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Nipple Preference and Contestsin Suckling Kittens of theDomestic Cat Are Unrelated toPresumed Nipple Quality
Robyn Hudson1
Gina Raihani1
Daniel Gonzalez1
Amando Bautista2
Hans Distel3
1Instituto de Investigaciones BiomedicasUniversidad Nacional Autonoma de Mexico
Mexico, DF, MexicoE-mail: [email protected]
2Centro Tlaxcala de Biologıa de la ConductaUniversidad Autonoma de
Tlaxcala-Universidad Nacional Autonomade Mexico, Tlaxcala, Mexico
3Institut fur Medizinische PsychologieUniversitat Munchen, Munchen, Germany
ABSTRACT: We studied the development of suckling behavior and weight gain in11 litters (52 kittens) of free-ranging domestic cats until postnatal day 28 just beforethe start of weaning. In six of these litters, we also recorded milk intake and contestsfor access to nipples. Already within 12 hr of birth kittens showed a preference forposterior nipples, and by postnatal day 3 each had developed a preference forparticular nipples. In fact, 86% of kittens used one particular nipple most often, andeven when the mother changed the side she lay on to nurse. Contests for access tonipples occurred throughout the study period at an average rate of one to twocontests per kitten per hour of nursing. Contrary to suggestions in the literature thatkittens compete for more productive nipples, we found no relation between kittens’use of particular nipples and their weight gain, milk intake, or involvement incontests during suckling. We suggest that kittens’ preference for posterior nipples aswell as their establishment of an individual ‘‘teat order’’ might function to optimizethe number of nipples remaining productive across lactation, and to reduceenergetically costly scrambles and potentially injurious contests among littermates.� 2009 Wiley Periodicals, Inc. Dev Psychobiol 51: 322–332, 2009.
Keywords: sibling competition; teat order; suckling performance; aggression;domestic cat; Felis silvestris catus
INTRODUCTION
Most mammalian young grow up in the company of same
or different age sibs or half-sibs, and there is increasing
evidence that relations among them can significantly
influence individual development and survival (review in
Hudson & Trillmich, 2008). Emphasis to date has been on
competition for limited resources, and particularly for the
mother’s milk (Conley, 2004; Drummond, 2006; Mock &
Parker, 1997; Sulloway, 2001; but see Bautista, Drum-
mond, Martınez-Gomez, & Hudson, 2003; Hudson &
Trillmich, 2008; Rodel, Hudson, & von Holst, 2008 for
a consideration of positive effects of sibling presence).
Energetically, lactation is the most costly phase of a
female mammal’s reproductive cycle (Loudon & Racey,
1987) and milk, vital for the survival of all newborn
mammals, often represents a limited resource for which
siblings must compete. Evidence for this comes from
species as diverse as domestic pigs (review in Drake,
Fraser, & Weary, 2008), spotted hyenas (Hofer &
East, 2008; White, 2008), wild and domestic European
rabbits (Bautista, Mendoza-Degante, Coureaud, Martı-
nez-Gomez, & Hudson, 2005; Drummond, Vazquez,
Sanchez-Colon, Martınez-Gomez, & Hudson, 2000;
Rodel, Hudson, & von Holst, 2008; Rodel, Prager,
Stefanski, von Holst, & Hudson, 2008), Galapagos fur
seals and sea lions (Trillmich & Wolf, 2008), and various
rodent species (e.g., Fey & Trillmich, 2008; McGuire,
1998; McGuire & Sullivan, 2001; Rodel, Prager, Stefanski,
von Holst, & Hudson, 2008; review in Mendl, 1988).
However, while these studies provide valuable infor-
mation on the manner in which the young compete for
Received 27 November 2008; Accepted 3 February 2009Correspondence to: R. HudsonContract grant sponsor: CONACyTContract grant number: 48692-QContract grant sponsor: DGAPAContract grant number: IN229907Published online 3 March 2009 in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/dev.20371
� 2009 Wiley Periodicals, Inc.
access to nipples—by overtly aggressive contests in pigs,
hyenas, and seals, and by push-and-shove scramble in
rabbits and guinea pigs—little is known about how
mothers allocate milk across their multiple mammary
glands or about the possible consequences of this for
differential development of the young. Generally, it is
assumed that milk production across the glands is not
equal, that some nipples or teats therefore represent better
quality outlets than others, and that particular young,
usually the heaviest, out-compete their sibs in gaining
access to and monopolizing these (reviews in Drummond,
2006; Hudson & Trillmich, 2008). However, direct
evidence for this remains scarce (Drake et al., 2008;
McGuire, 1998).
The domestic cat (Felis silvestris catus) provides a
particularly good opportunity to examine such questions.
