Understanding Zebra Sociality, Survival & Movement
Daniel I. Rubenstein, Department of Ecology and Evolutionary Biology, Princeton University
Why are Grevy’s zebras doing so poorly?
Why do plains zebras live in herds?
Equids exhibit two types of sociality. The plains zebra and horses live in closed
membership groups of one male and many females and their young. Grevy’s
zebras and wild asses live in open membership groups in which associations
among males and females change frequently. Thus it is not surprising that in such
‘fission-fusion’ societies herds containing hundreds of individuals form. In horses,
core ‘harems’ remain apart and
when aggregations form they
dissolve quickly. In plains zebras
herds of core groups are common,
last for hours and even days and
can contain 50-100 individuals.
Why do horses and zebras, both
members of the same genus and whose societies consist of ‘harem’ groups, show
such different herding tendencies?
Over the lasts 25 years both numbers and the range of Grevy’s zebras have declined
markedly despite the cessation of poaching. Even on conservancy land, such as Lewa
Downs, numbers have been dropping. Yet plains zebra numbers have been on the rise.
Why the difference?
< .05 PC 1:‘Quantity’
< .10 PC 2:‘Quality’
< .60 PC 3:‘Diversity
0.16
0.01
0.07
< 0.05
< 0.90
< 0.50
Predator Index
-0.03
< .15 Predator Index
0.00
< 0.90
600
10,000
500
8,000
400
6,000
300
4,000
200
2,000
100
1995
1996
1997
< .0001 Social Factor:
Numb
F
4,227
0.5
0.5
0.4
0.4
0.3
0.2
0.1
0
(bites/hr)
1000
1000
750
750
500
250
500
0
1
2-4
0
With Bachelors
Without Bachelors
F3,89 = 3.1; p < .05 (males)
F3,142 = 2.3; NS (females)
2-4
Low
Herd Size
Herd Size
2003
2004
Lewa GZ
F 1,39 = .05; ns
0.3
0.2
0.1
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
% Grevy's zebras
Cuticle
larger
0.8
1
smaller
Preliminary data (n = 50 lions on 31 kills):
• 93% scat samples contain zebra hair
• 68% scat with zebra hair contain Grevy’s hair
• 56% zebra kills are Grevy’s ==> (E[GZ/PZ] = 1/4: c2 = 8.96; p < .01)
2500
3) Parasitism: Grevy’s less
infected with macroparasites
then plains zebras
2000
F 1,29 = 8.6; p < .007
3000
Sex: F 2.29 = .40; NS
Reproductive Class:
F 2.29 = 1.09; NS
2500
1500
2000
1000
1500
Plains Zebra
1000
500
500
0
0
Grevy's
Plains
BreedingLactating Non- BreedingLactating NonMale
Female Lactating Male
Female Lactating
Female
Female
Age/Reproductive Class
Zebra Species
Since plains zebras outnumber Grevy’s zebras by 4 to 1 on the conservancy, plains
zebras are likely to outnumber Grevy’s zebras wherever they are found and hence
depress Grevy’s zebra foraging on a continual basis. Moreover, high predation rates
Business as usual…
Population Projection Model
as previously, bachelor males (light green Xs) patrol the center of their home range
repeatedly making daily loop like forays out in search of females. Within the first 2 days
the collared bachelor male and his partners had found the three stallion groups with
randomly collared females. This underscores the ability of bachelors to put tremendous
cuckolding pressure on breeding males. Third, zebras move most quickly at dawn and
dusk and on average faster at night then during the day.
LEWA GREVY'S ZEBRA POPULATION
BIRTHS
FEMALES
TOTAL
500.00
Join herd to offset affect
(Insurance?)
harem
With
bachelors
1
400.00
300.00
200.00
Avg. = 326 +/- 10
100.00
2+
Number of Harems
0.00
0.00
10.00
20.00
30.00
The importance of females in
shaping herds emerges from
microsatellite genetic analyses.
