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Compensatory Reproduction in Feral Horses Author(s): Jay F. Kirkpatrick, John W. Turner and Jr. Source: The Journal of Wildlife Management, Vol. 55, No. 4 (Oct., 1991), pp.

1 Compensatory Reproduction in Feral Horses Author(s): Jay F. Kirkpatrick, John W. Turner and Jr. Source: The Journal of Wildlife Management, Vol. 55, No. 4 (Oct., 1991), pp Published by: Wiley on behalf of the Wildlife Society Stable URL: Accessed: :34 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at Wiley, Wildlife Society are collaborating with JSTOR to digitize, preserve and extend access to The Journal of Wildlife Management

COMPENSATORY REPRODUCTION IN FERAL HORSES JAY F. KIRKPATRICK, Department of Biological Sciences, Eastern Montana College, Billings, MT 59101 JOHN W. TURNER, JR.

2 COMPENSATORY REPRODUCTION IN FERAL HORSES JAY F. KIRKPATRICK, Department of Biological Sciences, Eastern Montana College, Billings, MT JOHN W. TURNER, JR., Department of Physiology, Medical College of Ohio, Toledo, OH Abstract: We estimated fetal loss rate in 2 separate herds of feral horses (Equus caballus) with signif different foaling rates that inhabited the same barrier island. We estimated pregnancy rates for 4 mares from Assateague Island National Seashore (NS), where the foaling rate was 32.5% and for 48 from Chincoteague National Wildlife Refuge (NWR) where the foaling rate was 62.5%. Fetal loss rate estimated by comparing fall pregnancy rates with foaling rates during the following spring and su The estimated fetal loss for mares from Assateague Island NS and Chincoteague NWR was 7.1 and respectively. Ten of the 40 mares from Assateague Island NS (25%) were lactating and only 2 produced whereas 2 of the 48 mares from Chincoteague NWR (4.1%) were lactating and neither produced foals data indicate that (1) the differential foaling rates are not the result of fetal loss after 90-days postconce (2) lactational anestrus is a contributing factor to differential foaling rates, and (3) the annual remo foals from the Chincoteague NWR herd results in compensatory reproductive mechanisms and higher f rates. J. WILDL. MANAGE. 55(4): Compensatory reproduction might be an the important adaptive response to various natural, only 57.1%, with an age-specific range of 40- unmanaged Assateague Island NS horses was population-limiting pressures such as predation, 70%. Since 1986, the Assateague Island NS foaling rate has dropped below 50% (J. F. Kirkpat- food availability, and disease. Human intervention into wildlife populations by means of rick, hunt-unpubling or removal of animals can also affect pop- The purpose of our study was to examine fall data). ulation growth. Two populations of feral horses pregnancy rates in the 2 herds and to compare inhabiting Assateague Island, a barrier these islandwith respective foaling rates during the off the Maryland/Virginia coast, provide following an excellent model for the study of compensatory mining the extent of fetal loss between the 2 spring and summer, thereby deter- reproduction. The island's feral horses are herds. maintained in 2 separate herds, one on the Maryland stresses of concurrent pregnancy and lactation We hypothesized that the physiological portion of the island and the other on the among Virginia portion. Peak breeding activity for greater both incidence of fetal loss than among the the Assateague Island NS horses cause a herds is May and June, with peak foaling horses activity during April and May (Keiper and Houpt iological stress of lactation is removed annually. of Chincoteague NWR, where the phys- 1984). Weaning occurs at age 1 year for mares The authors gratefully acknowledge the logistical at support provided by G. Olson. We also foaling 2 consecutive years but often occurs ages 2 or even 3 years for mares that do not thank foal B. L. Lasley and S. E. Shideler for assistance in establishing the urinary steroid assays. on consecutive years. The Maryland herd consists of approximately Our study was supported in part by National 150 horses living on Assateague Island National Park Service grant CA Seashore, and, in keeping with National Park METHODS Service policies of nonintervention, management is minimal. The Virginia herd consists Forty of mares whose home ranges were exclusively confined to Assateague Island NS and 48 approximately 200 horses inhabiting Chincoteague NWR; these horses are intensively mares managed through the annual removal of approxifined to Chincoteague NWR were included in whose home ranges were exclusively conmately 80% of the foals, a practice dating our back study. All mares were sexually mature and >30 years. In an 8-year study of reproduction >3 years old. The ages of 29 of the Assateague among the Assateague Island horses conducted Island NS mares were known, but the ages of between 1975 and 1983, Keiper and Houpt the Chincoteague NWR mares were not known (1984) reported an annual foaling rate of exactly. 74.4% Ten of the 40 Assateague mares (25%) among sexually mature mares on the Chincoteague NWR. In contrast, the foaling rate foals for at their sides and were lactating. and 2 of the Chincoteague mares (4.1%) had 649

650 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner J. Wildl. Manage. 55(4):1991 Table 1. Pregnancy and foaling rates (%) for lactating and nonlactating feral mares in 2 separate herds.