It can be easily observed, handled, and bred under free-
ranging, semi-natural conditions, as one of the world’s
most popular mammalian pets it is universally available,
and as a significant species in biomedical research much
is known about its anatomy, genetics, physiology, and
behavior, including during development (e.g., Leyhausen,
1982; Turner & Bateson, 2000). There have been,
however, few studies of kittens’ suckling behavior (Blass,
Shuleikina-Turpaeva, & Luschekin, 1988; Kovach &
Kling, 1967; Larson & Stein, 1984; Rosenblatt, 1971,
1972; review in Mermet, Coureaud, McGrane, & Schaal,
2008), and almost none outside the laboratory in
unmanipulated conditions. An exception, and the starting
point for the present study, is the pioneering work of Ewer
(1959, 1961).
Based on detailed observations of two litters of
house cats, Ewer (1959) concluded that kittens establish
individual use of particular nipples within a few days of
birth, that they preferentially use the most posterior rows,
and (speculatively) that these are the most productive
and represent a superior resource for which littermates
aggressively compete. However, only two litters from two
mothers were studied, each litter containing four kittens,
and the suggestion that rear nipples are more productive
was not actually tested.
It was therefore our aim to re-investigate and extend
Ewer’s findings by studying the development of nipple use
in a larger sample of kittens from birth to the start of
weaning at 4 weeks, and like Ewer, in animals kept in
private homes and under free-ranging conditions. Con-
sistent with Ewer, we found that kittens show a gradient in
nipple use from anterior (least used) to posterior (most
used), and that littermates rapidly establish individual use
of particular nipples (nipple ‘‘ownership’’). However,
already in the course of the study it became clear that the
data did not support the proposition that rear nipples are
more productive, that is, that kittens using these show
greater weight gain. We therefore decided to obtain
additional information from the last six litters we studied
on the competition of kittens for access to and actual milk
intake from individual nipples.
METHODS
Animals
We collected data from 52 kittens in 11 litters from seven cross-
bred mothers kept as pets in three private homes. Five were first
litters, five were second, and one was a third litter (Tab. 1). The
mean size of first litters was somewhat smaller (4.2, SD
1.2 kittens) than second or third litters (5.2, SD 1.0 kitten). The
cats were free to mate with local free-ranging males, which from
direct observations of mating and differences in location were
different individuals. Mothers were fed daily dried and canned
commercial cat food and fresh meat and fish. Milk, water, dried
food and litter trays were always available. Mothers shared the
three homes with other intact and neutered male and female cats
and were free to leave the house at will. All kittens survived the
4 weeks of the study, and at about 8 weeks of age were given
away as pets with the help of local veterinarians.
Procedures
Females always gave birth in the house, and in five cases in the
presence of the first author, allowing close observation of
parturition and immediate postpartum behavior of mothers and
kittens. If the location was not suitable for observation, the
kittens were transferred, depending on the household, to an oval
wicker cat basket or to a foam rubber bed (area for the litter
70 cm� 40 cm), both lined with flannel and located in a quiet
part of the house such as a cupboard or spare room allowing a
clear view of the litter during nursing. Eleven kittens (from five
litters) that could not be readily distinguished by coat color were
marked on the head, nape, or back with white correcting fluid in
the case of dark kittens, and with gentian violet spray in the case
of white or ginger kittens. This did not appear to affect the
behavior of either mothers or young, and the daily weight gain
of marked kittens did not differ significantly from that of 11 of
their randomly selected, unmarked littermates (unpaired t-test:
t22¼ 1.25, p¼ 0.22).
The time of birth was recorded in six litters (Tab. 1) and
arbitrarily defined as midnight in the other five litters. The
first day of postnatal life (day 1) was defined as 24� 12 hr after
the time of birth; subsequent days were defined accordingly.
Nine litters (n¼ 41 kittens) were observed on day 0, 6 of the
litters several times a day, 10 litters (n¼ 47 kittens) from day 1,
and all 11 litters (n¼ 52 kittens) from day 3. Kittens were
weighed individually to the nearest 0.1 g on an electronic balance
once a day when possible. On average, 35.7 weight measure-
ments were taken until the end of the study period on
postnatal day 28. Furthermore, to estimate milk intake, in the
last six litters, kittens were weighed before and after observation
sessions (Tab. 1). We were initially reluctant to disturb kittens by
weighing them soon after birth, and only after day 2 were all
litters regularly weighed. We therefore refer to kittens’ weight
Developmental Psychobiology Nipple Preference in Kittens of the Domestic Cat 323
on day 3 as their starting weight. Similarly, after day 16, missing
data did not allow us to compare weights across all 11 litters. We
therefore based estimates of kitten growth on their weight gain
during days 3–16.
