Wild and Free-Roaming Horses and Burros Current Knowledge and Recommended Research. (1980) / Chapter Skim
Currently Skimming:
BIOLOGY OF HORSES AND BURROS
BIOLOGY OF HORSES AND BURROS
Pages 19-130
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 19... ...
that an overpopulation exists on a given area of the public lands, and that action is necessary to remove excess animals, he shall immediately remove excess animals from the range so as to achieve appropriate management levels .... This heavy emphasis on the definition of "excess" animals, and the fact that the Committee was invested with the responsibility for helping the Secretaries of Interior and Agriculture make that determination, necessitate a clarification of the term "excess," and of its implications for informational needs.
|
From page 20... ...
It must be determined how these vary with population size, adequacy of forage, weather conditions, competitors, and other environmental variables. Physiology, as well as food preferences and consumption, must also be understood.
|
From page 21... ...
Native Americans incorporated equids into their cultures and horse and ass populations -- both wild and domesticated -- spread rapidly throughout the western states. Feral populations increased when animals escaped and were released from ranching and mining activities in the l9th and 20th centuries.
|
From page 22... ...
. Thus although feral horses and asses are considered alien or 'exotic1 today, they represent lineages that have a long paleohistory in North America.
|
From page 23... ...
In particular, resource availability, feeding ecology, daily activity patterns, and demography are critical in determining social behavior, social bonding, and the type of social organization maintained. Most reviews of social organization and ecology have been on the interspecies level.
|
From page 24... ...
at ^ * •U • £ rH IH A rH 3 California Death Valley Nat'l.
|
From page 25... ...
. The largest home range observed was l03.6 km2 in Death Valley National Monument.
|
From page 26... ...
. Mother asses in Death Valley (Moehlman l974)
|
From page 27... ...
Information on daily activity patterns is available from two studies: one conducted at Death Valley, California and the other at Ossabaw Island, Georgia (Moehlman l974, l979)
|
From page 28... ...
A basic element of the daily activity pattern -- percentage of time spent feeding -- was quite different in the two populations. The Death Valley asses were primarily browsers and spent 5l.0 percent of their time feeding; Ossabaw Island asses were primarily grazers and spent 38.l percent of their time feeding.
|
From page 29... ...
Thus, when feral asses did form permanent groups they followed the pattern for Equidae. Behavioral and ecological information on the two feral populations of asses indicates that although Equus asinus normally displays low sociability, this species does have the behavioral plasticity, given a favorable environment, to form highly social and stable harem groups.
|
From page 31... ...
Anecdotal accounts of Asiatic wild horses (Mohr l97l) suggest that their home ranges are centered around water sources during spring foaling and the summer dry season, but that they greatly increase their range during the winter when snow is available.
|
From page 32... ...
. A great deal of data is available on equid social organization, demography, seasonal distribution and feeding ecology, daily activity pattern, and reproduction.
|
From page 33... ...
, mean generation length, and average reproductive value of an individual female at a given age all provide insights into the species' potential evolutionary plasticity. From the practical standpoint of equid management, estimates of herd increase rates disclose the rates at which animals must be removed in order to hold them at some decided-upon level.
|
From page 34... ...
in a 3-year study of 227 to 306 wild horses on Sable Island, Nova Scotia. Youngest mare seen copulating was 26 months of age.
|
From page 35... ...
She states that "Most mares foaled for the first time when 3 or 4 years old, but some not until they were 5 years old." But since the foaling performance of these different age classes was not given, no other rates can be calculated. It should be pointed out that the colt foals were removed each year from this population, somewhat similar to the situation on Chincoteague National Wildlife Refuge, as discussed below.
|
From page 36... ...
One might also speculate that the annual foal removal could have produced an older mare population with higher fecundity. The CNWR age composition was not given, and hence it is impossible to calculate age-specific fecundity rates for the population.
|
From page 37... ...
Since 3-year-olds made up 9 percent of the herd, they numbered 80 x .09 = 7. If we apply the 40 percent female percentage, then the number of mares in each of these age classes approximated 4, l7, and 3, and totaled 24.
|
From page 38... ...
Judging by the abundant representation of rates for 4-year-olds and older, and for 5-year-olds and older, the rates in this latter class commonly rise above 60 percent, in individual years exceeding 80. Because of these age-specific differences in fecundity, the rate one uses to express the performance of a given herd depends on the age classes included.
|
From page 40... ...
One veterinarian has told us that if range-reared domestic mares are bred at 2 years of age and foal at 3, they are not likely to foal again at 4. It is not known whether these tendencies explain the fact that none of the herds summarized in Table 2.4 continued to foal during each year of observation at rates markedly above or below the means for the age classes.
|
From page 41... ...
