1. Population Projection Model Business as usual… -- 27% foal survival rate -- 88% adult survival rate …Grevy’s numbers decline gradually (Lewa’s foal patrol and program MARK) Avg. = 326 +/- 10 (F(N) = density dependence;  = age 1st reproduction; S = survival) 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 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. 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 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. 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? 4010.2.0 ‘Quantity’ PC 2:‘Quality’ PC 3:‘Diversity 0.16 01 07 < 05 < 90 < 50 Predator Index-0.03<.15Predator Index0.00< 90 Social Factor: No.Bachelors/ Stallions 2.44<.0001Social Factor: Numb VariableSlopeP valueVariableSlopeP value Zebra Density 0.09<.01Zebra Density0.00< 40 PC 1:‘Quantity’ PC 2:‘Quality’ PC 3:‘Diversity 0.18 -0.09 -0.66 <.05 <.10 <.60 PC 1: Herd Size Herd Composition (% B Males) F 4,227 = 77.9; p <.0001; r 2 =.67 F 4,227 = 3.0; p <.001; r 2 =.14 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 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. Understanding Zebra Sociality, Survival & Movement Daniel I. Rubenstein, Department of Ecology and Evolutionary Biology, Princeton University 0 250 500 750 1000 1250 1500 Ingestion Rate (bites/hr) 12-4 Herd Size Females 0 250 500 750 1000 1250 1500 12-4 Herd Size Stallions With Bachelors Without Bachelors F 3,89 = 3.1; p <.05 (males) F 3,142 = 2.3; NS (females) High Grazing Intake Rate Low Number of Harems 12+ Solitary harem (high grazing rate) harem With bachelors Multiple harems with or without bachelors (intake ≈ solitary harem) Join herd to offset affect (Insurance?) RanchBachelors/Stallions Herd Size Segera 1.54 ±.0617.1 ± 1.3 El Karama.64 ±.0212.9 ±.9 F 1,256 = 6.7; p <.01; r 2 =.28 Predation risk affects group composition ==> % Young foals F 3,678 = 3.89; p <.01; r 2 =.18 GLM Variable Slope F value p value No. Stallions -.004 1.09 <.30 Visibility index -.003 4.40 <.04 No. Lions.005 4.21 <.05 The composition of groups is affected by predation risk. As habitat visibility declines or number of lions sighted per ranch increases, the fraction of young foals comprising herds increases. Since females initiate most group movements—hence most group fusions—females are not passive players when it comes to influencing group composition. The importance of females in shaping herds emerges from microsatellite genetic analyses. 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 population Relatedness within herds Relatedness within core groups Why are Grevy’s zebras doing so poorly? 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? 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. 1) Competition: Plains zebras out-compete Grevy’s zebras 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) larger smaller Medulla Cortex Cuticle 2) Predation: Lions prefer Grevy’s zebras over plains zebras 3) Parasitism: Grevy’s less infected with macroparasites then plains zebras 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 Asubuhi Adhuhuri Jioni Usiku in late afternoon. Wildlife is gaining value and after one year human behavior is changing: Grevy’s zebras are now seen with livestock 50% of the time. Speed (m/min) DayNight How do zebras move about landscapes? 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 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. 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. Data Base station (car or plane ) Data Store-and-forward communications Data Tracking node with CPU, FLASH, radio and GPS - Mobile sensor net - GPS fix & swap - Data percolates - Solar powered Electivity = (r-p)/(r+p) + preference - avoidance Zebras also tend to make more long distance movements during the night as opposed to during the day. More- over, zebras show strong habitat preferences that are shaped by sex and time of day. All zebras use open grass plains and bushland in pro- portion to their abundance. All tend to avoid dense forest and bareground 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. 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. Yes No 0 0.2 0.4 0.6 0.8 1 Percent Association Livestock Association TM BM NLF LF - = - + Chi Square = 12.1 p <.05 + = + - All Samburu Communities 2004