When circumstances permitted, litters were observed at least
once per day, and when not interrupted by the mother leaving,
for 30 min per session. This resulted in a mean observation
time per litter of 15.7 hr distributed across a mean of 23.3
observation days (Tab. 1). Litters were observed directly and
behavior recorded using continuous scan sampling. This was
possible because kittens usually remained attached to the same
nipple for several minutes or more. Three observers participated
in data collection (Tab. 1).
Throughout the study, animals were kept and treated
according to the guidelines for the care and use of animals in
research of the Instituto de Investigaciones Biomedicas,
Universidad Nacional Autonoma de Mexico, and according to
the current laws of Mexico.
Behaviors Recorded
For each observation session, in addition to the date, time of day,
identity and location of the litter, and identity of the observer, we
recorded the following behaviors:
Mother’s posture: Whether during nursing the mother was
lying on her right or left side. All mothers always nursed lying on
their side, and all used either side.
Nipple use: Mothers had four symmetrical pairs of nipples,
which we labeled from anterior to posterior 1–4, and whether the
nipples were left or right. We recorded to the nearest minute
the time at which kittens attached to particular nipples and
when they left them. From this we calculated the frequency of
attachment to and time each kitten spent on each nipple. As
kittens often spent long periods attached to nipples without
visibly sucking, we refer to nipple attachment rather than to
suckling.
Contest behavior: For the last six litters (Tab. 1) we also
recorded all incidents of unattached kittens pushing with
their head and muzzle against the muzzle of attached kittens,
apparently trying to dislodge them. Incidents of one or both
kittens swiping with their forepaws, usually at the head region of
the other kitten, sometimes accompanied by vocalization, were
recorded separately as contests of higher intensity. We refer to
the two kittens involved in a given contest as the pusher and the
defender kitten, and we recorded whether the displacement
attempt of the pusher kitten was successful, and whether it
subsequently attached to the nipple. We also recorded the
identity of the nipple to which the defender kitten was attached
and at which the pusher kitten appeared to aim.
Data Treatment and Analysis
Because of differences in litter sizes, number of observation
sessions, and total observation time across litters (Tab. 1), we
needed to normalize and reduce the data to comparable values.
For this, we first calculated for each kitten and day the sum of
attachments and attachment durations on each nipple, the
number and outcome of contests per nipple and per hour of
Developmental PsychobiologyT
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324 Hudson et al.
nursing, and the daily average of weight measurements. We then
calculated for each kitten the average of these measures for six
postnatal time bins: 0–12 hr, 24� 12 hr, 48� 12 hr, days 3–
8, days 9–18, and days 19–28. Occasionally, we combined the
first and last three bins to one bin (0–60 hr and days 3–28,
respectively), and regularly, we calculated for each kitten
and day value an overall average for the entire study period
(days 0–28).
Since in mammals growth of littermates generally depends
on litter size (review in Rodel, Prager, Stefanski, von Holst, &
Hudson, 2008), we compared intra-litter differences in starting
weight (day 3) or weight gain (days 3–16) by calculating for
each kitten its percent difference as aþ or� value from the litter
mean. To estimate kittens’ milk intake from particular nipples,
we calculated the difference in body weight before and after
observation sessions. We then selected sessions in which kittens
had increased their weight and noted the nipples they had been
attached to. Further, to be certain about the source of weight gain,
we discarded all records in which kittens attached to more than
one nipple, or in which mothers licked kittens’ anogenital region,
possibly inducing them to urinate.
We analyzed the data in two ways, first by comparing values
across nipple rows, and second by comparing values according to
nipple rank, defined for each kitten as the percent of its total
attachment time it spent on particular individual nipples. To this
end, we ranked for the entire study period for each kitten its total
attachment times to nipples twice, when the mother was lying on
her right side and when she was lying on her left side.
Graphical Representations and Statistics
For this we used the statistics program GraphPad Prism 5
(GraphPad Software, Inc). Since litter averages were the
statistical unit in most cases, and since the number of litters
was small, the data are depicted throughout as vertical dot plots
with mean values as horizontal bars. To compare the results
across nipple rows and ranks, we checked first for normality
of the data using the Kolmogorov–Smirnov test and then,
depending on the outcome, calculated parametric repeated
measures ANOVAs or Friedman nonparametric repeated
measures ANOVAs. However, we have refrained from reporting
post hoc tests since the variance of the data and differences
between rows and ranks or, in many cases, the lack of difference,
can be fully appreciated from the graphical representations. The
statistical tests reported were two-tailed and we took p< 0.05 as
the level of significance.