In large areas where emigration and immigration are not important variables, the age composition of an animal population -- numbers and percentages of animals in each age class -- is produced by the interaction of its reproductive and mortality rates. Hence, in a general way it is indicative of the demographic characteristics of a population.
|
From page 42... ...
42 TABLE 2.5 Monthly Distribution of Foals Born in Five Studies of Wild Horse Herds Percent of Foals Born by Location and Source Wyoming (Boyd 1980)
|
From page 43... ...
Furthermore, the percentages are potential annual rates of increase from herd size just before foaling to herd size at that point in the following year, again on the condition that no mortality occurs during the year. They therefore set upper limits on potential increase rates in the herds and years for which they are calculated.
|
From page 44... ...
44 % 0 -H -- OOOOOO0sNOO o o o o ooo OOOOOOo^tmoo 0 O O o o O *
|
From page 45... ...
45 to 9 OOOOOOOO vD (N n V rH •3 a O'H OOOOO CO O O u s r- CM co vo r00 CM i-l VD O vD n |
From page 46... ...
46 r0) o o cooo oo in CD oocncM j.
|
From page 47... ...
, the potential increase rates are l5 and 30 percent. The average is 2l.
|
From page 48... ...
A second means of calculating survival rates is with age-composition measurements. In wild populations, the difference between number of foals at birth and the number of yearlings at the same date can be used to estimate first-year survival rate provided the population is a long unchanging one that has assumed a stationary age distribution from constant natality and mortality rates over a preceding period of several years.
|
From page 49... ...
However, Wolfe (l980) used similar BLM roundup data to calculate survival rates from age structure.
|
From page 50... ...
+ mares foaling 78 53 % foals surviving, 0-2 months 82 98 Hence there appears to be some evidence that natality rates and foal survival rates may be inverse to each other, at least in some cases. Welsh further reported that the survival of foals born to 3- and 4-year-old mares is lower than that of foals from older mares, In this case, age-specific fecundity and subsequent foal survival are positively correlated, once again reflecting the dependency of the parameters on the mares' resources.
|
From page 51... ...
This is 43 out of a 3-year composite population of 979, a mortality rate of 4 percent (43/979 x l00) , and an annual survival rate of 96 percent.
|
From page 52... ...
But adult survival rates can be crudely calculated for those age classes from 2 through l2 years by transforming the numbers of animals in each class to logarithmic form, and regressing these values on age. The slope of the line estimates the mean, annual instantaneous mortality rate.
|
From page 53... ...
emphasized the reproductive hazards to which the mares are subject, suggesting that the costs of pregnancy and lactation may increase female mortality rates. It may be no coincidence that the shift toward males in the sex ratios begins after the 4 to 6 age classes.
|
From page 55... ...
As mentioned above, the question of the rates at which horse herds increase has been one of the more disputed ones in the controversy surrounding the wild horse issue. Annual increase rates of 20 percent or more, apparently originating from the management agencies, have been cited fairly widely, but their origin and basis have been difficult to track down.
|
From page 57... ...
57 S 8 n iijlj tl j! il ijllii I1!
|
From page 64... ...
concedes that the prevailing evidence points to increases in horse herds since l97l despite her suspicions about the precise validity of the agency censuses. In the 72 herds shown in Table 2.ll, there are 25 sets of consecutive census years that (a)
|
From page 65... ...
Hence it is difficult to say whether this apparent chronological trend is a function of changing techniques or actually declining increase rates. While helicopter counts appear to be more efficient at finding animals, and a change from fixed-wing to helicopter censuses obviously increases the number of animals seen, no such change is involved in the 25 tests.
|
From page 66... ...
Only with unrealistically high fecundity rates in Wolfe's calculations, and survival rates approaching l00 percent in Conley's case, could annual increase rates well above l0 percent be attained. They concluded that rates of herd growth as high as 20 percent per year, like those suggested by both agency personnel and a number of authors, were beyond the biological capability of large, established herds which had assumed remote approximations of geometric age structures.
|
From page 67... ...
To what extent do these rates simulate increase rates of real-world herds? They do so to the extent that the fecundity and survival rates in real herds are similar to those in Table 2.l2, and age structures in the herds approximate the exponential series
|
From page 68... ...
68 TABLE 2.12 Annual Finite Rates of Population Increase (X) Possible Under Differing Rates of Fecundity and Survival Annual Survival Rates Annual A Values by Fecundity Rates 1 Yr.+ Foal Max.
|
From page 69... ...
