with season, sex, and neighborhood. Ecologically,aggres-
sion increases regionally and locally with optimal habitats
and with the correlated diversity and productivity of the
environment (fig. 25.5c). Demographically,aggression in-
creases with population density. Life-historically,aggres-
sion is higher in males than in females and in the breeding
than in the nonbreeding season. Physiologically,aggression
decreases in climatically xeric and harsher environments
(fig. 25.5c). Finally, ethologically,intermale aggression is
highest between neighbors within populations and between
contiguous rather than noncontiguous species (fig. 25.5c).
Can highly territorial, solitary, and aggressive Spalax
evolve sociality?
Our extensive work on the Spalax ehrenbergisuperspecies
revealed four species (Nevo et al. 2001) in which aggression
plays a significant role in both speciation and adaptation
(Nevo et al. 1986). Spalaxare strongly territorial, solitary,
and innately polymorphic for aggression (fig. 25.5a –b).
However, the levels of aggression decrease gradually to-
ward the Negev Desert in Israel, presumably to minimize
overheating, water, and energy expenditure and to prevent
thermal death (fig. 25.5c). It was therefore a mixed surprise
and expectation when we encountered total pacifistic be-
havior in Egyptian isolates of theS. ehrenbergisuperspe-
cies. These Egyptian isolates separated presumably 10,000 –
25,000 years ago (Lay and Nadler 1972) from the Israeli
complex and were described by us as a new species (Nevo,
Heth, and Pratt 1991, Nevo, Heth, and Simson 1991; Nevo
et al., 1992). This extraordinary discovery supported our
ecological evolutionary theory of aggression in Spalax,
which predicts that aggression should decrease in the des-
ert (Nevo et al. 1986), also catalyzing social evolution
(Nevo et al. 1992). Here, I describe field and lab experi-
ments with the new pacifistic species of the Spalax ehren-
bergisuperspecies.
Predominating pacifism is a prerequisite
for social evolution in Spalax
We were able to test and verify the prediction of increas-
ing pacifism toward the desert when we obtained 14 mole-
rats from Burj-El-Arab in northern Egypt, in May 1989
(Nevo et al. 1992). The mole-rats all arrived in one wooden
crate (120
70
40 cm) instead of each animal being caged
separately, the usual routine with Israeli and Eurasian
Spalaxspecies. Furthermore, the mole-rats displayed atypi-
cal, non-Spalax-like social interactions, with no aggression,
rather amicable and pacifistic behavior (fig. 25.1c). By con-
trast, even pacifist-behavioral phenotypes of S. ehrenbergi
from Israel must be kept in separate cages to prevent them
from killing each other.
I suggest that this pacifistic behavior evolved by natural
selection in the Egyptian isolates, as an adaptation for sur-
vival in the environmentally harsh, northern, marginal Sa-
hara Desert. Overheating, loss of water, and excessive en-
ergy investment may be fatal in the desert. We tested this
unique behavior of the Egyptian mole-rats and compared it
to a control experiment with Israeli S. ehrenbergi, 2n60,
now named S. judaei,the least aggressive species (Nevo
et al. 1992).Geographical location and habitat
The two Spalaxisolates (Burj-El-Arab and El-Hammam,
25 km apart, near El Alamein) are located in agricultural
valleys in the xeric Mediterranean zone of northern Egypt
fringing the Sahara Desert (fig. 1 in Nevo et al. 1992). Phy-
togeographically, the region is xeric-Mediterranean, sur-
rounded by Saharo-Arabian vegetation.Experimental animals
We carried out two field expeditions to the northern Egyp-
tian localities (fig. 1 in Nevo et al. 1992) and conducted
three sets of experiments and observations in 1989 –1991.
All observations, experimental designs, and detailed results
appear in Nevo et al. (1992).
In general, behavioral phenotypes in Israel displayed dis-
tinct clinal, geographical variation in both sexes: pacifists
increased southward in males —16%, 24%, 28%, 34%, as
well as in females —39%, 45%, 55%, 50%, whereas mili-
tants decreased southward in males —33%, 31%, 23%,
18%, and in females —32%, 16%, 16%, 11%, for 2n
52, 54, 58, and 60, respectively. The Egyptian population
showed 100% pacifism and 0% militarism.Experimental results
All experiments in which Egyptian animals were put to-
gether, either in small plastic containers in the field for
hours, or in larger aquaria in the laboratory (for up to
12 days) resulted in pacifistic behavior and amicable inter-
actions between animals (fig. 25.1c). The Egyptian mole-
rats were characterized by huddling, tactile contact, touch-
ing, and warming each other in cohesive aggregates without
aggression. Some pushing and screaming before settling
down occurred, but the occurrence of contact cohabitation
was in strong contrast to usual behavior of Spalax. More-
over, animals were easily handled by us and caressed freely
like pets, an inconceivable act with other knownSpalaxspe-
cies. We observed aggressive behavior on just two occasions.300 Chapter Twenty-Five