RESULTS
Behavior of Mothers and Kittens
Mothers gave birth and raised their young to weaning
without apparent difficulty. The five births that were
directly observed followed the same general pattern.
Kittens were born at approximately 10–20 min intervals.
Once parturition was completed, mothers lay quietly on
their side in the typical nursing posture, often maintaining
it for hours, and only occasionally leaving the kittens for a
few minutes during the first 2 or 3 days to feed or use the
litter tray. Mothers then spent progressively less time in
direct contact with their young, and by the end of the study
period, just before the age when the kittens started to feed
independently, spent several hours at a time away from the
litter, often out of the house. At no time was any behavior
observed on the part of mothers that could be interpreted
as guiding kittens to nipples, nor were mothers ever seen
to respond in any way to even the most vigorous and vocal
struggles between kittens.
In the case of the five births that were observed, the
kittens (n¼ 23) crawled around the moment they were
born and usually started nuzzling the mother’s ventrum
within minutes. Although often displaced by the mother’s
licking and her postural changes, some kittens attached
to nipples while parturition was still in progress. Once
parturition had finished and the mother had adopted a
near-motionless nursing posture, kittens usually attached
within a few minutes. Time spent on any one nipple was at
first relatively long, but decreased as kittens became more
active. Thus, during the first 12 hr individual nipple
attachments lasted a mean of 13.1 min, during the next
24 hr a mean of 10.9 min, and subsequently declined more
slowly to means of 7.1, 6.5, 6.4, and 5.9 min on days 2, 3–
8, 9–18, and 19–28, respectively (n¼ 8 litters, repeated
measures ANOVA: F6,8¼ 6.6, p< 0.001).
When mothers returned to the nest and adopted the
nursing posture, kittens usually attached to nipples within
less than a minute. After some minutes they started to suck
in a strong, rhythmic manner, accompanied by rhythmic
backward–forward movements of their ears, presumably
coinciding with milk letdown. Approximately 15–25 s
later they stopped sucking and usually dropped from the
nipples within a few seconds (McVittie, 1978 for similar
observations in snow leopards). Weighing kittens imme-
diately after such episodes showed that they obtained
around 2–5 g of milk each, and older kittens up to 10 g.
Kittens started to open their eyes between days 9 and
10, and to engage in rough-and-tumble play, to climb the
walls of the nest, and occasionally to leave it towards the
end of the fourth week at the end of the study period.
Gradient in Nipple Use
A clear pattern of preferential use of posterior nipples
was already seen soon after birth. Thus, during the
first 12 hr kittens spent on average 7.7% of their total
attachment time on nipples of row 1 compared to 18.9%,
28.3%, and 27.6% of their time on nipples of rows 2, 3, and
4, respectively (Friedman ANOVA: Fr4,9¼ 9.0, p< 0.05;
Fig. 1, left panel). The preference for posterior nipples
became even more pronounced with age (24� 12 hr:
Fr4,10 ¼ 13.1, p< 0.01; 48� 12 hr: Fr4,10 ¼ 19.6,
Developmental Psychobiology Nipple Preference in Kittens of the Domestic Cat 325
p< 0.001). On days 3–28, kittens spent on average 4.6%
of their attachment time on nipples of row 1 compared to
19.4%, 36.4%, and 39.4% of their attachment time on
nipples of rows 2, 3, and 4, respectively (days 3–28;
Fr4,11 ¼ 26.9, p< 0.0001; Fig. 1, right panel).
Individuals’ Use of Particular Nipples
In contrast to row preference, preferential use of particular
nipples was not present at birth but developed during the
next 3 days. Thus, during the first 12 hr kittens spent on
average 13.0% and 14.8% of their total attachment time on
the nipple which later became their most used nipple (rank
1) and their second most used nipple (rank 2), respectively,
corresponding to random use of any one of the eight
nipples at their disposal (¼ 12.5%). In addition, they spent
on average 31.2% of their total attachment time on one or
more of the six other nipples which they later used the
least (F3,9¼ 2.17, p> 0.14; Fig. 2, left panel). During
the two subsequent 24-hr periods kittens spent 31.4%
and 52.5%, respectively, of their total attachment time on
their rank 1 nipple (n¼ 10 litters). By days 3–8, mean
attachment time to rank 1 nipples increased to 77.6%.
Repeated measures ANOVAs showed significant differ-
ences across nipple ranks to be already developing during
the 24� 12 and 48� 12 hr periods (F3,10 ¼ 5.5, p< 0.05
and F3,10¼ 11.2, p< 0.001, respectively), reaching a
maximum by days 3–8 (F3,11¼ 84.1, p< 0.0001).
