Printed in Great Britain
1991 Journals of Reproduction & Fertility Ltd
Pasture mating behaviour of donkeys (Equus minus) at
natural and induced oestrus
M. Henry, S.M. McDonnell*, L.D. Lodi and E.L. Gastalt
School of Veterinary Medicine, University of Minas Gerais Cp
Belo Horizonte, Brazil; *Section
of Reproductive Studies, University of Pennsylvania, New Bolton Center, School of Veterinary Medicine,
Kennett Square, Pennsylvania 19348, USA and
de Zootecnia, Post0 de Equideocultura de Colina
Cp II, 14770 Colina, Brazil
The mating behaviour of 2 jacks, each with 21 non-pregnant jennies, was studied
when the jennies were in natural oestrus and simultaneously induced oestrus. The main
observations were: efficient pasture breeding at natural and induced oestrus, a territorial
sociosexual structure, prolonged pre-copulatory interaction, gradual increase of mating
activity up to 2 days before ovulation, a copulatory sequence similar to that of horses,
vocalization of the jack as a conspicuous behaviour initiating pre-copulatory interaction,
frequent heterotypical behaviour of jennies and active involvement of jennies in initiating
Numerous authors have described sociosexual behaviour of domestic horses (Collery, 1969; Pickett et al.,
1970; Collery, 1974; Asa et al., 1979; Bristol, 1982; Ginther, 1983; McDonnell, 1986; Bristol, 1987), feral
McCullough, 1975, 1976; Klingel, 1982; Keiper
and plains, mountain and Grevy’s zebras (Klingel, 1975). However,
studies on social organization and reproductive behaviour of donkeys are few. A study of wild African and
Asiatic asses revealed a territorial, rather than harem, type of social organization. These jacks defend their
boundaries against neighbouring territorial males only under specific conditio& such as when a jenny is
in oestrus (Klingel, 1977). In a 2-year study of domestic jack semen collection (M. Henry, M.M.F.
Oliveira, C. Meira & E.L. Gastal, unpublished observations), the mean time from approach to ejaculation
of jacks running free with an oestrous jenny varied from 12-26 min. During this time, prolonged Flehmen
episodes, mounts without erection, partial drop of the penis, rapid and sudden withdrawal from the female
and grazing were observed. In that study, unexplainable, repeated failure to collect semen from some
individual jacks was noted. The rather long pre-copulatory interval in domestic jacks and the
collecting semen, compared with horses was noted also by Nishikawa (1959) and Kreuchauf (1984).
These findings, together with subject observations of slow sexual response of jacks, prompted
questions concerning their reproductive management. This experiment was conducted to describe
reproductive behaviour of domestic donkeys. To permit comparison with horses, our project design
included pasture breeding of donkeys under simultaneously induced oestrous conditions similar to those
described for horses by Bristol (1982, 1987).
Materials and Methods
The trials were conducted on donkey bneding farms near CarIos Chagas (Trial 1) and
2). In Trial
Jack A (IZyears-old, 280 kg, Pega) was kept at pastu~. for 60 days starting in February 1989, with 21 grade jennies. In Trial
2, Jack B (3.5years-old, 360 kg, Brazilian breed) was pastured for 39 days during January and February 1990 with
Brazilian jennies that had been together for at least
years before the experiment. Three jennies fmm Trial 1 and 10 from
78 M. Henry et al.
Trial 2 had a foal at foot throughout the study. The jacks and jennies at pasture together were observed for 6 daylight hours
Day 1 and Days 7-10; and 12 daylight hours on Days 2-6 and II-16 for Trial 1. For Trial 2, they were observed for 12
daylight hours on Days I-18. For the
remahing hours of these days, the jacks were
from the jennies. On Days 17-60
and Days 19-39 in Trial 2, the animals remained at
In each trial, Jennies cycled naturally from Days
To induce oestrus,
F2u (10 mg, intramuscularly
was administered on Day 6 to all females not in oestrus.
The pastures were rectangular and approximately 15 hectares (* 37 acres) each. One was divided longitudinally by a
creek (Trial 1) and the other had a pond near the fence (Trial 2).
pasture was divided into 6 sectors by imaginary lines
based on landmarks or easily observed fence marks made for this purpose. During the trials, ambient temperature varied from
26 to 42°C. Rain of less than 30 min duration occurred once in each trial.