All studied herds have been found to experience some mortality, and a 0.95 survival rate for the older animals and 0.9 or 0.8 for foals would seem justified, if not conservative, from the available studies. Yet these combinations produce increase rates well below l0 percent (x - l.l0)
|
From page 70... ...
A second way in which increase rates are relevant to management is the rates at which herds must be reduced in order to maintain them at decided-upon levels. If a policy decision determines that a given herd must be maintained at some specified level by removing the annual increment, the number of animals to be removed annually and the associated cost will obviously be very different depending on whether the herd increases by 7 percent or l6 percent each year.
|
From page 71... ...
Shorter periods of study, smaller samples, and failure of most studies to subdivide the mares into as many age classes as have the horse investigators make the discernment of age-specific fecundity schedules in burros more difficult. But further analysis of the different authors' data permits a fragmentary and tentative picture: (i)
|
From page 72... ...
concluded that 56.8 percent of females foaled in the l2-month period l November l975 to 3l October l976 in the Wildrose-Emigrant population of Death Valley. Since he observed one mare foaling before 3 years of age, we assume the rate applies to all 2-year-olds and older females.
|
From page 73... ...
It seems reasonable to suspect that this is part of an age-specific increase in fecundity up to some optimum age class or range, as discussed above for horses. Although the data are too few to compare fecundity rates of the older age classes in burros and horses, the net effect of the earlier burro reproduction appears to result in higher rates in two or more of the younger age classes.
|
From page 75... ...
This comparison suggests that fecundity rates, at least in age classes 2 through 4, may be higher in burros than in horses. The age structure of those portions of the herds that are 2-year-olds and older appear similar in horses and burros, as we will see shortly.
|
From page 76... ...
Grand Canyon (Ruffner S Carothers 1977) January 21 4 0 February 0 1 0 March 7 1 40 April 7 7 25 May 14 15 0 June 7 19 20 July 7 2R 10 August 0 6 0 September 21 6 0 October 7 7 0 November 7 3 5 December 0 3 0 No.
|
From page 77... ...
. While these are generally subdivided into only three age classes, they give some insight not only into age composition, but also into herd dynamics as we discussed above for horses.
|
From page 78... ...
» S 3 8 | § « .2 r- en SrH • 8 C 5 M T3 3 tT1 Co in en O C 0) rl • r4 0 ^ -- 4J « Unweighted Means ^•Number of foals 1 Number of foals : October percental Bedrock Canyon H< Lower Canyon Here 6September-Novemb< August- November ( 8From Table 2.16 ( ^ vo cn -- B O M P git' r*
|
From page 79... ...
observed three pregnant jennies during her study of the Wildrose-Emigrant Canyon population in Death Valley. These animals were not accompained by foals at a later date, suggesting that they had aborted or lost their young.
|
From page 80... ...
calculated a first-year survival rate of 0.66 between l977 and l978 for the Butte Valley population in Death Valley. Dead foals are occasionally found.
|
From page 81... ...
Because the demographic parameters we have been reviewing are both more variable and less precisely known than those for horses analyzed above, life-table calculation of increase rates becomes more problematical. In review, we have seen that: (a)
|
From page 82... ...
We have performed life-table calculations on annual increase rates of both horses and burros, using a hypothetical rate of first foaling in 3-year-old horses and in 2-year-old burros of 25 percent. All older age classes were assigned 50 percent foaling rates.
|
From page 83... ...
, excellent physical condition, and presumed population increase prior to their removal effort. But the Lower Canyon Herd had not been subject to human interference, had only 49 percent of its individuals in the first 4 age classes, was not in as good physical condition, and was presumed to be roughly stationary or growing only slightly.
|
From page 84... ...
If these hypotheses are correct, they may have important management implications. Burros appear to have materially higher potential increase rates than horses, and to be capable of populating an area rapidly.
|
From page 85... ...
A DATA FROM PRESENT STUDY I DATA 1NCOMPLETE FOR YEARU972) IOCH 80<60o u111 _J |40' UJ .2 .4 .6 DENS1TY/Km2 1.0 Figure 2.l Relationship of the percentage of jennies foaling to population density in Wildrose-Emigrant population of Death Valley (after Douglas and Nonnent l977)
|
From page 86... ...
The pony mare is more strictly seasonal (as are the Pryor Mountain wild horses) , with all animals cycling by May and none after September.
|
From page 87... ...
Multiple ovulations and twinning very rarely occur in ponies. It is unknown whether multiple ovulations and multiple implantations occur in the wild horse.
|
From page 88... ...
