Functional Ecology 2007
doi: 10.1111/j.1365-2435.2007.01354.x
Infant carrying behaviour in dolphins: costly parental
care in an aquatic environment
Blackwell Publishing Ltd
S. R. Noren*
Institute of Marine Science, Center for Ocean Health, University of California at Santa Cruz, 100 Shaffer Road, Santa Cruz,
CA 95060, USA
Summary
1. Infant carrying behaviour occurs across diverse taxa inhabiting arboreal, volant and aquatic
environments. For mammals, it is considered to be the most expensive form of parental care after
lactation, yet the effect of infant carrying on the energetics and performance of the carrier is
virtually unknown.
2. Echelon swimming in cetacean (dolphin and whale) mother–infant dyads, described as calf in
very close proximity of its mother’s mid-lateral flank, appears to be a form of aquatic ‘infant
carrying’ behaviour as indicated by the hydrodynamic benefits gained by calves in this position
which enables them to maintain proximity of their travelling mothers. Although this behaviour
provides a solution for minimizing separations of mother–infant dyads, it may be associated with
maternal costs.
3. Through kinematic analyses this study demonstrates empirically that ‘infant carrying’ impacts
the locomotion of dolphin (Tursiops truncatus) mothers as evident by decreased swim performance
and increased effort.
4. The mean maximum swim speed of mothers swimming in echelon only represented 76% of the
mean maximum swim speed of these mothers swimming solitarily. In addition, there was a
concomitant 13% reduction in distance per stroke for mothers swimming in echelon compared to
periods of solitary swimming.
5. Thus, ‘infant carrying’ in an aquatic environment is associated with maternal costs, and could
ultimately impact maternal energy budgets, foraging efficiency and predator evasion.
Key-words: cetacean, echelon position, hydrodynamics, kinematics, swimming
Functional
doi: 10.1111/j.1365-2435.2007.0@@@@.x
Ecology (2007) xx, 000–000
Introduction
Maternal investment in mammals includes gestation,
lactation and other forms of parental care. Infant carrying is
considered to be the most costly form of parental care after
lactation (Altmann & Samuels 1992; Kramer 1998) and has
been described in 6 of 19 eutherian mammalian orders (for
review, see Ross 2001). This behaviour provides a solution for
mothers of diverse taxa that must manoeuvre within their
environment to forage and avoid predators while accompanied by their young offspring (Ross 2001), which are
handicapped by small body size, undeveloped tissues and
naïveté (Carrier 1996). This behaviour is only thought to
evolve when offspring are unable to independently follow
their mothers, as in arboreal (i.e. primates) and volant (i.e.
bats) environments (Ross 2001).
*Correspondence author. E-mail: snoren@biology.ucsc.edu
The aquatic environment also appears to require ‘infant
carrying’ to ensure that mother–infant dyads remain intact
during travel as manatees and sea otters are observed
physically carrying their young. Cetaceans (whale and dolphin),
however, cannot physically carry their young. Yet similar to
that observed for primates (Altmann & Samuels 1992; Doran
1992; Wells & Turnquist 2001), mature locomotor performance
in dolphins is precluded for several years postpartum (Noren,
Biedenbach & Edwards 2006). Echelon position is the
predominant behaviour displayed by cetacean mother–infant
dyads (Fig. 1; McBride & Kritzler 1951; Tavolga & Essapian
1957; Norris & Prescott 1961; Au & Perryman 1982; Taber &
Thomas 1982; Mann & Smuts 1999; Noren & Edwards 2007)
and appears to represent an aquatic form of ‘infant carrying’
because it enables neonatal cetaceans to maintain close
proximity to their mothers during travel (Norris & Prescott
1961; Lang 1966) by increasing the swimming efficiency of the
infant (Kelly 1959; Weihs 2004; Noren et al. 2008). Thus
cetaceans, like primates, appear to ‘carry’ their young.
© 2007 The Author. Journal compilation © 2007 British Ecological Society
2
S. R. Noren
Fig. 1. Three bottlenose dolphin mother–calf pairs swimming in echelon
position. Echelon position is described as calf in very close proximity
with its mother’s mid-lateral flank in the region near her dorsal fin.
Photo © and courtesy of Dolphin Quest Hawaii.
Only a few studies have examined the energetic and
locomotor consequences of infant carrying for the carrier,
and these studies have focused on primates (Altmann &
Samuels 1992; Schradin & Anzenberger 2001) and marsupials
(Baudinette & Biewener 1998). Meanwhile the maternal
consequences of infant carrying in other taxa and environments
(i.e. volant and aquatic) remain unexplored. In view of this, I
examined the kinematics of dolphin mothers swimming with
their calf in echelon (Fig. 1) and swimming solitarily (> 1 m
from their calf and all other dolphins) to elucidate the effect of
‘infant carrying’ on maternal locomotor performance and
effort in an aquatic environment. The advantages gained by
calves in echelon position are examined in the accompanying
paper (Noren et al. 2008).