When mothers changed their nursing posture, lying
down either on their left or their right side, 43 of the 52
kittens (82.6%) continued to attach to their same rank 1
nipple (binomial test: p< 0.0001). Of the kittens that
changed their preferred nipple with the change in their
mother’s posture, seven used the opposite nipple, and two
a neighboring nipple (Tab. 2, litters 5, 8, 9, and 11). Thus,
maintenance of nipple preference was not perfect, and as
ranking of nipples by attachment time showed, kittens
attached to the rank 2 nipple on average 20.5% of their
total attachment time. Nevertheless, attachment to other
Developmental Psychobiology
FIGURE 1 Nipple row preference across age. Differential use
of nipple rows was little affected by age as the pattern remained
the same whether kittens were observed soon after birth
(left panel) or after the second postnatal day (right panel). Dots
give the average kitten values for each litter, and horizontal bars
represent means; 0–12 hr: n¼ 9 litters; days 3–28: n¼ 11 litters.
FIGURE 2 Development of individual nipple use. Differential
use of particular nipples ranked for each individual according to
sums of attachment time was not seen immediately after birth
(left panel) but was evident after the second postnatal day (right
panel). Ranks refer to the nipples which subsequently became
each kitten’s most used and second most used nipple. Dots give
the average kitten values for each litter, and horizontal bars
represent means; 0–12 hr: n¼ 9 litters; days 3–28: n¼ 11 litters.
326 Hudson et al.
nipples was minimal (on average 9.3% of total
attachment time) (days 3–28, F3,11¼ 56.7, p< 0.0001;
Fig. 2, right panel).
Development of Contest Behavior
From birth, kittens in all litters engaged in contests by
pushing at the head and muzzle region of littermates
attached to nipples, apparently trying to dislodge them.
Taking litters as a whole, the overall frequency of suckling
contests for the last six closely observed litters was rather
high—on average 7.9 contests per litter per hour of
nursing (Tab. 1). Considering the rate for individual
kittens across development, during the first 60 postnatal
hours the frequency of contests was somewhat lower (0.89
per hour of nursing per kitten) but not significantly so, than
during the subsequent periods (2.0, 1.6, and 2.0 contests
per hour of nursing for days 3–8, 9–18, and 19–28,
respectively; F4,6¼ 1.4, p> 0.27; Fig. 3).
In contests of higher intensity both the pusher and
defender kittens engaged in paw swiping, vocalizing
loudly during particularly vigorous interactions. In six
litters one or more kittens had an infected eye before eye
opening, which from the visible scratch marks, was
apparently the result of such swipes. Although all kittens
recovered without treatment, opening of the infected eye
was sometimes delayed by several days. It was notable
that paw swiping was only observed during suckling
contests. Pushing, however, also occurred when kittens
were huddled together or otherwise interacting in the nest.
Contests of higher intensity contributed about 35% to all
contests observed, but the variance of the data was too
great across litters and time bins to detect any consistent
developmental changes in this (Friedman ANOVA:
Fr4,6 ¼ 2.6, p> 0.51).
Contest Frequency in Relation to Nipple Rowand Nipple Rank
Comparing the number of contests across nipple rows
showed that fewer contests occurred on anterior than on
posterior nipples, that is, only 0.5% of the 834 contests
Developmental Psychobiology
Table 2. Location of Nipples of Rank 1 According to Litter Size
Litter no. Nipple side Row 1 Row 2 Row 3 Row 4
Litter of three kittens
2 Left 1fa
Right 2f 3m
11 Left 1m 2f
Right 3f, 2f
Litter of four kittens
3 Left 1m
Right 2m 3m 4m
9 Left 1f, 2f 3f
Right 2f, 1f 4m, 3f
Litter of five kittens
5 Left 1m 2f, 3f 3f 4m
Right 5m 4m
6 Left 1m 2m 3m
Right 4f 5m
7 Left 1f 2m
Right 3f 4m 5m
8 Left 1f 2f 3f, 4m 4m, 5m
Right 3f 5m
Litter of six kittens
1 Left 1f 2f 3f, 4m
Right 5m 6m
4 Left 1m 2f 3m
Right 4f 5m 6f
10 Left 1f 2m 3f
Right 4f 5f 6m
No. of kittens Left 2 6.5 10.5 8.5
Right 1 4 9.5 10
aIndividual kittens are numbered; f, females, m, males. Kittens that attached to different nipples when the
mother changed her nursing posture are represented twice.