3 650 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner J. Wildl. Manage. 55(4):1991 Table 1. Pregnancy and foaling rates (%) for lactating and nonlactating feral mares in 2 separate herds. Assateague Chincotea.gue Islan9d National National Wildlife Seashore Refuge n % n % Total mares tested Diagnosed pregnant Live foals Lactating mares Diagnosed pregnant Live foals We used methods described by Kirkpatrick et al. (1991) to measure fecal EC and ipdg concentrations. Fecal total estrogens were assayed by radioimmunoassay as described by Bamberg et al. (1984) and Kirkpatrick et al. (1990c). Fecal steroid metabolites and free estrogens were reported as ng/g feces. Pregnancy was predicted on the basis of >1.0 ng/g EC, > 150 ng/g ipdg, and 5.0 ng/g total estrogens (Kirkpatrick et al. 1990c, 1991). During the summer and fall of 1990 we located all pregnancy-tested mares and recorded the presence or absence of foals. Ownership of foals was based on witnessed lactation. We used a binomial During October 1989 probability urine distribution and/or (Freedman fecal et al. 1978) sa ples were collected from to test for differences each mare in pregnancy, as lactation, descr by Kirkpatrick et al. and foaling (1988, between 1990b). the 2 herds. Urine w aspirated directly from the ground immedi following elimination RESULTS by a mare, or it was h centrifuged from sand, A significantly placed (P < 0.001) on greater ice at proportion the of collection, and stored of Chincoteague frozen mares tested at positive the for end pregnancy, and were a significantly placed (P < 0.001) in greater pl each day. Fecal samples bags, kept on ice during number of the foals day were produced and frozen by Chincoteague Assateague mares than by Assateague mares, we (Table evening. For the 40 lected urine samples 1). from There was a % mares, and a 6.2% fetal fecal loss rates ples from 17 mares, among and the 40 both Assateague urine and 48 Chincoteague and fe samples from 5 mares. mares, respectively, For with the no significant Chincote difference between were the 2 herds. collected Annual foal removal from mares, urine samples animals and fecal samples resulted in a significant from (P < 0.01) 25. difference All in hor had distinctive markings lactating mares which between the 2 we herds, with recorde only drawings or with photographs 2 lactating Chincoteague mares for and 10 later lactating ide fication. Assateague mares. Collectively, the 12 lactating Unextracted urine mares samples produced only were 2 foals. assayed f (1) creatinine (Cr), (2) estrone conjugates DISCUSSION by enzyme immunoassay as previously scribed by Shideler Early et neonatal al. (1990) death is almost and impossible mod to for use in the horse detect (Kirkpatrick on Assateague Island habitat et al. because 1990 of and (3) nonspecific dense immunoreactive vegetation and the pregnant proges mares' behavior of isolating as described themselves at parturition. by K one metabolites (ipdg) patrick et al. (1990a). Thus, The we could reliability not distinguish between for the fetal assays and the 100% loss and accuracy neonatal loss. Nevertheless, for predic the nearly pregnancy, which has doubly been great pregnancy tested rate for in Chincoteague dome horses and confirmed horses by indicates rectal that the differential palpation foaling rates (E et al. 1984, Kasman between the al. 2 herds 1988) is determined and by has Octoberb demonstrated in feral pregnancy horses rates, and (Kirkpatrick not by fetal loss after ap-proximately been reported days postconception. elsewh 1990a,b,c, 1991), have Urinary steroid metabolite Among domestic concentrations mares, Ginther (1979) reported to an 80% account seasonal pregnancy for differ rate and a w indexed to urinary Cr in urine concentrations, 70% live foal and crop. they Van Niekerk were (1965) reported as ng/mg or og/mg a 12.5% Cr. pregnancy We considered loss among domestic mares, con trations of EC in excess with greater of pregnancy 1.0 Ag/mg loss during the first Cr half (E et al. 1984, Kasman of et pregnancy. al. 1988, The suggested Kirkpatrick critical period for al. 1988) and ipdg pregnancy in excess loss was of Days Estimates ng/mof (Kirkpatrick et al. 1991) abortion rates as in indicative the domestic mare, after of Day pr nancy. 45 of pregnancy, range from 7 to 12% (Platt

J. Wildl. Manage. 55(4):1991 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner 651 The most probable cause for the differences particularly young horses.

