PARENTAL BEHAVIOR 



1305 



tioii and other events in the reproductive 

 cycle is typically somewhat different in the 

 two groups. The obvious functional rela- 

 tionships between nest-building and incuba- 

 tion behavior led us to include a treatment 

 of nest-building behavior, even though a 

 strict (and artificial) attempt at definition 

 might result in the conclusion that nest- 

 building was a part of sexual, rather than 

 parental behavior. In birds, ovulation oc- 

 curs between the building of the nest and 

 the incubation of the eggs, and since the 

 physiologic events associated with ovula- 

 tion are very much involved both in nest- 

 building behavior and in the onset of 

 incubation behavior, various aspects of ovu- 

 lation in birds necessarily attracted our at- 

 tention. In mammals, on the other hand, 

 ovulation and fertilization occur long be- 

 fore the emergence of the fetus from the 

 mother. Since nest-building tends to occur 

 late in pregnancy (see below), the physiol- 

 ogy of ovulation does not play the role in 

 establishing parental behavior in mammals 

 that it typically does in birds, and we will 

 therefore need no discussion of ovulation 

 in this part of the chapter. 



A. NEST-BUILDING 



1. Xest-building Patterns in Mammals 



Many mammals build shelters or "nests" 

 of various types, the building and occupa- 

 tion of which is not necessarily closely as- 

 sociated with any particular time in the 

 breeding cycle, as it is in birds. Although 

 laboratory data and some scattered field 

 observations indicate that, in some cases at 

 least, changes in the pattern of building be- 

 havior do occur in association with repro- 

 duction, most of the data from field ob- 

 servations are not sufficiently detailed to 

 permit any differentiation between the 

 building of shelters and reproductive nesting 

 behavior. 



Some types of mammals live the year 

 round without constructing any type of 

 nest or shelter. This mode is characteristic 

 of aquatic mammals in general (Fisher, 

 1940; Bourliere, 1954) and most ungulates 

 (e.g., Murie, 1951). Other mammals make 

 temporary shelters, which they may change 

 from day to day, or at somewhat longer in- 



tervals. The European hare digs small 

 trenches in the ground or in grass, in which 

 it spends most of its time during the day 

 (Eibl-Eibesfeldt, 1958). Many tropical bats 

 take shelter during the day in abandoned 

 bird nests or mammal burrows, or cut the 

 blades of large leaves with their teeth so 

 that part of the leaf falls around the cling- 

 ing animal (Allen, 1939; Bourliere, 1954). 

 Many of the higher primates build sleeping 

 shelters of light branches, twigs, leaves, etc., 

 in which they take shelter during the night ; 

 in most cases a new one is built each day 

 at a different location. The orang-utang 

 and chimpanzee build such nests in trees 

 (Reichenow, 1921; Aschemeier, 1922; Nis- 

 sen, 1931), whereas the gorilla may build 

 either in trees or on the ground (Yerkes 

 and Child, 1927) . 



The most common type of shelter among 

 mammals is the burrow (Wunder, 1937; 

 Bourliere, 1954; Krumbiegel, 1955) which 

 is used by mammals of many different or- 

 ders. These burrows range from the simple 

 holes used by many carnivores (Hamilton, 

 1939) to the elaborate underground net- 

 works of tunnels and galleries dug by many 

 rodents (Eisentraut, 1928; Grasse and De- 

 keyser, 1955; Eibl-Eibesfeldt, 1958). 



Some mammals, mostly rodents, build 

 nests of grass, twigs, and leaves in trees 

 (American red squirrel, Hatt, 1929) , on the 

 ground about the bases of trees (dusky- 

 footed wood-rat, Linsdale and Tevis, 1951), 

 on the surface of the water (European water 

 vole, Wunder, 1937), or partly submerged in 

 the water (American beaver, Warren, 1927) . 



Thorough descriptions of the building, 

 burrowing, and nesting behavior of many 

 species of mammals may be found in the 

 monographs and textbooks by Wunder 

 (1937), Hamilton (1939) , Bourliere (1954), 

 Grasse (1955), and Krumbiegel (1955). 



2. Hormonal Basis of Nest-building 



Timing of nest-building during the repro- 

 ductive cycle. We have already noted that 

 the building of shelters, burrows, and 

 "nests" usually seems not to be particularly 

 related to pregnancy or to the care of the 

 young. There are some indications, however, 

 even in the observations on the behavior of 

 free-living wild mammals, that behavior 