Nipple Preference in Kittens of the Domestic Cat 327
recorded during the entire observation period took place
on row 1 compared to 15.4%, 42.2%, and 40.8% on rows
2, 3, and 4, respectively (Friedman ANOVA across rows 2,
3, and 4: Fr3,6¼ 9.0, p< 0.01). However, this was due to
the fact that kittens approached and attached to anterior
nipples less frequently than to posterior nipples. Thus,
when for pusher kittens the number of contests for nipples
of a given row was calculated according to the number of
attachments to these nipples, row differences largely
disappeared (F3,6¼ 1.9, p> 0.20; Fig. 4, left panel),
suggesting little effect of nipple position on contest
behavior.
Similar results were obtained in relation to nipple rank.
Pusher kittens contested other kittens more frequently
when these were attached to the pusher’s rank 1 nipple,
less frequently when these were attached to the pusher’s
rank 2 nipple, and least when these were attached to other
nipples (F3,6 ¼ 7.1, p< 0.05). However, taking into
account the differential use of nipples of pusher kittens
by dividing the number of contests for nipples of different
rank by the number of pushers’ attachments to these
nipples it became apparent that the relative frequency of
contests was not determined by nipple rank (Friedman
ANOVA: Fr3,6 ¼ 1.9, p> 0.20; Fig. 4, right panel).
Outcome of Contests in Relation to Nipple Rank
In about half of all contests (46.9%), pusher kittens
displaced other kittens from the nipple these were
attached to, but surprisingly, in only 57.1% of all
successful displacements pusher kittens then attached to
the nipple which was now available. Nevertheless, when
the distribution of successful displacements was com-
pared across nipple ranks it was apparent that pusher
kittens tended to be on average more successful in
displacing kittens from their own rank 1 nipple (65.6% of
contests) than from their rank 2 nipple (41.8% of contests)
or from other nipples (46.6% of contests) (F3,6¼ 5.7,
p< 0.05; Fig. 5, left panel). Furthermore, after success-
fully displacing the defender kitten, pushers attached on
average significantly more often to their own rank 1 nipple
(76.1%), than to their rank 2 nipple (48.3%), or to other
nipples (35.5%) (Friedman ANOVA: Fr3,6¼ 9.3,
p< 0.001). Conversely, kittens were also on average
more successful in defending their rank 1 and rank 2
nipples (62.8% and 54.0% of contests, respectively)
compared to other nipples (29.4% of contests) (F3,6¼ 8.4,
p< 0.001; Fig. 5, right panel).
Developmental Psychobiology
FIGURE 4 Frequency of contests per attachment according to
nipple row and nipple rank of pusher kittens. The frequency of
contests was largely dependent on nipple use (see text), and
differences were not apparent after dividing the number of
contests by the number of attachments observed for each row
(left panel) and for each nipple rank (right panel). Kittens rarely
attached to nipples in row 1 and consequently contests were so
rare that the data have been omitted. Dots give the average kitten
values for each litter, and horizontal bars represent means; days
0–28 for rows and days 3–28 for ranks; n¼ 6 litters.
FIGURE 3 Contest frequency across age. Contest frequency
per hour of nursing was rather similar across age with the
possible exception of lower frequencies during the first 60 hr.
Dots give the average kitten values for each litter, and horizontal
bars represent means; n¼ 6 litters.
328 Hudson et al.
Body Weight and Growth in Relation toNipple Use
Mean weight of kittens on day 3 (starting weight) was
124.6, SD 42.2 g, and mean daily weight gain from days 3
to 16 was 12.1, SD 1.7 g (n¼ 11 litters). Mean starting
weight, and by inference birth weight, was negatively
correlated with litter size (Spearman rank correlation
coefficient: rs¼�0.65, n¼ 11, p< 0.05), as was mean
daily weight gain (rs¼�0.71, n¼ 11, p< 0.05). Within
litters, the starting weight of individual kittens differed
from the litter mean by as much as �14.8% and þ12.6%,
and weight gain by as much as �19.3% and þ11.8%.
Despite such large intra-litter differences, we found
little evidence that heavier kittens preferentially estab-
lished their rank 1 nipples on posterior rows, or that kittens
which predominantly attached to more posterior nipples
grew better than kittens which predominantly attached to
anterior nipples (Fig. 6). Thus, whether kittens mainly
used nipples on rows 2, 3, or 4, the intra-litter differences
in starting weight (�0.7%, 1.0% and 1.2%, respectively)
did not deviate significantly from 0%, the litter mean
(Wilcoxon signed ranks tests: p> 0.43, p< 0.76,
p< 0.17, respectively), nor were kittens which mainly
used row 2 nipples lighter than kittens which mainly
used row 4 nipples (Mann–Whitney test: U7,11¼ 32.0,
p> 0.58; Fig. 6, left panel). Likewise, intra-litter differ-
ences in weight gain (row 2: �2.2%, row 3: �1.7%,
and row 4: 0.9%) did not deviate significantly from
0% (Wilcoxon signed ranks tests: p> 0.31, p> 0.57,
p> 0.32, respectively), nor were nipples of row 2 inferior
to nipples of row 4 in supporting growth (Mann–Whitney
test: U7,11¼ 23.0, p> 0.17; Fig. 6, right panel).