Two observers were stationed at elevated sites on opposite sides of the pasture (Trial 1) or at 3 different elevated sites
within the pasture (Trial 2). Primary focus was on the jack and secondary focus on the jennies. The observers recorded all
specific interactive responses between adult donkeys (i.e. olfactory, aggression, vocalization. mounting, insertion, thrusting
and ejaculation for the jack, approaching, posturing, jawing, ear and tail position for the jennies), and maintenance behaviours
drinking, elimination, grooming) on a time base using a microcomputer event recorder, audio recorder and
written notation. Ejaculation was identified by tail flagging and seminal reflux. The location of each animal relative to the
sectors was recorded at 2-h intervals. At 15-min intervals, the jennies close to the jack were identified as part of the ‘proximal
group’ and were considered to be sexually active if they stayed in the vicinity of the jack and postured to the jack at least once
during the 15min interval.
Observers recorded their subjective appraisal of each
oestrous or dioestrous condition each day based on jawing,
ear pinning and posturing to the jack.
Semen collection .
Attempts were made to collect semen from the jacks by artificial vagina on the day before the start of the trial and every
other day (alternating morning and afternoon) during natural and induced oesuus. Semen was also collected on Day 39 of Trial
2. All semen samples were evaluated as described by
Ovarian activity was monitored by palpation per rectum
day during oestrus and every third day during dioestrus.
For Trial 2, follicular growth was monitored by ultrasound. At least once a week, the jennies were bled by jugular
venepuncture. Plasma was stored at -2o’C until progesterone analysis was performed by radioimunnossay (Vermeulen
Verdonck, 1976). The sensitivity of the assay was 0.5 @ml and the
and inter-assay coefficients of variation were 3.6
and 7.38, respectively. Palpation per rectum, collection of blood samples and ulttasonography were performed during the
evening of each observation day.
Proportional and quantitative data were evaluated by Chi squared tests and analysis of variance procedures, respectively.
Pierson correlation techniques were used to evaluate associations.
General breeding results
During Trials 1 and
(95%) and 21 (100%) of the jennies, respectively,
and 14 (67%) and 18 (85%) ovulated. The mean
s.d.) length of ovulatory oestrus was 6 days
and ovulation to end of
20). Length of anovulatory oestrus was 2.6 (k 2.2) days (n
12). One split ovulatory oestrus was
observed. One of the jennies showing anovulatory
ovulated during a subsequent oestrus that
started 8 days later, and 1 was cycling but neither a large follicle nor ovulation was detected. The
remaining jennies were not cycling, as indicated by the progesterone assay results (<
Plasma progesterone concentrations (> 3
indicated that the detected ovulations resulted in active
Pregnancy was diagnosed for all jennies by a Single palpation per rectum at least
ovulation. .Of the 14 jennies that ovulated during Trial
(78%) were diagnosed as pregnant after the
Pasture breeding behaviour of donkeys
11 12 13 14 15 16 17 18
Number of jennies showing oestrus per day during Trials 1
first cycle and 3 (22%) after the second cycle. For Trial 2, ultrasonography of the
began on Day 10
after ovulation. An embryo was detected by Day 12 after ovulation in 11/16 (69%) jennies mated during
the first cycle. Two jennies were not mated during the first ovulatory
All but 1 jenny became
pregnant within 2 cycles. Early embryonic death was detected in 2 jennies during the first cycle and in
the second cycle.
In both trials, arrival of the jack at the paddock each morning evoked
vocalization and vigorous general interaction with all jennies, followed by a more thorough individual
teasing of one of the jennies. Teasing behaviour included naso-nasal contact, nibbling the head, neck, knee,
and flank and sniffing parts of the body, mostly the perineal area. Teasing generally concluded with 1 or
more mounts without erection. As the jack interacted with an individual jenny, other jennies
(predominately those in oestrus) gathered in the vicinity. After each mount, the jacks grazed or rested for
14.2) min before the next vocalization or period of teasing. During the rest period, the sexually
active jennies remained nearby, but if they approached the male they were generally rebuffed by
threatening or kicking behaviour.
.Spontaneous vocalization by the male, and less frequently by a female in oestrus, was the signal for a
period of pre-copulatory interaction. Occasionally, the approach of a jenny in oestrus was the stimulus for
the jack to vocalize after his rest. Immediately after the jack’s vocalization, the jennies in oestrus
approached the male. Intense teasing usually followed. Vocalization by jennies was more frequent when
they were in induced, as opposed to spontaneous, oestrus. During Trial 2, 12 of 21 jennies vocalized at
least once; 78% of the total number of vocalizations came from jennies in oestrus.
