uted in their main ranges corresponding to the isolation by
distance model, but become semi-isolated and isolated in
the marginal habitat at the southern border of S. judaei
(fig. 25.4; and fig. 47 in Nevo et al. 2001).
These results contrast with the commonly held view that
the population structure of subterranean mammals always
involves numerous geographically partially isolated demes
across their ranges. Moreover, the results suggest that clinal
differentiation characterizes the main distributional ranges
and speciation may primarily originate in the peripheral,
small isolates where the initial fixation of spontaneous
chromosomal mutations may take place by natural selec-
tion (Nevo et al. 1992; see speciation theory, chapter 23 in
Nevo 1999).
Aggression Patterns in Speciation and Adaptation
of Blind Subterranean Mole-Rats in Israel
Speciation requires reproductive isolation and ecological
compatibility (Mayr 1963). Yet, while species-specific be-
havioral signals are well-known reinforcers of reproductive
isolation, the role of aggression as a factor in species for-
mation was poorly known. Aggressive behavior is common
and adaptive withinmany animal species, chiefly in spacing
out individuals, but its evolutionary significance between
species is known primarily as an ecological effect rather
than as a determinant of speciation, i.e., as a premating re-
productive isolating mechanism (Wilson 1975).
The objective of long-term studies on aggression in the
four species of S. ehrenbergisuperspecies was to evaluate
the evolutionary significance of interspecific aggression as a
factor during final stages of speciation (Nevo et al. 1975;
Guttman et al. 1975). Can species recognition be reinforced
by natural selection through high levels of aggression at a
stage when both reproductive isolation and ecological com-
patibility are still incomplete? Likewise, is the parapatric
distribution among the four allospecies due to aggressive
behavior? To solve these problems, we followed earlier sug-
gestions (Nevo 1969) and investigated aggression patterns
in mole-rats belonging to the S. ehrenbergisuperspecies as
a model of active and prolific speciation.
The evolutionary significance of interspecific aggression
as a factor in speciation was tested among three species
of the actively speciating superspecies, Spalax ehren-
bergi. Laboratory experiments testing intra- and interspe-
cific aggression were conducted on 48 adult animals from
10 populations comprising three species: S. galili, S. car-
meli,and S. judaei. Twelve agonistic, motivational-conflict,
and territorial-behavioral variables were recorded during
72 combats involving homo- and heterospecific encounters
between opponents. Analysis of the data matrix was carried
out by the nonmetric multivariate Smallest Space Analysis
(SSA-II; Nevo et al. 1975; Guttman et al. 1975). Results in-
dicated that (1) aggression patterns, involving agonistic con-
flict and territorial variables, are higher in heterospecific
encounters than in homospecific ones; and (2) aggression is
higher among contiguous species (2n58 – 60, and 2n
52 –58) than among noncontiguous ones (2n52 – 60).
Both (1) and (2) suggest that high interspecific aggression
appears to be adaptively selected at final stages of specia-
tion in mole-rats as a premating, isolating mechanism,
which reinforces species identification and establishes para-
patric distributions among the evolving species. This con-
clusion was substantiated in our later studies (fig. 25.5d).Adaptive aggressive behavior within and between species
in S. ehrenbergisuperspecies
Resource competition in subterranean mammals gener-
ally takes the form of intra- and interspecific aggression
(Vaughan and Hansen 1964; Grant 1972; Nevo 1979;
see references in Nevo 1999). The results are usually the
following.- Solitariness accompanied by strong territoriality and
competition within species, mediated by aggressive be-
havior between individuals (or between colonies in
social mole-rats; see Reeve and Sherman in Sherman
et al. 1991) in defense of their exclusive territories. - Competitive exclusion between species, also mediated
by aggression, resulting in parapatric, that is, contigu-
ous distributions primarily between evolving species
(Nevo 1979; see fig. 25.4).
Species-specific aggression
Aggressive behavior augments other behavioral patterns
(activity, exploratory, and habitat selection patterns) in op-
timizing the energetic balance of S. ehrenbergiby deter-
mining territorial size and accessible resources (Nevo 1999;
Nevo et al. 1975, 1986). Aggression also functions as a
premating isolating mechanism (Nevo 1990). All four spe-
cies and populations ofS. ehrenbergiare genetically poly-
morphic for aggression, each involving trimodal behav-
ioral phenotypes: pacifists, intermediates, and militants
(fig. 25.5a –b). Aggressive behavior displayed geographic
gradients where pacifism increased and militarism de-
creased southward, as follows: S. galiliS. golaniS. car-
meliS. judaei(2n 52 54 58 60, species, respec-
tively; see fig. 25.5d). Aggression intensity was correlated
with climatic determinants as well as with productivity and
territory size. Aggression increases with competition for re-
sources, as evidenced in highly dense northern populationsEvolution of Pacifism and Sociality in Blind Mole-Rats 297