altered through selection for meat traits, with massive muscle development particularly in the breast.
Although some studies found male displays to be important in mate choice by Ž
females Zuk et al., 1992, 1995; Collins, 1994; Chappell et al., 1997; Leonard and .
Ž Zanette, 1998 , others did not Lill and Wood-Gush, 1965; Zuk et al, 1990c; Ligon and
. Zwartjes, 1995; van Kampen, 1994 . In our previous study, broiler breeder strain males
Ž .
performed less courtship behaviour than laying strain males Millman et al., 1996 . It is also possible that limitations of conformation may make courtship displays by broiler
breeder males less effective in releasing female sexual motivation than displays by laying strain males.
The objectives of this study were to tease apart mechanisms of female mate Ž .
Ž . preference involving 1 male physical characteristics and 2 male behaviour. We also
wanted to determine if sexual experience would affect mate preferences by females. Our hypothesis was that females would discriminate against broiler breeder males.
2. Materials and methods
Procedures carried out in this experiment were approved by the University of Guelph Animal Care Committee, according to the guidelines of the Canadian Council for
Animal Care. 2.1. Treatments
This experiment was conducted as a repeated measures design with two treatments. Ž
. Broiler breeder females Arbor Acres were individually placed in a Y-maze to test mate
preferences. In the first test, females chose between male models of a broiler breeder Ž
. Ž
. strain Ross and a commercial laying strain ISA Brown . Broiler breeder males had
white plumage and laying strain males had buff plumage with some dark brown feathers. In the second test, models were exchanged for a tethered live male of each strain.
Females were tested when mature, but sexually inexperienced, and again after they had been housed with males for 6 weeks.
2.2. Birds Ž
. Hens used in this experiment were mature broiler breeder females Arbor Acres , that
had had no physical or visual experience with males. They had been reared in single-sex Ž
groups of 20–30 individuals according to management guidelines Arbor Acres Farms, .
1995 . At maturity, they were housed in eight 3.35 = 3.35 m pens of 30 individuals, with nest boxes and pine shavings for litter. Visual access between pens was restricted
by plywood partitions, since pairs of males were to be housed with these females in these pens between Inexperienced and Experienced Tests. A standard laying ration
Ž
. Leeson and Summers, 1991
was provided daily at 0900 h, with feed restricted Ž
. according to management guidelines Arbor Acres Farms, 1995 . Target weights were
Ž .
achieved, with mean body weight S.E. for hens being 3.9 0.1 kg just before testing started when they were 47 weeks of age. Eggs were collected daily at 1500 h. A
16L:8D lighting regimen was provided, with lights on at 0500 h and off at 2100 h. Hens were 48 weeks of age when testing began and were 57 weeks of age when testing
concluded. Birds had been wing-banded at 1 week of age for individual identification and three females from each of eight pens were used in the preference tests.
The males used in this experiment had been reared in single-sex groups of 20–30 Ž
. individuals. The broiler breeder males Ross had been reared and maintained with feed
Ž .
restricted according to the management guidelines Ross Breeders, 1997 . The laying Ž
. strain males ISA Brown had also been reared with feed restricted to give a propor-
tional body weight reduction equivalent to the reduction experienced by the broiler Ž
. breeder males. Further details on rearing may be found in Millman and Duncan 2000 .
Males were individually housed at sexual maturity in pens with visual, but no physical access between pens. Males of each strain were housed in separate rooms, with
pens for laying strain males measuring 1.8 = 1.2 m and pens for broiler breeder males measuring 1.5 = 1.5 m. All pens had pine shavings as litter. Males had been used in
Ž .
previous experiments Millman and Duncan, 2000 involving behavioural observations and blood sampling. As a result, males were familiar with being handled and had
received limited sexual experience. Broiler breeder males were feed-restricted according Ž
. to management guidelines Ross Breeders, 1997 . Laying strain males were feed-re-
stricted to give a proportional body weight reduction equivalent to the reduction that should have been experienced by the broiler breeder males. We were able to achieve the
target body weights for the laying strain males, but were less successful in restricting the body weights of our broiler breeder males. Although they were maintained on the
recommended growth curve when group-housed during the rearing phase, broiler breeder males gained weight when they were moved into individual pens. This was
likely due to large variation in body weights during rearing of broiler breeder males, which did not occur with feed-restricted laying strain males. Thus, when individually
housed, underweight broiler breeder males grew, whereas overweight males did not lose weight. Mean body weight of broiler breeder males when testing began at 39 weeks of
age was 5.4 kg, which was 25 above the recommended body weight at that age and
Ž .
similar to the body weight of ad libitum fed males Millman, 1999 . Conversely, laying strain males weighed 2.3 kg at 39 weeks of age when testing began, which was 82 of
Ž .
the body weight of ad libitum fed laying strain males Millman, 1999 . This trend persisted through the experiment and at 57 weeks of age when testing concluded, body
weights were 5.8 kg and 2.6 kg for broiler breeder and laying strain males, respectively.
2.3. Male models Male models were developed according to the method described by Millman and
Ž .
Duncan 2000 . Two mature males of each strain were euthanized and each male was suspended in a sling from dowling rods. Prior to rigor mortis, limbs and head of each
male were adjusted, in the manner of a marionette, into a non-threatening, standing posture and eyes were closed to avoid possible effects of freezing on eyeball structure.
Models were placed in a freezer for 24 h, after which time strings and sling were
Ž removed. Once frozen, models were able to stand alone, without supporting rods Fig.
