Litter Size and Latitude in North American Mammals

The population ecology of Rattus exulans on Tiritiri Matangi Island, and a model of comparative population dynamics in New Zealand

“…However, there 1S no correlation between latitude and the annual productivity of mature females ( (Moller 1977) in R. exulans. Lord's (1960) and Jackson's (1965) explanation for latitudinal increases in litter size subscribes to group selection arguments since it implies that females in tropical areas have lower reproductive rates than they could otherwise attain. Spencer and Steinhoff (1968) state:…”

“…Lord (1960)and Jackson (1965) suggestedthat an increase in litter size (for R. exulans and for small mammals in general) with latitude indicates that animals at higher latitudes have evolved higher reproductive rates. Higher reproductive rates were supposed to balance the higher mortality that r~sults from increased predation.…”

EVOLUTION OF LITTER-SIZE IN THE PIKA,OCHOTONA PRINCEPS(RICHARDSON)

“…Species with large litters are generally larger than those with small litters (Table 2) . Lord (1960) predicted a positive correlation between litter-size and latitude in non-hibernating prey species. This is not true of the pika; litter-size is relatively constant in North America (Table 1).…”

“…These generally view litter-size as only part of an overall reproductive strategy, but differ in the parameters considered important in determining litter-size. For example, "resources," length of breeding season, food supply, body size, altitude, latitude, mortality, population stability, and competition have all been suggested to influence litter-size directly or indirectly (Lord, 1960;Cody, 1966;Gibb, 1968;Smith and McGinnis, 1968;Spencer and Steinhoff, 1968).…”

INTRASPECIFIC LITTER SIZE VARIATION IN MICROTUS CALIFORNICUS II. VARIATION BETWEEN POPULATIONS

“…Patterns of interpopulation variation in litter size can be used to study the effects of spatial variation on the phenotype. In general, litter sizes of small mammals increase with latitude (Lord, 1960), and in an apparently analogous fashion, with altitude (Dunmire, 1960;Fleming and Rauscher, 1978;Bronson, 1979).…”