The pattern of vocalization, general and individual teasing and retiring of the male was repeated
several times. Vocalization appeared to attract oestrous jennies to the vicinity. Occasionally, intense teasing
of an individual occurred, usually not culminating in mating with that particular individual. The long,
intermittent periods of sexual stimulation ended with the jack achieving an erection and mating with a
jenny, not necessarily the closest one. Erection and copulation occurred suddenly at the end of an
intermittent ‘rest’ period.
M. Henry et al.
Proximal group and sexually active jennies. A group of jennies stayed
to the jack and expressed
both heterotypical and homotypical sexual behaviour. Heterotypical behaviour included mounting,
herding/chasing, teasing and Flehmen response, which are infrequently observed in mares. At least one
incident of heterotypical behaviour was exhibited by 37 of the 42 (88%) jennies. For Trials 1 and
combined, 169 mounts, 286 herding/chasing episodes, 62 teasing episodes and 99 Flehrnen responses
were observed. Mounting by jennies was more frequent during the second part of each trial when more
jennies were showing ocstms. All mounts during Trial 1 and 52/63 (82%) in Trial 2 involved one jenny
mounting another. During Trial 2,1 jenny mounted the jack 10 times and another jenny mounted the jack
once. Also during Trial 2, 46 of the 52 (88%) mounts involved jennies that were both in oestrus. On 3
occasions, only the jenny that mounted was in oestrus, on 2 occasions only the mounted jenny was in
ocstrus, and on 1 occasion, neither was in oestrus. Most heterotypical herding/chasing episodes occurred
during intense mutual teasing, increasing in frequency when the jack was about to mate a jenny.
Occasionally one jenny prevented the jack from mating another by kicking, biting, striking and pushing
the jack off the jenny.
The proximal group was comprised mostly of jennies in oestrus. During natural oestrus in both
there was a clear separation of the proximal group from the remainder. During induced oestrus, the
division was less distinct. The remaining jennies continued to graze independently in all sectors of the
pasture. All pasture sectors were used by the jennies in a frequency varying from 14% for 1 sector to 75%
for another in Trial 1 and from
in Trial 2.
From Day -8 to Day -3 before ovulation, individual jennies showed an increase in the time spent in the
proximal and sexually active groups, as well as an increase in the number of approaches to the jack (Fig.
2). There was a significant difference among days during the periovulatory period (P c OQOl) in the
number of approaches to the jack and in the sexually active group. Mean number of approaches by the
jennies and by the jack from Day -8 to
were, respectively, 24.7 (3
and 6.3 (9.3) (P < O-001)
and per jenny.
During induced oestrus in both trials, strong relationships were formed between certain jennies who
often approached the jack in pairs even if 1 of them was not in oestrus. Three such pairs were identified in
Trial 1, and 6 in Trial 2; possibly because jennies in Trial 2 were pastured together before the experiment.
With the increased number of oestrous jennies in the sexually active group during induced
number of jennies in the proximal group also increased.
-6 -7 -6 -5 -4 -3 -2 -1
Peri-ovulatory period (days)
2. Frequency (%) in the proximal (m) and sexually active (*) groups and percentage of the total
number of approaches to the jack (
) during the peri-ovulatory period. Day 0 = day of ovulation.
Pasture breeding behaviour
There was no correlation
> O-05) between number of approaches and number of matings per day of
oestrus per jenny; or between the presence of a jenny in the proximal group per day of oestrus and number
of matings. The number of matings per jenny was correlated (r = 0.43,
< O-01) with the frequency of her
presence in the sexually active group.
Compared with horse mares, jennies played an active role in mating. The first sign indicative of
impending sexual receptivity was increased time in the proximal group: some joined this group just a few
hours before showing the first overt signs of oestrus. Some jennies displayed prolonged periods of
alternating sexual receptivity and rejection of the jack. During early oestrus, jennies frequently kicked at
the jacks approach. Kicking gradually decreased during the following days.
Copulatory frequencies and intervals.
The mean interval from introduction of the jack to fast mount
with ejaculation was 39.9
30.4) mm for Jack A and 25.9
17.8) min for Jack B
ejaculatory interval was 88.4
71.5) min for Jack A and 93.3
54.5) min for Jack B
of the activity of the 2 jacks during natural and induced oestrus are given in Table 1.