. 1 . Models were stored in plastic bags in the freezer and were only removed from the
freezer for 1.5–2.0 h periods, to avoid any damaging effects of thawing. As some
Fig. 1. As viewed by the female from the starting box, a broiler breeder strain male model is on the right and a laying strain male model is on the left.
frosting up of comb and wattles occurred immediately after removal from the freezer, tests were not initiated until 5–10 min had passed, by which time these fleshy
appendages thawed enough to appear reddish-pink and life-like. Half of the females Ž
. tested per day were exposed to one pair of models Pair A , after which models were
returned to the freezer and the remaining females were tested with the other pair of Ž
. models Pair B .
2.4. Testing procedure Tests were conducted from 1700 to 2100 h, as sexual behaviour has been shown to be
Ž .
most frequent late in the day Lake and Wood-Gush, 1956 . Three females in each of eight home-pens were tested. Four females were tested per day and randomized so that
females from the same home-pen were not tested on the same day. Each female was first tested with male models, after which models were replaced with live, tethered males.
Ž .
Male strains were randomly assigned to compartment A or B of the Y-maze Fig. 2 , balanced across females, and each female was presented with male strains in the same
compartments in all tests. Similarly, each female was always allowed to choose between the same individual live males and models in all tests. Live males were only used in one
test per day and were always presented as the same pair of individuals, so that Laying strain male 1 was always paired with Broiler Breeder strain male 1. Eight males of each
strain were used, with each male pair used in tests with three females from different home-pens.
Ž .
Our Y-maze and testing procedure was modified from that of Zuk et al. 1990a . The female to be tested was removed from the home-pen and carried in an upright position
to the start box. A plywood partition was used to obscure the view of the test female
Fig. 2. Schematic diagram of the Y-maze used during preference tests. Solid lines represent wooden partitions. Dashed line represents the marker, delineating compartment of each male. Dotted line represents the gate,
releasing female from the starting box.
while models were placed in the Y-maze. Models were placed centrally over the rings, which secured the tether in the live male tests and were oriented to face the start box.
The plywood partition was removed allowing the female to observe the models for 2 min, after which the starting gate was raised by the observer hidden behind the female.
Following testing with male models, the female was caught, returned to the start box and the wooden partition replaced to obscure her view of the compartments. Male
models were replaced with live males. Live males were gently carried in an upright position from their home-pens and were secured to tethers using black Velcro leg-bands.
As compartments were separated by a wooden barrier, males were not able to see each other during the test. Care was taken to obscure visual contact between males when
placing them in the compartments and after both males were tethered, 1 min was allowed for males to acclimitize to the environment before the wooden partition was
removed, exposing them to the test female. The female was able to observe the males for 2 min and then the starting gate was raised to release her.
Direct observations were taken with the observer behind a wooden partition, for 5 min during tests with male models and for 15 min in tests with live males. A female was
considered in residence with a male when she entered the 1 = 1-m compartment in which the male was tethered. The male that was first approached by the female was
recorded as were the number of 30-s time periods the female spent in residence with each male. In addition, solicitations performed by the female and copulations were noted
when they occurred. A solicitation was defined as the female approaching the male and
crouching, prior to courtship displays by the male. A copulation was defined as the male mounting, gripping and treading the female and appearing to achieve cloacal contact.
In an attempt to decrease the confounding variable of fearfulness during testing, birds were acclimatized to the test conditions. At 1 week prior to testing, males were
individually released in the test pen for 30 min. The following day, males were tethered for 15 min in each side of the Y-maze. Males had been handled frequently in previous
experiments and displayed little fearfulness or alarm in the test surroundings. They explored the test pen thoroughly and spent the remainder of the time foraging. Crowing
and wing-flapping were often performed as males could hear, but not see, males in other rooms of the barn. The males adjusted to tethers rapidly and they rarely pulled against
the tether.
Females were less accustomed to being handled than the males and had no experience outside their home-pen. A trial run of the test, using surplus birds, indicated that females
remained tonic in the starting box and were extremely fearful. Hence, the three test females from each home-pen were placed together in the start box and released
simultaneously into the Y-maze for 30 min. The presence of social companions successfully facilitated exploratory behaviour and appeared to reduce fearfulness in the
females. The following day, the procedure was repeated for females individually and again during the morning of the day they were tested.
After all females had been tested once with models and live males, pairs of broiler breeder or laying strain males were randomly placed in each of the female home-pens.
As males have been shown to maintain social relationships with only visual and auditory Ž
. contact Mench and Ottinger, 1991 , we paired males with their immediate neighbour in
an attempt to decrease fighting. Females were not housed with the individual males they were exposed to during preference tests. Males were fed from raised hoppers to maintain
feed-restriction and female feeders contained male exclusion grills. Although the head width of laying strain males was small enough to pass through the exclusion grill, no
males in this experiment had their combs dubbed and large combs restricted access to the female feeder. After 6 weeks, males were returned to their individual pens.
Preference testing commenced 2 weeks later, with birds familiarized to the test pen during the morning of the day they were tested.
Two weeks following testing, all males were euthanized. Males were weighed and comb length was measured immediately by placing the comb on a ruler, laterally and
recording the distance from the front tip to the back tip, in millimeters. Males were autopsied and examined for evidence of abnormalities, such as deep pectoral myopathy
and abnormalities of the hock cartilage, which might impede movement or be suggestive of pain.
2.5. Data analysis Our experiment involved a repeated measures design with 24 females tested before
and after receiving sexual experience. Test results included three possible outcomes; broiler breeder strain preferred, laying strain preferred, or neither strain preferred. As
nominal data were involved, McNemar’s test for significant changes was used to determine if sexual experience affected the choices females made. Within Inexperienced
and Experienced groups, differences in the responses of females were determined using
Ž .
a Chi-Squared Goodness-of-Fit test Zar, 1984 . Since we could not be sure of the motivation of females preferring neither strain, results were further examined with
females choosing neither strain removed from the analysis to determine male strain bias among the females that made a choice.
3. Results