4 J. Wildl. Manage. 55(4):1991 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner 651 The most probable cause for the differences particularly young horses. Because our results in pregnancy and foaling rates is lactational anestrus, a condition in mares during which ovu- suggest that feral horses increase conception rates in response to the removal of foals before weaning, a policy of removing young, unweaned lation does not occur (Day 1939, Burkhardt 1948, Roberts 1971, Lieux 1973). In domestic mares, horses can lead to greater fecundity among their this condition persists in a large percentage of mothers. Conversely, the removal of weaned lactating mares until the foal is weaned, usually horses will be unlikely to have any significant at 4-6 months. This period of an ovulatory lactational anestrus has been characterized by very effect upon fecundity of the mothers. Future small or absent ovarian follicles (Day 1939). In management policies for the control of feral horse populations should consider compensatory controlled experiments with domestic mares, removing the foal at parturition hastened the re- reproductive responses of the mares. Regardless turn of estrus in 3 of 4 mares, whereas all 4 of whether actual removal or contraception is mares that nursed their foals normally were anestrus (Ginther et al. 1972). On Assateague Island, weaning seldom occurs sooner than 1 year, is fairly common at 2 years, and in some instances has been observed at 3 or 4 years. In contrast, about 80% of the foals of Chincoteague NWR mares are removed annually in July. LITERATURE CITED Our results differ significantly from those of Wolfe et al. (1989), who examined pregnancy rates among 553 feral mares >2 years old, in Nevada, Oregon, and Wyoming. The pregnancy rates for horses in these states were 57.0, 61.0, and 81.4%, respectively, with no differences between lactating and nonlactating mares. Some of the differences between the results of our study and those of Wolfe et al. (1989) could be the result of different pregnancy testing procedures and the accuracies of these tests. Wolfe 1973, Irwin 1975, Sullivan et al. 1975, Chevalier and Palmer 1982), and major causes of this loss are thought to include foal heat conceptions in lactating mares (Platt 1973) and periglandular fibrosis of the uterine mucosa (Kenney 1978). In our study fetal loss after Day 90 postconception in both herds of feral mares was not dif- et al. (1989) reported accuracy rates of 81.8% for plasma progesterone concentrations, 63.0% for plasma estrogen, and 53.7% for plasma pregnant mares' serum gonadotropin (PMSG). The use of only plasma progesterone measurements ferent from that reported for domestic mares for pregnancy detection in horses is potentially after Day 45 postconception. inaccurate (Hyland 1990), and the same is true Among island feral horses, age has also been for urinary ipdg measurements alone, largely shown to be a major factor in fetal loss rates. because of the high incidence of persistent corpora lutea in mares. Urinary E,C concentra- Lucas et al. (1990) demonstrated that fetal loss among mares of Sable Island, Canada, betweentions, however, reflect the presence of a viable Days 120 and term was 9.7, 40.3, 25.0, andembryo and drop precipitously within hours of 26.3% annually over 4 years. The fetal loss rates the death of the fetus (Kasman et al. 1988). were age-dependent, with a 16.7% loss for 3 year Consequently, accuracy of pregnancy detection olds and a 5.6% loss for 4 year olds. In our study, in mares approaches 100% with urinary E,C only mares >3 years old were included, thus concentrations it alone (Evans et al. 1984, Kasman is unlikely that age had a significant effect upon et al. 1988), 100% for urinary E,C and ipdg the 2-fold difference in pregnancy and foaling concentrations together (Kirkpatrick et al. rates. Furthermore, an environmental basis for 1990a), and 95% for total fecal estrogens (Bam- et al. 1984, Kirkpatrick et al. 1990c). the differences seems unlikely, because the 2berg herds inhabit the same island. Photoperiod, climate, and food supplies are equivalent, and both MANAGEMENT IMPLICATIONS herds originate from the same genetic forebearers. Control policies for feral horses have historically centered around the removal of horses, employed as the control method, efforts should focus on mares >3 years old, and those without foals. Conversely, the removal of foals or yearlings should be avoided because of the probability of increasing fecundity among their mares. BAMBERG, E., H. S. CHOI, E. MOSTL, W. WURM, D. LORIN, AND K. ARBEITER Enzymatic determination of unconjugated oestrogens in faeces for pregnancy diagnosis in mares. Equine Vet. J. 16: BURKHARDT, J Some clinical problems of horse breeding. Vet. Rec. 60: CHEVALIER, F., AND E. PALMER Ultrasonic echography in the mare. J. Reprod. Fert. (Suppl. 32):

652 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner J. Wildl. Manage. 55(4):1991 DAY, F. T. 1939. No. 2-sterility in the mare associated with irregularities of the oestrous cycle. Vet. Rec.

