Anthony K. Lee

BERGMANN'S RULE AND CLIMATIC ADAPTATION IN WOODRATS (NEOTOMA)

Within species and genera of homeotherms the body size of individuals of different populations is often negatively correlated with environmental temperature, and the relative size of appendages is often positively correlated with environmental temperature. Such trends are sufficiently widespread to have been labeled the ecogeographic rules of Bergmann (body size) and Allen (appendages). More than a decade ago, Scholander (1955, 1956) and Mayr (1956) debated the validity of the inference that these rules reflected physiological adaptations to climate. To that time there had been no study of the thermoregulatory physiology of a large number of conspecific or congeneric populations so that their arguments were theoretical or based on indirect evidence. Subsequently, Hayward (1965a, b) studied the temperature regulation of representatives of several races of Peromyscus, but concluded that the observed size related variation was not adaptive to climate. Other papers have recently discussed ecogeographic rules (Irving, 1957; Hamilton, 1961; McNab, 1966; Herreid and Kessel, 1967; Kendeigh, 1969), but none provide data on heat exchange of closely related homeotherms of different sizes from different climates. We now have data on the temperature regulation of representatives of 10 populations of woodrats. These populations vary three-fold in body weight, occur in diverse climates in western North America, and include representatives of four species of the genus N eotoma. Our data permit a direct evaluation of the relationships between metabolism, heat loss, body size, and environmental temperature, and indicate the major avenues of climatic adaptation in this group of rodents. MATERIALS AND METHODS

Corticosteroid levels and male mortality in Antechinus stuartii

Current views on the regulation of numbers of small mammals have developed mainly from studies of murid and cricetid rodents (Christian, 1971; Krebs et al., 1973; Krebs and Myers, 1974), and particularly those species which provide spectacular examples of cyclical fluctuations. In spite of these studies, the causes of cycles and intrinsic population regulatory mechanisms remain poorly understood. Among the difficulties encountered has been the complex age structure of many populations, and problems of ageing and identifying cohorts, and of finding the causes of differing mortalities within cohorts. With these difficulties in mind we have begun to examine populations with simple, easily determined age structure, which also show abrupt changes in numbers.

MACROGEOGRAPHIC VARIATION IN LITTER SIZE IN ANTECHINUS (MARSUPIALIA: DASYURIDAE)

The best documented cases of geographic variation in brood size are the tendency of avian clutches to increase with latitude (Lack, 1947, 1954; Klomp, 1970), and mammalian litters to increase with both latitude (Lord, 1960), and altitude (Dunmire, 1960; Fleming and Rauscher, 1978; Bronson, 1979). There is no consensus on the cause of this phenomenon (Stearns, 1976; Ricklefs, 1980). Lack (1947) proposed that parents produce broods which correspond with the maximum number of young they can nourish. Numerous modifications to this hypothesis have been suggested (Ashmole, 1963; Cody, 1966; Owen, 1977; Hogstedt, 1980), and a number of theoretical models have been developed that suggest brood size should be reduced below the most productive size (Stearns, 1976). Two classes with wide application can be recognized among the latter models. The first includes those based on the tradeoff between fecundity and adult mortality resulting from increased reproductive effort (Charnov and Krebs, 1974). According to this approach adults should limit the size of their brood in certain environments to extend their opportunities of breeding again (Ricklefs, 1977a). This viewpoint has been criticized because it fails to predict what size smaller broods should be to ensure iteroparous reproduction (Ricklefs, 1977b). The second includes those models which predict tradeoffs between demands for resources on the part of reproduction and other functions, particularly avoidance of predation (Cody, 1966; Skutch, 1967; Perrins, 1977). Ricklefs (1977a) presented evidence that predation is not sufficiently important to account for the magnitude of variation commonly observed along geographic gradients. Several methodological difficulties have hindered investigation of these hypotheses. The genetic basis of variation in brood size has rarely been established (Krohne, 1980, 1981). Further, the influence of predation and the proposed trade-off between reduced clutch size and increased adult longevity have been difficult to quantify in field populations. While it is possible to distinguish between semelparous and iteroparous species, an index of the extent of iteroparity remains elusive and is confounded by difficulties in ageing wild individuals and the production of more than one brood within a season. A study of variation in brood size within semelparous species might be informative, but brood sizes in semelparous invertebrates and lower vertebrates and monocarpic plants are enormous and difficult to quantify. In this study we report macrogeographic variation in the litter size of Antechinus, a genus of mammals in which the frequency of iteroparous versus semelparous reproduction can be precisely quantified, and in which we believe litter size differences are genetically determined. Litter size is manageably low and variable (6-13), and only one litter is produced each year. This combination of characters provides a tractable system for resolution of the separate factors which might influence litter size.