Total ejaculatory and non-ejaculatory mounts were, respectively, 107 (33%) and 217 (67%) for Jack A,
and 133 (32%) and 286 (68%) for Jack B. The numbers of mounts with and without ejaculation per day
and jenny for Trials 1 and 2 are shown in Fig. 3. Mating frequency was not related to time of day
O-05) for either trial. Matings per jenny per ovulatory oestrus ranged from O-18 in Trials
and 2 together.
individual jennies did not affect the number of mounts without ejaculation or matings per day of ovulatory
> O-05). Of 10 jennies in anovulatory oestrus, 3 were mated, compared to 30 of 32 in ovulatory
oestrus. Also, there were fewer
< O-05) matings per day among jennies in anovulatory oestrus (5/31)
than those in ovulatory oestrus (165/212, Jack A + Jack B). Jack A exhibited fewer mounts without
ejaculation per day of anovulatory oestrus
0.01). Significant correlations were observed between
number of jennies in oestrus and number of matings per day for each jack individually and for both
2 3 4 5 6 7
8 9 101112131415~
2 3 4 5 6 7
Number of mounts with (m) and without (0) ejaculation by Day and Jenny in Trials 1 and 2.
82 M. Henry et al.
Activity of the jacks and mean number of jennies in natural and induced
Natural oestrus Induced oestrus
s.d. Jack A
s.d. Jack B
Jennies in oestrus
0.8 9.5 rt
Mount without erection 14.8
1.6 9.1 iz 1.2
= Days 2-6; induced oestrus
Days 11-16. There was no significant difference in
these measures (P > 0.05) between natural and induced oestrus for both jacks and between jacks for
natural and induced oestrus.
2. Copulatory sequence of 2 jacks mating 21 jennies each in a free-range breeding
Jack ‘A’ Jack ‘B’
Approach-erection* 0.5 1.7 98
Approach-mount* 13 19 98
Mount-ejaculation 19 5.5 98 19 5
Mount-dismount* 25 598
Number of thrusts 5.5 1.4 98
n = number of observations.
*Significant difference between jacks, P < OGOl.
together (P < 0.01; r = 0.61; Jack A and Jack B) and between number of jennies showing oestrus and
different number of jennies mated. (P
0.71; Jack A and Jack B).
A gradual increase in frequency of matings and number of jennies mated per day and per number of
jennies showing oestrus per day, from Days -8 to -2 (of ovulation) was observed (P
O-05; Figs 4 & 5).
For statistical analysis, the data of both trials were combined. Frequency of matings and proportion of
jennies mated among days were compared using combined data for Days -8 to
to -3, -2 to -1 and 0 to
Copulatory sequences for both jacks are shown in Table 2. For Jack A, there was a difference among
jennies in the interval from approach to mount and from approach to ejaculation (P < 0.05). For Jack B,
there were differences among jennies for all measures except the interval from approach to erection and
number of thrusts (P < O-05). Except for the mean number of thrusts, Jack A demonstrated no significant
differences related to day. Jack B showed an effect related to day for all copulatory measures except
number of thrusts (P < 0.05). Throughout the trials, there was no trend toward increasing or decreasing
copulatory performance. There was no effect of time of day on any copulatory measures for either jack (P
> 0.05) except for mean number of thrusts for Jack
(5.5 in the morning, 5.9 in the afternoon; P
Masturbation and spontaneous erection. This behaviour was observed during both trials. Incidences of
masturbation and spontaneous erection, respectively, were 24 and 8 (Jack A) and 56 and 24 (Jack B)
Pasture breeding behaviour of donkeys
-7 -6 -5 -4 -3
Fig. 4. Number of matings per day (0) and per jenny in oestrus (x 10)
ovulatory period. Day 0 = day of ovulation.
Percentage of jennies mated per day (Cl) and per number of jennies showing oestrus per
day (B). Day 0 = day of ovulation.
distributed over the duration of the trials with no discernible pattern.
Territorial social structure
observed in horses (McDonnell, 1986); were not observed in the present study. Infrequently, prolonged
chasing of an individual jenny (almost to exhaustion) was observed.