Developmental Psychobiology
FIGURE 6 Intra-litter differences in starting weight and
weight gain in relation to the location of kittens’ rank 1 nipples.
Initial body weight on day 3 had little effect on kittens’ choice of
nipple row when establishing nipple preference (left panel). In
addition, nipple row had little or no effect on kittens’ weight gain
from days 3 to 16. Rank 1 nipples were rarely established in row 1
(Tab. 2) and data have therefore been omitted. Dots give the
average kitten values for each litter and horizontal bars represent
means; n¼ 11 litters.
FIGURE 5 Frequency of successful displacements or
defenses according to nipple rank of pusher or defender kittens,
respectively. Kittens were more successful in displacing other
kittens from their rank 1 nipple than from nipples of lower ranks
(left panel). Conversely, defender kittens were more successful
in retaining their rank 1 or rank 2 nipples compared to other
nipples (right panel). Dots give the average kitten values for
each litter, and horizontal bars represent means; days 3–28;
n¼ 6 litters.
Nipple Preference in Kittens of the Domestic Cat 329
Milk Intake in Relation to Nipple Use
Milk intake, estimated by kittens’ weight increase during
observation sessions, was quite varied. One or more
kittens obtained milk in about 83.2% of nursing sessions,
but in only 21.5% of these did all kittens obtain milk.
Thus, on average in every second session (49.4%) each
kitten received milk. Nevertheless, intake appeared to be
rather uniform and only to be affected by age. During the
first eight postnatal days kittens received on average 1.4 g
at each intake, from days 9–18 on average 2.5 g, and
from days 19 to 28 on average 4.0 g (F3,6¼ 4.6, p< 0.05).
When milk intake on days 0–28 was compared across
nipple rows, the average amount of milk kittens obtained
on row 2 (1.6 g) tended to be smaller than on rows 3 and 4
(2.6 and 2.7 g, respectively). However, differences across
rows were not significant (F3,6¼ 1.6, p> 0.25) suggesting
that kittens could obtain similar amounts of milk from
any row and nipple, irrespective of the location (Fig. 7,
left panel). Similarly, milk intake was only marginally
affected by nipple rank (on average 2.8, 2.7, and 2.3 g from
nipples of rank 1, 2, and other ranks, respectively), and
differences across nipple ranks were not significant
(F3,6¼ 0.6, p> 0.58; Fig. 7, right panel).
DISCUSSION
The present study confirms previous reports that newborn
kittens of the domestic cat preferentially suckle posterior
nipples, that they rapidly establish a ‘‘teat order’’ in which
each kitten predominantly uses one or sometimes two
particular nipples, that this order is maintained even after
eye opening, and that vigorous contests often occur at
nipples during suckling (Ewer, 1959, 1961; McVittie,
1978; Pfeifer, 1980; Rosenblatt, 1971, 1972 for similar
behavior in snow leopard and mountain lion cubs,
respectively). Unexpectedly, however, our findings do
not support the suggestion (Ewer, 1959) that in the cat
the posterior nipples are the most productive, or that
particular (e.g., high birth weight) kittens are more
successful in obtaining and defending these (McVittie,
1978 for a similar finding in snow leopards). Specifically,
we found no evidence for differential nipple quality as
measured by general indicators of milk production.
Weight gain of individual kittens was unrelated to the
location of their primary nipple(s) (Fig. 6, right panel),
and amounts of milk ingested were largely independent of
nipple location (Fig. 7, left panel; Jacobsen, DePeters,
Rogers, & Taylor, 2004 for evidence of similar milk
composition across nipples in the domestic cat). Accord-
ingly, we also failed to find a relation between kittens’
starting weight—in mammals a good predictor of early
growth and survival (review in Rodel, Bautista, Garcıa-
Torres, Martınez-Gomez, & Hudson, 2008)—and the
location of their primary nipples (Fig. 6, right panel). And
finally, after we controlled for differences in attachment
frequency across nipples we did not find that they were
differentially contested (Fig. 4, left panel).
