Age-specific reproduction in feral horses

Abstract

1991. Age-specific reproduction in feral horses. Can. J. Zool. 69: 738-743. Two age-specific reproduction schedules were constructed for feral horses (Equus caballus) on the basis of lactation status of 14 788 females captured during herd reduction programs and pregnancy rates of 667 horses determined by serum progesterone assays. The probability of detecting lactation progressively decreased for females captured further from the foaling season, indicating that these data resulted in substantial underestimates of true foaling rates. A third reproductive schedule was, therefore, constructed on the basis of a subsample of 1144 horses captured immediately after the foaling season in June. Characteristics common across all three data sets were first reproduction at age 2, an increase in the proportion of females foaling through age 6, highest foaling rates from 6 to 15 years, and a gradual decrease in foaling rates of females > 15 years. Variability in the proportion of reproductively active females in each age-class was detected among populations and among years within a population; however, the general trend was high reproductive rates, with 80-90% of the prime-age females foaling. The reproduction m$el suggested by L. L. Eberhardt, provided a close mathematical approximation of the observed age-specific changes in foaling rates, providing a useful tool for the construction of reproduction schedules required for age-structured population models.GARROTT, R. A., EAGLE, T. C,, et PLOTKA, E. D. 1991. Age-specific reproduction in feral horses. Can. J. Zool. 69 : 738-743. Nous avons klabork deux calendriers de reproduction spkcifiques a l'ige chez des chevaux sauvages (Equus caballus), calendriers basks, d'une part sur la lactation chez 14 788 femelles capturkes au cours de programmes de rkduction des troupeaux, d'autre part sur les taux de grossesse dkterminks par des tests sur la progestkrone skrique de 667 chevaux. La probabilitk de dktecter la lactation diminiue progressivement dans le cas des femelles capturkes plus longtemps aprks la saison de mise-bas, ce qui indique que les rksultats comportent des sous-estimations importantes des taux rkels de mise-bas.

Achieving population goals in a long-lived wildlife species (Equus caballus) with contraception

“…Normally, 20 years represents an extreme old age among wild horse herds. Eberhardt et al (1982) considered 20 years the maximum age for wild horses inhabiting ranges in Oregon, and Garrott et al (1991) found that among 14 788 wild horses across Nevada, only 115 (0.007%) were older than 20 years. Linklater et al (2004) found that among New Zealand's Kaimanawa wild horses, only an average of 1.0% would reach 20 years.…”

“…Several models were developed with which to determine the various levels of effort and the concomitant effects of contraceptive management on entire populations of large ungulates, and specifically, wild horses (Garrott 1991;Garrott and Siniff 1992;Hobbs et al 2000). These various models examined possibilities within the parameters of (1) duration of contraceptive effect, (2) reproductive rates, (3) mortality rates and (4) contraceptive efficacy.…”

Age-related reproductive effort in reindeer (Rangifer tarandus): evidence of senescence

“…This is probably because reproductive senescence seems to occur at a later stage than survival senescence (Bérubé et al 1999), and it is therefore even more difficult to obtain such data. Earlier studies nevertheless indicated a decline in the proportion of females reproducing after prime-age was reached, but this was based on data where age determination was partly unreliable [for horses Equus caballus (Garrott et al 1991)] or based on either tabular data or many ages pooled in classes [for red deer Cervus elaphus (Langvatn et al 1996); bighorn sheep Ovis canadensis (FestaBianchet 1988)]. Only recently has convincing evidence of a declining proportion of females lambing [in bighorn sheep (Bérubé et al 1999)], a reduced litter size [in moose Alces alces ] and decreased productivity [in roe deer Capreolus capreolus (Gaillard et al 1998b;Hewison and Gaillard 2001)] with increasing age of female been reported.…”

Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

“…Gestation period in wild and feral horses on average is 336 (range 331 -352) days and only one young is born (Monfort et al 1991;Duncan 1992;. Female fertility gradually increases with age, reaching its peak between 6-10 years of age (Keiper and Houpt 1984;Berger 1986;Duncan 1992;, with a gradual reduction in fertility until the onset of senescence at about 15-20 years of age (Eberhardt et al 1982;Garrott and Taylor 1990;Garrott et al 1991a). …”

“…While growth rates in some populations are limited by human management Eberhardt et al 1982;Putman 1986;Garrott and Taylor 1990;, in a majority of populations, food limitation and predation are believed to be the most important factor driving the rate of population change over time (Keiper and Houpt 1984;Greger and Romney 1999;. Observed population growth rates vary significantly between studies with negative rates (<1.0) being recorded by Greger and Romney (1999), slow (1.03 -1.09) by ; ; Dawson and Hone (2011);Keiper and Houpt (1984), moderate (1.12 -1.20) by Garrott and Taylor (1990); Garrott et al (1991a);Berger (1983); Eberhardt (1982), and rapid (>1.20) by Lopez (2010), andDuncan (1992). These variations were due to differences in annual survival and fecundity influenced by climatic conditions, predation or its absence, human management, and removal of domestic stock, which reduced competition for resources.…”