Jack A and B appeared to have
preferred area of pasture where they rolled; grazed, rested and
84 M. Henry et al.
Seminal characteristics of 2 jacks, before, during and at the end of a free-range breeding
Day Period of
Total no. of
6, 13, 15,
morning 90185 519.3
afternoon 80170 3.5 1.7
* The results are expressed as the mean for the days indicated
groomed, and where most matings occurred. Jacks A and B were within the range of 2 bordering sectors
of the pasture for, respectively, 73 and 65% of the 2-h interval recording times. Notwithstanding the
absence of harem behaviour, the jacks were alone in a given sector only 10% (Jack A) and 1% (Jack B) of
Quality of semen
Semen was collected from Jack A in the morning on Days 1 and 7 and in the afternoon on Days
and 15; and from Jack B in the morning on Days
and 39 and in the afternoon on Days 7,10
and 18. As shown in Table 3, all ejaculates yielded at least
spermatozoa. Morphologies were
evaluated on 4 samples from Jack A and 9 samples from Jack B. The mean (+
abnormal heads, middle pieces and tails were, respectively, 2.6
(It 0.9) for Jack
(2 2.3) for Jack B.
Although the experiment was not designed to evaluate the breeding potential of donkeys, the findings
indicate that donkeys mate efficiently at pasture. All but 2 jennies showing ovulatory oestrus were mated
at least once. Of the 2 that were not mated, one showed overt oestrous behaviour for only 2 days and did
not approach the jack frequently, and the other had foaled recently and kicked the jack vigorously at the
beginning of her period of sexual receptivity. The jack rejected her later when she was in
cycle pregnancy rate was 78% for Jack A (more than 40 days after ovulation) and 69% for Jack B (Day 12
after ovulation), which decreased to 56% with 2 early pregnancy losses. The final pregnancy rates at 40
days after ovulation were 100% and 89% for Jack A and B, respectively. This indicates, as semen quality
suggested at each collection, that high frequency of mating was not detrimental to fertility. Similar findings
were reported for mares mated at pasture under simiiar conditions (Bristol, 1982, 1987) and under farm
and feral conditions (Collery, 1974).
The observations of this study indicate that donkeys have a territorial, non-harem type of sociosexual
organization. The jacks spent most time and did most mating in two contiguous sectors. Jennies moved
freely around all sectors without any apparent concern or interference by the male. On one occasion,
during Trial 1, a second jack approached the fence; the original jack confronted the intruding male but did
not attempt to herd the jennies together.
Pasture breeding behaviour
Another prominent featute was the sexually active group, which showed intense heterotypical and
homotypical behaviour. Gestrous responses appeared similar to those described by Clayton et al. (1981)
for domestic jennies, and by Trumler
for zebras r&d donkeys&. jawing, ears depressed
extended neck, tail slightly~ed.~urinati,onar~ posturing
jack*+ proximal group
pasture-bred maresi(Asa et
but mare&how much less heterotypical
behaviour. In contrast to Klingel’s
1975) studies of wild horses, during which mares assumed the
oestrous stance only when the stallion approached and mounted, jennies in this study exhibited
spontaneous external signs of oestrus and approached the jack frequently. There was a significant
difference between jennies in the number of approaches to the jack and frequency in the proximal and
sexually active groups. Cows in oestrus form a sexually active group, differing from jennies in that they
were extremely mobile and not restricted to the proximity of the male (Blockey, 1978). Another similarity
between cows and jennies was the high frequency of heterotypical behaviour, particularly mounting, a
behaviour rarely observed in mares. Heterotypically active jennies interfered with mating and, as described
in mares (Ginther et al., 1983), their efforts were directed toward the jennies before mounting and toward
the jack during mating.
Vocalization was more pronounced in donkeys than has been reported for horses and appeared to play
a role in initiating pm-copulatory interaction. The jacks
to disregard vocalization between dam
and foal. Vocalization was not noted as a prominent aspect of reproductive behaviour among wild African
and Asiatic asses (Klingel, 1977). In wild plains zebras
quagga), a contact call that serves to keep
the herd members together was noted (Klingel, 1967). Feist & McCullough (1976) suggest that whinnying
in feral horses may be a prelude to aggressive interactions rather than cohesion.
Pasture breeding performance and copulatory behaviour were very efficient for both jacks, although’
their performance during attempts to collect semen was slow and inconsistent, as is observed in hand-
mated or intensely managed jacks. Mounts without erection, as seen in feral horses (Heist & McCullough,
1976), pmceded ejaculatory mounts.