F3,678 = 3.89; p < .01; r2 = .18
Figure . Mean relatedness scores (R) and standard errors for pairs of individuals in
various demographic categories from the Sweetwaters plains zebra population (n=37)
0.5
Relatedness within
population
0.4
Relatedness Score (R)
In equids, it has been thought that both
sexes disperse before breeding, but the
more philopatric sex could be responsible
for forming associations based on kinship.
Using DNA extracted from dung of many
members, both male and female, of many
solitary harems and of harems in the same
herd, it appears that not only do many
sibling females end up as members of the
same harem, they end up as associates in
the same herd. Females appear to be the
herding agents of plains zebras.
Relatedness
within core
groups
Relatedness within
herds
0.3
0.2
0.1
0
-0.1
All
M-M
(666)
(171)
F-M
F-F
(342)
(153)
All
All
All
(627)
(655)
(39)
Across
Herds
Across
core
M-M
(7)
F-M
(22)
F-F
(10)
All
F-M
(11)
(7)
F-F
(2)
-0.2
-0.3
Across 37 SW individuals
Within Herds
-- 27% foal survival rate
-- 88%
adult survival rate
(Lewa’s foal patrol and program MARK)
…Grevy’s
numbers
decline
gradually
on Grevy’s zebras are likely to reduce numbers by 50%
over the
next 30 years
unless
lion numbers are reduced. Only then will population sizes increase fast enough to
allow translocations of ‘excess’ Grevy’s zebras to parts of its historic range.
Within Core groups
0
Yes
No
Livestock Association
Day
0.8
Open Grass
0.6
Electivity Index
On pastoral lands outside conservancies, competition
with livestock is thought to be the major reason that
Grevy’s zebra numbers are not increasing in what was
formerly the center of its range. By employing pastoral
men and women as scouts we are enhancing local
economic development. Two women and one man from
each community are taught basic monitoring techniques
so that they can gather data while carrying out daily
activities. During workshops in which scouts discuss
what they do and we share their data with them we
begin discussions on the consequences of Grevy’s
zebras avoiding livestock and forming large herds
in late afternoon.
All Samburu Communities
1
Wildlife is gaining
Chi Square = 12.1 p < .05
2004
value and after one
0.8
+
=
+
year human
0.6
behavior is
changing: Grevy’s
0.4
=
+
zebras are now seen
with livestock 50% of
TM BM NLF LF
0.2 TM BM NLF LF
Asubuhi
the time.
Zebras also tend to make more long
distance movements during the night
as opposed to during the day. Moreover, zebras show strong habitat
preferences that are shaped by sex
and time of day. All zebras use open
grass plains and bushland in proportion to their abundance. All tend to
avoid dense forest and bareground
0.4
Dirt
0.2
Grass-Trees
0
Bushland
-0.2
Dense Forest
-0.4
Water
-0.6
-0.8
-1
Bachelors
Harems
Types of Zebra Groups
Electivity =
(r-p)/(r+p)
+ preference
- avoidance
0.6
Open Grass
0.4
Electivity Index
< .05
(F(N) = density dependence;  = age 1st reproduction; S = survival)
Percent Association
4.21
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
3500
Grevy's Zebra
Medulla Cortex
Multiple harems
with or without bachelors
(intake ≈ solitary harem)
reductions in foraging. On ranches where hunting of bachelor males lowers the
bachelor/stallion ratio herds are significantly smaller than on ranches where hunting
removes stallions, increasing this ratio.
The composition of groups is
Ranch
Bachelors/Stallions
Herd Size
affected by predation risk. As
habitat visibility declines or
Segera
1.54 ± .06
17.1 ± 1.3
number of lions sighted per ranch
increases, the fraction of young
El Karama
.64 ± .02
12.9 ± .9
F1,256 = 6.7; p < .01; r2 = .28
foals comprising herds increases.
Predation risk affects group composition ==> % Young foals Since females initiate most group
movements—hence most group
GLM
fusions—females are not passive
Variable
Slope
F value p value
players when it comes to
No. Stallions -.004
1.09
< .30
influencing group composition.
Visibility index -.003
4.40
< .04
.005
- Mobile sensor net
- GPS fix & swap
- Data percolates
- Solar powered
After two deployments and over 5000 GPS fixes some fascinating results are emerging.
