lively with tendencies toward ptiiyaiidry ( Kendeigh 

 and Baldwin 1937). Mating behavior is therefore to 

 some extent adaptable in order to compensate for lop- 

 sided sex ratios and to maintain high reproductive 

 capacity. 



BREEDING AGE 



The age at which young animals first at- 

 tain the ability for reproduction affects the reproduc- 

 tive capacity and rate of growth of populations. 

 Planktonic entomostracans are sexually mature in a 

 few days ; insects, often in a few weeks. Among birds, 

 a tropical sparrow is known to reach full reproductive 

 level in six to eight months (Miller 1959). Small 

 non-tropical song birds commonly nest during spring 

 and summer of the year following that in which they 

 hatched, but banding of nestling house wrens indi- 

 cated that 12 to 18 per cent failed to do so until the 

 second or third year (Kendeigh and Baldwin 1937). 

 Upland game birds probably nest as yearlings ; geese 

 and wild turkeys do not nest until they are two years 

 old ; common terns, commonly only after three 

 years. Some lizards and snakes require two to three 

 years to reach sexual maturity : turtles much longer. 

 Females of the F.uropean voles may mate at 13 

 days, even before they are weaned, and give birth to 

 their first litter when only 33 days old (Frank 1957). 

 Woodland white-footed mice born in spring may pro- 

 duce young late in summer ; but most small and me- 

 dium-sized mammals do not breed until one year old. 

 Beaver, wolf, lion, and whale breed when two years 

 old. Big game mammals, such as deer, bison, and 

 bear, reach maturity only after three years. The 

 elephant is said to require 8-16 years, and the rhi- 

 noceros 20 years (Spector 1956: 115, 119). 



NON-BREEDING POPULATIONS 



Although the breeding population of a par- 

 ticular mammal, bird, or other animal is the only 

 fraction of the total population of a species concerned 

 with its reproductivity, there is often present in an 

 area a substantial, though inconspicuous, non-breed- 

 ing population that must be considered in any under- 

 standing of community dynamics (Zimmerman 

 1932). In a 16-hectare (40 acre) tract of spruce-fir 

 forest 'in northern Maine, there were, in 1950, 308 in- 

 dividuals (154 pairs) of nesting birds present during 

 the first half of June. By the use of fire-arms the 

 population was reduced to 21 per cent by June 21, 

 and held at this level until July 11. This involved a 

 removal of not only 228 breeding birds plus 49 of 

 uncertain status, but also of 250 new birds appearing 

 to take over the territories and places of the nesting 



10 12 

 AGE 



13 I 15 I 17 

 14 16 I 



19 1 21 

 20 



FIG. 15-4 Age composition of a breeding population ot com- 

 mon terns (Austin and Austin Jr. 1956). 



birds that were removed (Hensley and Cope 1951). 

 This surprisingly high non-breeding reserve may not 

 be typical of all species of birds (Bendell 1955). 

 Other studies have shown that the non-breeding pop- 

 ulation, especially of birds, consists principally of 

 young animals that have been slow to reach sexual 

 maturity, of surplus individuals of either sex in 

 monogamous species, and of adults which, for one 

 reason or another, have lacked reproductive vigor or 

 have been unsuccessful in establishing proper breed- 

 ing relations. 



LONGEVITY AND MORTALITY RATE 



When protected in captivity, animals are 

 capable of living surprisingly long periods (Spector 

 1956: 182). Definite physiological limits of life are 

 characteristic of each species and are occasionally 

 realized under natural conditions (Cooke 1942), but 

 invariably the potential longevity of a species is many 

 times greater than the mean longevity actually at- 

 tained by wild populations (Bourliere 1946). 



Finding the mean length of life for wild popula- 

 tions requires the working out of life tables, accom- 

 plished for only a few species. In birds older than the 

 juvenile stage, it commonly varies from one to five 

 years (Farner 1955), although in some large species 

 it is considerably longer. In rodents, usually not more 

 than 6 per cent of the population reaches one year of 

 age (Blair 1953). The larger Dall mountain sheep 

 has a mean length of life of 7.09 years. Adult barna- 

 cles have a mean life of 12.1 months (Deevey 1947), 

 and difTerent species of rotifers variously from 3 to 

 35 days (Edmondson 1946). Longevity may often 

 differ between the sexes. Thus in the male flour 



Reproductivity and structure 215 