Foal heat is likely to occur in wild horses, but its modulation by nutrition has not been studied. Thus, it is important to test captured mares that have accompanying foals or are lactating for pregnancy (milk progesterone, PMSG, serum progesterone)
|
From page 89... ...
Opposition groups, such as livestock and wildlife interests, argue that most wild horses today stem from escaped or released draft horses, cavalry mounts, and saddle animals. This dispute could be settled by genetic studies.
|
From page 90... ...
These two applications of blood-group genetics illustrate the resolving power of genetic studies. An early investigation of genetic variation in horse proteins other than those involved in blood-group determination was also published by Stormont and Suzuki (l963)
|
From page 91... ...
9l TABLE 2.l9 Estimates of the Frequencies of Alleles of Eight Blood-group Loci in Shetland Ponies and Thoroughbreds Frequencies in Loci Alleles Shetlands Thoroughbreds .M 0.3107 0.7050 A' 0.2852 0.0290 A a" 0.0358 0.0036 A'H 0.060l 0.0000 • 0.3082 0.2624 1D 0.1392 0.0000 D dJ 0.12l5 0.l503 d• 0.7394 0.8497 EPl 0.34l5 0.2058 EP' 0.0483 0.09l0 E 0.6102 0.7031 ae 0.1519 0.5082 R 0.3869 0.0000 51 s 0.0103 0.l038 a Q a 0.1306 0.0756 RS 0.l893 0.3l25 a a 0.13l0 0.0000 C*
|
From page 92... ...
92 TABLE 2.20 Number of Alleles Associated with Various Blood Proteins in Horses Protein Number of Alleles References Serum proteins Albumin Prealbumin Postalbumin Esterase Transferrin 2 Stormont and Suzuki (l963) ; Gahne (l966)
|
From page 93... ...
of the wild populations and the domestic breeds.
|
From page 94... ...
Table 2.2l displays a representative, though not exhaustive, summary of food-habit studies conducted on burro populations in several environments. Variations in the grass component, compared with that of horses (see Table 2.22 and later sections of this report)
|
From page 95... ...
(stomach contents) Mohave Desert (Death Valley)
|
From page 96... ...
96 TABLE 2.22 Diets of wild, Free-Roaming Horses 0ver a Range of Vegetation Types and Seasons, as Determined by Fecal Analysis Dietary Composition (%) Study Vegetation Type(s)
|
From page 97... ...
Equid Forage Requirements and Nutrition Knowledge of forage requirements and nutrition is important to effective management of horse and burro populations and the ranges they occupy. From the standpoint of animal welfare, the horse or burro biologist must understand the animals' nutritional needs for maintenance and reproduction in relation to the nutritional plane that the range is capable of providing seasonally.
|
From page 98... ...
Considerable energy from easily digestible foods is absorbed as glucose in the foregut of the equid. Undigested material, consisting largely of plant cell walls, is subsequently fermented in the cecum, yielding volatile fatty acids (VFA)
|
From page 99... ...
99 requirements of the two species. However, such comparisons must be interpreted with caution.
|
From page 100... ...
The NFS (l979) assumed that the daily intake by Grand Canyon burros was 5.l kg of forage per l67-kg animal, versus l.6 kg of forage per 64-kg sympatric bighorn sheep.
|
From page 101... ...
At any rate, now that wild horses and burros are established on many ranges, it must be determined whether the seasonal quality and quantity of their food are currently creating limitations. Generally, forage availability and nutritional quality are greatest during the season(s)
|
From page 102... ...
It is conceivable that, in the case of wild horses and burros, the question of mere survival may occasionally be the important one. Studies with ruminants have shown that nutritional deficiencies in animals cannot necessarily be determined by comparing the nutritional value of hand-harvested forage plants with animal requirements.
|
From page 103... ...
This type of information is scarce for domestic ruminants (Pieper and others l959, Cook and others l962) and is totally lacking for wild horses and burros.
|
From page 104... ...
Basically, we assume that wild horses and burros are limited by the same constraints in food supply as are grazing ruminants, the main difference being that horses and burros may be more resistant to depleted forage supplies than are ruminants. This presumption, however, must be verified.
|
From page 105... ...
Possible competition between wild horses and cattle was reviewed before the following comments were made. Broadly viewed, habitat analysis and evaluation -- the techniques of which are reviewed in deVos and Mosby (l97l)
|
From page 106... ...
This point seems to have escaped the writers of much of the literature on competition. The extensive writing on dietary similarities among wild equids, livestock, and wildlife often infers competition without evidence of resource limitation or population effect.
|
From page 107... ...
l07 All of this discussion comes down to the point of discerning and measuring competition between wild equids, livestock, and other wildlife so that it can be provided for in management plans. Since an essential criterion of competition is the creation of a population effect, its existence cannot absolutely be established without experimentally manipulating one species and ascertaining whether the other responds.
|
From page 108... ...