First, the network is working. For animals ranging in remote areas, data on their
movements arrives at the base station indirectly via contacts. Second, stallion groups
and bachelor groups move across the landscape differently illustrating large sex
differences. While stallions and their females move slowly and consistently over about 1
km per day and then ‘hop’ to a new area and there move just as slowly and consistently
TIME (YEARS)
No. Lions
Base station
(car or plane)
= 3.0; p < .001; r2 = .14
Grazing
Intake
Rate
1
2002
Data
2) Predation: Lions prefer Grevy’s zebras over plains zebras
.001
Solitary harem
(high grazing rate)
250
2001
NUMBERS
1250
Ingestion Rate
1250
Data
Data
0
0
High
1500
2000
1) Competition: Plains zebras out-compete Grevy’s zebras
Behavioral observations reveal that pairs of stallions repel bachelor intruders 92% of the
time while solitary stallions do so less then 10% of the time. Foraging success reveals
that females do not suffer significant differences in intake rate when in moderate size
herds or alone on the landscape. As a result, their indifference facilitates males forming
alliances to increase their defensive success without incurring reductions
Stallions
1500
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Observations on the Lewa conservancy demonstrate that at the southern edge of the
Grevy’s zebras range, two out of three of ‘Darwin’s hostile forces’ impact Grevy’s
zebras more then plains zebras.
variation in plains zebra herd sizes. And with respect to herd composition as measured
by the percentage of bachelor males in the herd, the number of females present made
the largest difference. In both cases, social forces dominated ecological ones.
Females
Store-and-forward
communications
Tracking node
with CPU,
FLASH, radio
and GPS
Speed (m/min)
F 4,227 = 77.9; p < .0001; r2 = .67
0.24
1999
Parasite Load (eggs/gram)
2.44
1998
Kenya's Rangelands
% Plains zebras
Social Factor:
No. Bachelors/
Stallions
Data
0
Plains zebra bite rate
0.18
- 0.09
- 0.66
12,000
1978
Slope P value
0.00
< 0.40
PC 1:‘Quantity’
PC 2:‘Quality’
PC 3:‘Diversity
700
0
Herd Composition (% B Males)
Slope P value Variable
0.09
< .01 Zebra Density
14,000
ZebraNet—a mobile sensor GPS tracking network—is making the invisible, visible. By
enabling us to follow zebras at all times of the day, across all seasons, through all
diverse habitats and in real time, we will be able to answer basic questions as to
whether zebras migrate back and forth over long distances or simply move about
nomadically or whether zebras pull predators around or predators push zebras about.
Numbers (Lewa)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Grevy's zebra bite rate
Variable
Zebra Density
Numbers (Kenya)
Grevy's zebra status in Kenya's Rangelands and on Lewa Wildlife
Conservancy: 1978 & 1995-2004
Three selective forces could be involved: the ‘top-down’ force of predation; ‘bottom-up’
forces associated with vegetation abundance, quality or diversity; or socio-sexual
forces associated with intruders and cuckoldry pressure. Of these, intruder pressure
as measured by the ratio of bachelor males to stallions accounted for most of the
Herd Size
How do zebras move about landscapes?
Dirt
Jioni
Usiku
and all zebras prefer grasslands with
a few acacia trees and water holes,
although stallions with females prefer
water holes less strongly than all
male groups. For stallion groups the
attraction to water holes is weaker at
night then during the day. Most
interesting is the fact that harem
groups tend to switch from avoiding
bushland during the day to preferring
it at night. Perhaps the asymmetry in
detection might be sufficiently
reduced to permit use of these
important habitats during the night.
0.2
Grass-Trees
0
Bushland
-0.2
Dense Forest
-0.4
Water
-0.6
Adhuhuri
Night
-0.8
Day
Night
Time of Day
Challenges remain in order for
ZebraNet to function for long periods.
But when these are overcome,
additional insights will emerge into
the lives of zebras and the ways
wildlife and livestock interact on a
common landscape.