I'arental ran' 



The miiiiljcr of eggs or young produced per 

 litter is correlated inversely with the amount of at- 

 tention that they require. \\'hen parental care is al- 

 together lacking, invertebrates may lay 1 ,(XX) to 500,- 

 000,000 eggs at one maturation : where there is some 

 l)rotection afforded by brood pouches, 100 to l.(K)0 

 eggs may be laid ; with a high degree of brood pro- 

 tection, 10, or less, to 100 eggs may be laid. Mam- 

 mals seldom have more than a dozen young in a single 

 litter and, in larger species, usually only one. Char- 

 acteristic clutch size among birds varies from 1 to 

 1 .=; ; rarely. 20. 



There is a limit on the size of the brood or litter 

 that adult warm-blooded animals can successfully 

 feed and raise to maturity. There is no advantage, 

 for instance, for starlings to have broods larger than 

 tive (Table 15-1). In larger broods, each individual 

 receives less food, and hence has less vigor and weight 

 on leaving the nest. Mortality increases either before 

 fledging or in immediately subsequent months. The 

 larger broods raised during years of abundance fur- 

 ther indicate that food is a critical factor (Lack ct al. 

 1957). The variability in clutch and litter size for 

 most species allows them to take advantage of tempo- 

 rarily improved conditions. There is some evidence 

 that in the tropics the number of young raised may 

 be limited not by food but by increased predation on 

 the larger sized broods (Skutch 1949). 



laid, or whether laying is undertaken at all (Ken- 

 deigh 1941). 



Among invertebrates, clutch size also varies under 

 different conditions and in different localities. The 

 copepod Eitdiaptomus gracilis commonly carries 11 

 eggs in April, 3 in early August, 9 in early Novem- 

 ber, and 5 or 6 over the winter. There is a decrease 

 between spring and summer in the number of eggs 

 carried in its brood pouch by the cladoceran Daphnia. 

 Diaptomus siciloides carries but 4 eggs in mountain 

 lakes of California, as many as 18 in the Illinois 

 River. These variations appear to be correlated with 

 differences in temperature and food supply (Hutchin- 

 son 1951). 



Weather 



Death rate 



Clutches laid by birds during periods of hot 

 weather are usually smaller than those laid when 

 temperature is moderate (Kendeigh 1941). Clutches 

 laid by related species in temperate latitudes tend to 

 be larger than those laid in the tropics (Moreau 

 1944, Lack 1947-48). The fecundity of white-tailed 

 deer is higher with good forage than with poor forage 

 (Cheatum and Severinghaus 1950). Reproduction is 

 generally more successful after periods of high mor- 

 tality than during years of abundance. 



The mobilization of energy, usually within a defi- 

 nite period of time, is a limiting factor in warm- 

 blooded organisms. The house wren, for example, 

 lays 5, 6, or 7 eggs per clutch, the total weight of 

 which is 7.0, 8.4, or 9.8 g respectively ; yet the adult 

 female herself weighs only 11.5 g. It is estimated 

 that under average conditions about one-third of the 

 daily energy intake of the bird, above its needs for 

 existence, is deposited in the eggs being produced. 

 Any appreciable change in temperature or rate of 

 feeding thus affects the size of the egg, the number 



Death rates vary between species and are cor- 

 related with rates of reproduction (Table 15-2). The 

 death rate of a species is influenced by a number of 

 factors, but of fundamental importance is the num- 

 ber of young that are born in relation to the carrying 

 capacity of the habitat. When more young are born 

 than the habitat can support, the surplus must either 

 die or leave the area. When populations are stabil- 

 ized at a constant level, the death rate must fluctuate 

 with the birth rate. Evolutionary adaptation tends to 

 lower the frequency at which the population replaces 

 itself and to raise reproduction to the highest rate 

 compatible with the energy resources both of the spe- 

 cies (Table 15-1) and of the habitat (Lack 1954). 



SURVIVAL OF YOUNG 



Success in raising young depends not only 

 on the ability of the adults to care for the young, but 

 also on the vitality of the embryo and on the chance 



Reproductivity and structure 21 1 