Although the number of different jennies mated per day increased with the number of jennies in
oestms, the jacks still continued to mate individuals up to 6 times per day when the number of jennies in
oestrus per day was at its highest. Preference to mate certain individuals has been suggested in other
studies (Bristol, 1982;
1979) and was observed during Trial 2, when the jack persisted in
attempting to mate a particular jenny that did not promptly accept mounting. A high number of matings of
individual jennies (up to 18) did not appear to be due to preference of the male exclusively: jennies also
appeared to influence the number of times they were mated. There was a positive correlation between
participation in the sexually active group and number of matings per day per jenny. The number of
matings per day of anovulatory oestrus was less than for ovulatory oestrus and mating activity increased
graduahy up to the 2 days before the ovulation. Independent of the participation of the female, the jack
seemed to determine the exact time of mating and the target jenny. This had been reported in stallions
The copulatory behaviour sequence appeared similar to that of stallions. The mean number of thrusts
for stallions was 7 (Asa et al., 1979) compared with 5 in these jacks. Copulation time was about 30 set for
a stallion breeding at pasture (S.M. McDonnell & F. Bristol, unpublished observations) and varied from 25
set for stallions in a semen collection programme (McDonnell, 1986; Pickett et al., 1970). compared
with 25 and 30 set found for the jacks in this study.
We thank A. Vieira de Axemdo Coutinho, veterinarian and farmer, for partial financial
providing the animals and facilities used in Trial 1; H. M. Piedade, veterinary student, for substantial
assistance during Trial 2 and M. Armstrong for assistance with preparation of the manuscript. This work
was supported partly by grants from the National Research Council
86 M. Henry et al.
C.S., Goldfoot, D.A.
Ginther, O.J. (1979)
Sociosexual bebavior and tbe ovulatory cycle of ponies
(Equus caballus) observed in
Blockey, M.A. de B. (1978) The influence of serving
capacity of bulls on herd fertility. J.
Anim. Sci. 46, 589-
Bristol, F. (1982) Breeding behaviour of a stallion at pasture
with 20 mares in synchronized oestrus. J.
Bristol, F. (1987) Fertility of pasture bred mares in
oeshus. J. Reprod. Fert. Suppl.
Clayton, H.M., Lindsay, F.E.F., Forms, A.C.
(1981) Some studies of comparative aspects of sexual
behavibur in ponies and donkeys.
Appl. Anim. Erhol. 7,
Collery, L. (1969) The sexual and social behaviour of the
Collery, L. (1974) Observations of equine animals under
farm and feral conditions.
Equine vet. J. 6,
McCullough, D.R. (1975) Reproduction in
J. Reprod. Fert. Suppl.
McCullough, D.R. (1976) Behavior patterns
and communication in feral horses. Z.
Ginther, O.J. (1983) Sexual behaviour following
introduction of a stallion into a group of mares.
Ginther, OJ, Scraha, S.T.
rates and sexual behavior under pasture breeding
conditions in mares.
Houpt, K. (1984) Reproduction in feral
horses: an eight-year study.
Am. J. vet.
Kenney, R.M., Hurtgen,
Pierson, R., Witherspoon, D.
Simons, J. (1983) Manual for Clinical Ferrility
Society for llteaiogenology.
Klingel, H. (1967) Sosiale organisation und verhalten
ffeilebender steppenzebras. Z.
Klingel H. (1969) Reproduction in the plains zebra,
behaviour and ecological factors.
Reprod. Fert. Suppl.
Klingel, H. (1975) Social organization and reproduction in
J. Reprod. Ferr. Suppl. 23.7-l
Klingel, H. (1977) Observations on social organization and
behaviour of African and Asiatic wild asses
E. hemionus). Z. Tierpsychol.
Klingel, H. (1982) Social organization of feral horses.
Reprod. Ferr. Suppl.
Zimmermann, W. (1988) Social behaviour of
Rzewalski (.Equu.s przewalskii) in the
Cologne xoo and
its consequences for management and housing.
Anim. Behav. Sci.
Kreuchauf, A. (1984) Reproductive physiology in the
jackass. Anim. Res. Devel.
McDonnell, S.M. (1986) Reproductive behavior of the
Vet. Clin. N. Am: Equine Pratt.
Studies on Reproduction in Horses pp.
227-229. Japan Racing Assoc., Tokyo.
B.W., Faulkner, L.C.
Sutherland T.M. (1970)
Effect of month and stallion on seminal characteristics
and sexual behavior.
J. Anim. Sci.
Trumler, E. (1958) Das rossigkeitsgesicht und ahnliches
ansdrucksverhalten bej einhufem.
Verdonck, L. (1976) Radioimmunoassay
dione, dehydroepiandrosterone, 17-hydroxy
progesterone and progesterone and its application to
human male plasma.
J. Steroid biochem. 7,