Demography of Wild Horses and Burros Project 6. Social Structure, Feeding Ecology, and Population Dynamics of Wild and Free-Roaming Horses and Burros Project 7.
|
From page 109... ...
The scientific observations to be made include range ecology, feeding behavior and nutrition, and measurement of various watershed parameters. Thus several disciplines will be needed, and the location and design of experiments will need to consider the availability of these capabilities.
|
From page 110... ...
ll0 Each of these experiments should be replicated three or four times, perhaps once in each of three or four states. It is not essential that the large-scale (l, l0)
|
From page 111... ...
Methodology l. A single experimental block should be a large area containing wild horses or burros.
|
From page 112... ...
Studies of this nature provide the critical link between the plant community and the demographic response of the animal population in question. Considerable research has been conducted on the nutrition and feeding ecology of sheep and cattle (e.g., Cook and Harris l968; Cook and others l953, l962, l967)
|
From page 113... ...
Detailed information on forage available (such as that derived from Project 8, the "grazing impacts" study) should be obtained in conjunction with dietary sampling.
|
From page 114... ...
Nutritional Plane, Condition Measures, and Reproductive Performance in Domestic Mares Rationale The nutritional condition of the female is considered to be of primary importance to estrus, ovulation, and early survival of the offspring from conception to weaning. The rate at which offspring are produced is a critical demographic parameter when estimating rates of population increase.
|
From page 115... ...
3. The following reproductive parameters should be measured to establish quantitative relationships between nutritional plane and reproductive success, which may then be used for demographic models of specific ranges:
|
From page 116... ...
5. Behavioral sampling should be performed for the following purposes: determining activity levels so that nutritional levels can be accurately generalized to free-roaming populations, whose activity levels are likely to be different; correlating differences in reproductive performance and condition to activity measures; and comparing nutritional treatment effects on activity.
|
From page 117... ...
These divisions are established because breakdown in the reproductive act can be attributed either to the mare or the stallion and the point at which failure occurs can be assigned to a particular phase of the reproductive interaction. The objective of this sample will be to monitor behavior of mares of different nutritional planes and that of their stallions to determine differences in each of these functional categories.
|
From page 118... ...
3. Systematically collect blood samples from mares in the reproductive-performance study to establish correlates with condition, reproductive success, and mechanisms of reproductive suppression.
|
From page 119... ...
As its numbers exceed the desired level, the excess can be removed periodically irrespective of what the increase rate may be. Another situation in which there is no need to know increase rates is if some populations are allowed to limit themselves, irrespective of their impacts on the ecosystem.
|
From page 120... ...
A limited research commitment, in this group's view, would not add materially to what is now known. To refine by a few percentage points the survivorship estimates, or sharpen somewhat the fecundity parameters, would not alter the basic picture of increase rates developed in the earlier demography section.
|
From page 121... ...
members of the Committee, not add a great deal to present levels of understanding. Objectives The objectives of this project are to measure age-specific fecundity rates, first-year survival rates, and adult survival rates to two different levels of precision.
|
From page 122... ...
5. Adult survival rates, Level l: Determine adult survival rates by: a.
|
From page 123... ...
6. Adult survival rates, Level 2: Determine the range of variation within and between populations in adult survival rates and the environmental causes of that variation by: a.
|
From page 124... ...
Methodology l. Establish three study areas each for wild horses and burros.
|
From page 125... ...
one of the two females should be "contraceptive" and the other "normal." 3. The sampling scheme for monitoring detailed feeding behavior is to be a continuous focal-animal sample.
|
From page 126... ...
The continuous samples will record the occurrence of sexual behaviors with a time record. From these samples the rates of behavior can be calculated and tested against estimates for wild populations.
|
From page 127... ...
On the contrary, if populations are genetically different from each other, the preservation of genetic variation requires that every one of the differentiated populations be preserved. In the case of wild and free-roaming horses and burros, it becomes important to study not only the wild populations but also the domestic breeds.
|
From page 128... ...
In order to obtain a valid estimate of the amount of genetic variation in a population, it is necessary to study a number of gene loci that are randomly selected with respect to how variable they are. That is, it is necessary that the probability of including a given locus in the sample be independent of how polymorphic the locus is.
|
From page 129... ...
The amount of genetic variation is best measured by the heterozygosity per locus or per individual, although the proportion of polymorphic loci per population also provides useful information. The degree of genetic similarity can be summarized with standard measures, such as Nei's genetic distance.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.