cessfiilly displace another species already occupying 

 it, as fret|uently occurs (Simpson 1953a). The search 

 for such niicroadaptations as might give one species 

 an advantage over another in a competition for a par- 

 ticular niche is a real challenge to ecological research. 



INHERITANCE 



The fact that all individuals of a species 

 behave almost identically in many of their activities 

 indicates that these behavior patterns or instincts are 

 in some form passed on or inherited from one gener- 

 ation to the next. Behavior that is learned after birth 

 is much more variable between individuals. Behavior 

 patterns are rooted in the structural arrangements of 

 neurons and synapses. Once a stimulus is received a 

 definite action results. Predisposition for a species" 

 behavior patterns could well be inherited genetically 

 through chromosomes and genes like any other struc- 

 tural or functional characteristic. These inherited be- 

 havior patterns are doubtless subject to evolutionary 

 development as much as are structural and func- 

 tional adaptations : indeed, the one may have de- 

 veloped synchronously with the others (Kendeigh 

 1952, Spieth 1952). 



The often large and elaborate nests built by ter- 

 mites are really manifestations of behavior patterns 

 inherent in the species. The nests of higher termites 

 are built specifically by the sterile workers ; plainly, 

 whatever js involved in the capacity the workers have 

 to build a certain type of nest must be transmitted 

 genetically, through the sexual forms only. Adaptive 

 modifications in nest structure occur and phylogenetic 

 sequences in nest structure that correlate with phylo- 

 genetic sequences in morphological characters can be 

 demonstrated (Emerson 1938). 



The two toads Bitjo aniericanits and B. ivood- 

 hoiisei differ in rate of embryonic development and 

 embryonic temperature tolerance but interbreed 

 freely. Embryos of hybrids show an intermediate rate 

 of development (Volpe 1952). The call of a natu- 

 rally occurring hybrid toad (Biifo americanus x B. 

 woodhoitsci) was found to be intermediate between 

 the calls of the two parent species (Blair 1956). 



Three sibling species of the cricket Nemobius 

 fasciafus differ slightly in structure and color but are 

 scarcely distinguishable except by the songs of the 

 males. Although they do not interbreed under nat- 

 ural conditions, they were induced to do so under 

 experimental conditions. The song of the Fi males 

 was intermediate between those of its parents, indi- 

 cating genetic influence (Fulton 1933). 



The spinning behavior of the flour moth has been 

 shown to be inherited on a Mendelian genetic basis ; 

 it is dependent on light and food factors. At least two 

 genes are involved : in the Fi generation the non- 



spiiniing behavior is almost, but not (|uite, dominant ; 

 in the F^ generation there is segregation of spinning 

 individuals (Caspari 1951). 



The segregation of the prairie deer mouse and 

 woodland white-footed mouse into different niches 

 is very definite (Table 2.1), as is also the segrega- 

 tion of related species occurring in chaparral (Mc- 

 Cabe and Blancliard 1950). The same segregation 

 fo different habitats holds even between prairie 

 {bairdii) and woodland {gracilis) subspecies of the 

 same species {Pcromyscus manicidatus). It is of in- 

 terest that laboratory-reared individuals not previ- 

 ously conditioned to their natural habitats were given 

 a choice under experimental conditions, bairdii se- 

 lected grass habitat ; gracilis, a tree-trunk habitat. 

 This suggests genetic inheritance of habitat prefer- 

 ence. In hybrids between the two subspecies, choice 

 of the grass habitat was dominant over choice of the 

 tree-trunk habitat (Harris 1952). 



There may be transmission of behavior patterns 

 to succeeding generations by tradition rather than 

 by genetic mechanisms ; i.e., training of young, young 

 imitating parents, conditioning or imprinting. It has 

 been demonstrated that some parasitoid wasps lay 

 their eggs only in the same kind of larvae as they 

 themselves became conditioned to during their early 

 growth and feeding. Young birds and other animals 

 become imprinted to their own parents, to their own 

 species, and perhaps to their proper niche, at critical 

 stages in their development (Baldwin 1896, Gushing 

 1941, 1944, Thorpe 1945). 



It is very probable that the manner in which 

 niche segregation is passed on to succeeding genera- 

 tions is not the same in all species. We may believe, 

 however, that most behavior of animals has a genetic 

 basis, but may become highly modified through prac- 

 tice, imitation, and experience. 



INTERSPECIFIC COMPETITION 



Segregating effect 



Charles Darwin stated in The Origin oj S/'e- 

 cics the case for interspecific competition as an instru- 

 ment segregating species into different niches as 

 follows (Crombie 1947) : 



1. The reproductive capacity of organisms is 

 greater than the carrying capacity of the envi- 

 ronment. 



2. The range of an organism's tolerance of physi- 

 cal conditions and choice of food is limited. 



3. The failure of an organism to survive, or be 

 born at all, may be a result of the direct action 

 of unfavorable habitat, predators, parasites, or 

 competitors. 



Niche segregation 251 



