AGE 



FIG. 15-2 Schemafie representation of different types of sur- 

 vivorship curves. The vertical scale may be graduated, arith- 

 nnetically or logarithmically. If graduated logarithmically, the 

 slope of the line will show the rate of change; a straight line is 

 indicative of a mortality rate equal at all ages (Deevey 1947). 



1 23456789 10 

 AGE 



FIG. 15-3 Survivorship curves for (A) the American robin, lero 

 age at November I (Farner 1945a), and for female mule deer 

 (males only are hunted) in (B) shrubland and (C) chaparral in 

 California (Taber and Dasmann 1957). 



those few individuals that survive have a high life 

 expectation thereafter. Types I and especially III are 

 not often observed under natural conditions. Nearly 

 all survival curves so far obtained are of type II, 

 although they seldom approach a straight line. Prob- 

 ably more curves approaching type III would be 

 found if data could be secured beginning with the 

 fertilized egg, as mortality in early life is often high, 

 especially in aquatic species that spawn many eggs. 

 Evolution of parental care in higher animals gives 

 greater protection and efficiency in raising the young. 

 This evolutionary trend should change survivorship 

 curves from type III towards type I. 



SEX RATIO AND MATING BEHAVIOR 



The primary sex ratio at the time the eggs 

 are fertilized should be approximately 50 S S : 50 

 9 ? in most species, although it has seldom been 

 measured. This ratio may be displaced in one direc- 

 tion or the other by differential mortality of the two 

 sexes during the period of growth, become manifest 

 in the secondary sex ratio at the time of hatching or 

 birth, and even more pronounced in the tertiary sex 

 ratio of the adults (Mayr 1939). Protozoa, some 

 coelenterates and flatworms are potentially immortal. 

 Recognition of sex and age in living animals is often 

 difficult, although criteria have been worked out for 

 many species (Taber, in Mosby 1960). 



The sex ratio of the adults is especially impor- 

 tant in understanding mating relations and reproduc- 

 tion potentials. For instance, the adult sex ratio in 

 ducks is often in the neighborhood of 60 ,J 3 : 40 

 9 9 (Johnsgard and Buss 1956). Since these birds 

 are largely monogamous under natural conditions, a 

 population of 100,000 birds does not furnish 50,000 

 breeding pairs but only 40,000. In polygynous spe- 

 cies, such as pheasants, some grouse, turkeys, deer, 

 and fur seals, breeding potential is probably not di- 

 minished under natural conditions if there are two, 

 three, or even ten times as many females as males. 

 On the other hand, polyandry has become charac- 

 teristic of some tinamous and bustards, and this is 

 correlated with a preponderance of males (Kendeigh 

 1952). 



The tertiary sex ratio is not a constant factor. In 

 the California quail it was found to vary monthly 

 from 51 $ S : A9 2 2 in early autumn to 53 $ $ : 

 47 2 2 during winter to 56 5 5 : 44 2 2 in June 

 (Emlen 1940). Game birds that are monogamous or 

 only slightly polygynous in the wild may become 

 highly polygynous after hunting seasons or in cap- 

 tivity, situations in which there is a preponderance of 

 females over males. Yearly variations in the ratio of 

 males to females in the house wren are correlated in- 

 versely with tendencies toward polygyny, and posi- 



214 Ecological processes and dynamics 