Consistent with the above, we also failed to find
evidence of privileged access to nipples by particular (e.g.,
heavier) individuals as has been reported for some
mammals (reviews in Hudson & Trillmich, 2008; Rodel,
Bautista, Garcıa-Torres, Martınez-Gomez, & Hudson,
2008). Indeed, there does not seem to be a resource for
which suckling kittens need compete. As outlined above,
nipples appear to be of equal quality, and in contrast to
species with very short and infrequent nursing bouts such
as the European rabbit (Bautista et al., 2005; review in
Bautista, Martınez-Gomez, & Hudson, 2008), the whole
litter has sufficient time to attach to nipples before milk
letdown. Nevertheless, the number of kittens establishing
their primary nipple on row 1 (three kittens from three
Developmental Psychobiology
FIGURE 7 Amount of milk ingested across nipple rows and
nipple ranks. There was little difference in productivity of
nipples, and kittens obtained per milk let down on average
similar amounts of milk across nipple rows and nipple ranks. Use
of row 1 nipples was rare and data have therefore been omitted.
Dots give the average kitten values for each litter, and horizontal
bars represent means; days 0–28; n¼ 6 litters.
330 Hudson et al.
litters; Tab. 2) was too small to meaningfully evaluate the
quality of row 1 nipples, although we can report that
the kitten with the greatest mean weight of the study
principally used a row 1 nipple. In any case, because litters
even of well-fed house cats rarely contain more than six
kittens the most anterior nipples are probably only rarely
used and so of little functional significance.
How, then, are we to account for kittens’ reliable and
well-structured pattern of individual nipple use? At an
ultimate level of explanation and as suggested by Ewer
(1959), preferential use of particular nipples might
function to ensure that in smaller litters at least some
nipples receive sufficient stimulation to remain produc-
tive, and enabling the mother to direct milk production to
fewer, optimally used outlets. That each kitten learns to
use a particular nipple or nipple pair might serve to reduce
presumably energetically costly scrambles for nipples as
well as potentially dangerous contests among these well-
armed young (De Passille, Rushen, & Hartsock, 1998
for a similar explanation of teat order in piglets). At a
proximate level of explanation, establishment of prefer-
ential nipple use in the cat almost certainly depends, at
least in part, on olfactory cues. Thus, in a study in progress
(Hudson, Raihani, Gonzalez, Arteaga, & Distel, 2008) we
have found that kittens respond with nipple-search
behavior and attachment when placed in contact with
the ventrum of late pregnant or lactating females but not of
mature, nonreproducing females (reviews in Bautista
et al., 2008; Hudson, Rojas, Arteaga, Martınez-Gomez, &
Distel, 2008 for a similar phenomenon in domestic
rabbits). Furthermore, in agreement with the suggestion
by Ewer (1961) and present observations of nipple
constancy when mothers changed the side they lay on to
nurse, that kittens use local olfactory cues rather than
topographical features of the mother to identify their
primary nipples, we have found that kittens do not
preferentially attach to the equivalent of their primary
nipple when tested on a female of similar lactational age to
their mother (Hudson, Raihani, Gonzalez, Arteaga, &
Distel, 2008).
Given such a well-organized suckling system, why do
kittens contest nipples at all, and then right up to the start
of weaning? This is particularly puzzling as we found no
evidence for a difference in resource quality across
nipples, that kittens displacing others only attached to
the vacated nipple in about 50% of contests, and that
even in large litters in which kittens on average grew
more slowly than kittens in smaller litters, they did so
independently of ownership of particular nipples.
Together, these findings raise the possibility that ‘‘con-
tests’’ during suckling may simply be a by-product of the
high arousal characteristic of the suckling context, and
leading kittens encountering an obstacle blocking their
search, to try and remove it.
In conclusion, information on the behavior of mam-
malian littermates during suckling is still surprisingly
scarce, and on the relation between individual differences
in behavior and the ability to obtain a resource as basic to
individual survival and development as the mother’s milk,
even scarcer. The discrepancy between the results of the
present study and previously proposed and generally
accepted explanations of the suckling system of the cat
points to the need for more information in this field,
including at a naturalistic, descriptive level.
NOTES
Financial support was provided by CONACyT (48692-Q),
DGAPA (IN229907), and to G.R. by the Postdoctoral Fellowship
Program of the Universidad Nacional Autonoma de Mexico.
G.R. also received a generous travel award from the International
Society for Developmental Psychobiology to present part of this
work at the 2008 Annual Meeting in Washington, USA. We
thank Carolina Rojas for excellent technical and bibliographical
assistance, Heiko Rodel for statistical advice, and Hugh
Drummond and other members of Los Lunaticos for valuable
discussion. We also thank neighbors, and particularly Martha
Rosa and Jorge Avelino Dahda Faour, for tolerating the presence
of the cats in their gardens and for helping care for them. Data
were collected with the help of Paulina Klein and Marıa Jose
Munozcano as part of their diploma theses.
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