5 652 COMPENSATORY REPRODUCTION * Kirkpatrick and Turner J. Wildl. Manage. 55(4):1991 DAY, F. T No. 2-sterility in the mare associated with irregularities of the oestrous cycle. Vet. Rec. 51: ,, AND J. W. TURNER, JR. 1990c. EVANS, K. L., J. P. HUGHES, M. COUTO, L. H. KASMAN, Pregnancy determination in uncaptured feral AND B. L. LASLEY Pregnancy diagnosis horses based on steroid metabolites in urine-soaked in the domestic horse through direct urinary snow estrone conjugate analysis. Theriogenology 22: and free-steroids in feces. Can. J. Zool. 68: 620. LIEUX, P Reproduction and genital diseases. FREEDMAN, D. B., B. PISANI, AND R. PURRES Pages in E. J. Catcott and J. F. Smith- Statistics. W. W. Norton, New York, N.Y. 506pp. Cors, eds. Equine medicine and surgery. Second GINTHER, O. J Reproductive biology of the ed. American Veterinary Publ., Inc., Wheaton, mare. McNaughton and Gunn, Ann Arbor, Mich. Ill. 413pp. LUCAS, Z., J. I. RAESIDE, AND K. J. BETTERIDGE , H. L. WHITMORE, AND E. L. SQUIRES The incidences of pregnancy and pregnancy loss Characteristics of estrus, diestrus, and ovulation in the feral horses of Sable Island based on field in mares and effects of season and nursing. Am. observations and determination of fecal oestro- J. Vet. Res. 33: HYLAND, J. H Reproductive endocrinology: its role in fertility and infertility in the horse. Br. Vet. J. 146:1-16. IRWIN, C. F. P Early pregnancy testing and its relationship to abortion. J. Reprod. Fert. (Suppl. 23): KASMAN, L. H., J. P. HUGHES, G. H. STABENFELDT, M. D. STARR, AND B. L. LASLEY Estrone sulfate concentrations as an indicator of fetal demise in horses. Am. J. Vet. Res. 49: KEIPER, R., AND K. HOUPT Reproduction in feral horses: an eight-year study. Am. J. Vet. Res. 45: in uncaptured feral horses by means of fecal steroid conjugates. Theriogenology 35: gen concentrations. Pages in Proc. fifth international symposium on equine reproduction, Deauville, France. PLATT, H Aetiological aspects of abortion in the thoroughbred mare. J. Comp. Pathol. 83: ROBERTS, S. J Veterinary obstetrics and genital diseases. Second ed. Edwards Brothers, Inc., Ann Arbor, Mich. 542pp. SHIDELER, S. E., L. TELL, G. OWITI, L. SHAHNAZARI- AN, R. CHATTERTON, AND B. L. LASLEY The relationship between serum estradiol and progestrone concentrations to the enzyme immunoassay measurements of urinary estrone conjugates and immunoreactive pregnanediol-3-glu- KENNEY, R. M Cyclic and pathologic changes of the mare endometrium as detected by biopsy curonide in Macaca mulatta. Am. J. Primatol. with a note on early embryonic death. J. Am. 22: Vet. Med. Assoc. 1: SULLIVAN, J. J., P. C. TURNER, L. C. SELL, H. B. KIRKPATRICK, J. F., L. H. KASMAN, B. L. LASLEY, GUTTERIDGE, AND D. E. BARTLETT Survey of reproductive efficiency in the quarter horse AND J. W. TURNER, JR Pregnancy determination in uncaptured feral horses. J. Wildl. and thoroughbred. J. Reprod. Fert. (Suppl. 23): Manage. 52: ,, B. L. LASLEY, AND S. E. SHIDELER. 1990a. VAN NEIKERK, C. H Early embryonic re- in mares. J. South Afr. Vet. Med. Assoc. Urinary steroid evaluations to monitor ovariansorption function in exotic ungulates: VII. Urinary pro-36:61-69gesterone metabolites in the Equidae assessed WOLFE, by M. L., L. C. ELLIS, AND R. MACMULLEN. immunoassay. Zoo Biol. 9: Reproductive rates of feral horses and burros. J. Wildl. Manage. 53: , I. K. M. LIU, AND J. W. TURNER, JR. 1990b. Remotely-delivered immunocontraception in feral horses. Wildl. Soc. Bull. 18: Received 7 November 1990., S. E. SHIDELER, B. L. LASLEY, AND J. W. Accepted 8 May TURNER, JR Pregnancy determination Associate Editor: Conner.

American Society of Mammalogists

American Society of Mammalogists Correction for Sprung Traps in Catch/Effort Calculations of Trapping Results Author(s): Lewis Nelson, Jr. and Francis W. Clark Reviewed work(s): Source: Journal of Mammalogy,