Energy and Water Budgets in Free-Living Antechinus stuartii (Marsupialia:Dasyuridae)

Rates of water flux and CO2 production were measured in Antechinus stuartii in the field using doubly-labeled water. Itemized dry matter, energy, and water budgets were determined for captive animals, and were used to estimate feeding rate and energy and water fluxes in free-living animals. In winter, steady-state, field A. stuartii (mean body mass 25.7 g) ingested 807 kcal/ kg/day and metabolized 670 kcal/kg/day. They consumed about 60% of their body weight in arthropods each day. There were no significant differences between metabolic rates of nonbreeding adult (July) males and females and breeding (August) females; satisfactory data for breeding males were not obtained. Energy expenditures of A. stuartii were similar to those of several free-living, small eutherians, after accounting for differences in ambient temperature and body size. However, this comparison is complicated by the use of burrows and occurrence of torpor in some of these mammals. The energetic impact of A. stuartii in temperate evergreen forest lies between 60 and 370 kcal/ha/day. These rates are similar to those obtained for small eutherian insectivores in other communities. Water fluxes did not differ between males and females in the field, but mean flux rate in steady-state, adult A. stuartii in August (734 ml/kg/day) was higher than in July (539 ml/kg/day), apparently because they ingested rain water that fell abundantly in August. Laboratory results indicate that field animals obtain much more water than they need to maintain water balance.

Variations in the hemoglobin concentrations of Great Basin rodents.

The hemoglobin concentration of the blood of seven species of Great Basin rodents varies inversely with body weight. Field values are higher in winter than in summer; part of this difference may be attributed to the effects of different ambient temperatures. Changes in hemoglobin concentration can be accounted for by changes in the number of circulating erythrocytes.

The marsupial life history

In this chapter we consider what it means to be a marsupial. We do this by contrasting the scope of the marsupial and eutherian radiations, and examine hypotheses which attempt to explain why the marsupial radiation appears to be conservative. It is not our intention to enter into the sterile debate over the general advantages of ‘marsupialness’ and ‘eutherianness’. The coexistence of these groups in South America and Australia clearly testifies to the evolutionary viability of both. However, we feel it is useful to point to a number of deficiencies in previous attempts to contrast the two taxa. Historically, it has often been assumed that marsupials are in some sense inferior to eutherians (Asdell, 1964; Lillegraven, 1975,1979), perhaps as a consequence of the widespread but fallacious belief that marsupials represent an intermediate grade of mammalian organisation between monotremes and eutherians. Both the original work and attempts to destroy the assumption (e.g. Kirsch, 1977a, b; Parker, 1977; Low, 1978), suffer from a lack of quantification and statistical examination, and have given rise to summary statements which are probably incorrect, for example, ‘the nonseasonal opportunism in most marsupials’ (Low, 1978, p. 206), or ‘even modest territorial behaviour in the marsupials is a comparative rarity’ (Lillegraven, 1979, p. 270). There is a further underlying and unrecognised assumption which pervades this literature. This may be paraphrased: all aspects of an organism are perfectly tuned or adapted to their environments. As we shall see, the case for interpretation of species characteristics in an adaptive framework is by no means always clear, and the application of similar assumptions to the characteristics which distinguish higher taxa is even less justified.