. This correlation suggests that trends in use of land should be considered among the factors influencing future epidemics of rabies in wildlife.

Circannual changes in nesting behavior of captive ground squirrels(Spermophilus beecheyi)

Abstract California ground squirrels (Spermophilus beecheyi) were kept for 5–7 years at 20°C with excess food. Presence or absence of nests, built from newspapers, was noted semiweekly, The squirrels were weighed semimonthly. Squirrels were kept in small rooms at natural (SB) length of day and at LD 12:12. Nests were built by squirrels in SB from May to January with a minimum in February to April. Squirrels in LD 12:12 ran freely. The date of the minimum building advanced steadily about a month a year. The persistence of rhythms in LD 12:12 conditions shows that building of nests is an endogenous rhythm and properly called circannual.