Materials and methods
Three captive bottlenose dolphin (Tursiops truncatus) mother–calf
pairs housed at Dolphin Quest Hawaii provided a controlled
experimental approach to investigate cetacean locomotor performance
and effort (Fish 1993; Skrovan et al. 1999; Noren et al. 2006, 2008).
Methodologies regarding the dolphin enclosure, placement of the
SCUBA diver-videographer, and placement of the dolphins in the
water column are described in detail elsewhere (Noren et al. 2008).
Experimental swim sessions included both opportunistic (no reward)
and directional swimming between two trainers (reward-based).
Echelon swimming was recorded only when the mothers’ calves
were 0–34 days postpartum. Thus, the present study only provides
details regarding the maternal costs of echelon for mothers with
very young calves (0 – 34 days postpartum). Solitary swimming was
recorded at several intervals 0 – 2 years past parturition. Thirty-three
hours of swimming were recorded and 334 short 1– 6 s video clips
were extracted and digitized (Fig. 2). Clips were divided into two
association categories: (i) echelon position (Fig. 1); and (ii) solitary
swimming (mother > 1 m away from calf and all other dolphins). In
all clips the mothers were continuously stroking.
A quantitative assessment of swim effort was obtained by
calculating peak-to-peak fluke stroke amplitude and tailbeat
oscillation frequency. Higher amplitudes and frequencies are
associated with greater energy expenditure (Kooyman & Ponganis
1998). Normalized tailbeat frequency (ratio of tailbeat frequency to
Fig. 2. A tracing from a digitized video clip of a solitarily swimming
mother dolphin. Anatomical points of interest (rostrum tip, cranial
insertion of the dorsal fin, and fluke tip) were digitized at a rate of 60
fields per second of video using a motion-analysis system (Peak
Motus 6·1; Peak Performance Technologies, Inc. Englewood, CO,
USA) following methods similar to Skrovan et al. (1999) and Noren
et al. (2006). A distinct trace represents the movements of each
digitised anatomical point. From left to right, the trace from the
rostrum leads (pink), followed by the trace from the cranial insertion
of the dorsal fin (yellow), and last is the trace from the fluke tip (blue).
The brown dot is a digitized reference point indicating that the
camera was steady while filming this video clip.
swim speed; Rohr & Fish 2004) and distance per stroke were also
calculated. Methods for video analysis and swim effort calculations
are described in detail elsewhere (Noren et al. 2006).
The goal of this study was not to address individual variation, but
to quantify changes in locomotor performance associated with swim
style (echelon vs. solitary), thus similar to other kinematic studies
data across individuals were pooled (Fish 1993; Skrovan et al. 1999;
Noren et al. 2006, 2008). Pearson product moment correlation
coefficients were used to determine the correlations of peak-to-peak
fluke stroke amplitude and tailbeat frequency with swim speed for
echelon swimming and also for solitary swimming; linear regression
analyses were then used to determine the relationship for the parameters
that demonstrated a strong correlation. Swim speed, normalized
tailbeat frequency, and distance per stroke during echelon swimming and solitary swimming were compared using student’s t-tests
when normally distributed, or Mann–Whitney rank sum tests when
normality failed (α = 0·05). The maximum swim performance of
each individual during echelon swimming and solitary swimming
was compared using a paired t-test (α = 0·10). Statistical analyses
were performed using S S 2·03 (Systat Software, Inc. Point
Richmond, CA, USA). Means ± 1 SEM are presented.
Results
Our experimental approach adequately captured swimming
behaviours representative of wild dolphins (Noren et al.
2008). The average swim speed of mother dolphins was
significantly slower during echelon swimming (2·11 ± 0·06 m s–1;
n = 178) than during solitary swimming (3·88 ± 0·09 m s–1;
n = 156; T = 36534·00, P < 0·001). Given that 53% of the
echelon swim data were from directional swim trials, compared to 94% for the solitary swim data, swim speed data from
directional trials only were also compared to ensure that the
previous result was not due to experimental design. The result
© 2007 The Author. Journal compilation © 2007 British Ecological Society, Functional Ecology
Infant carrying in dolphins 3
Fig. 4. ‘Infant carrying’ imparted maternal costs. Mother dolphins
swimming in echelon with their calf (white bar) demonstrated
significantly lower mean distance covered per stroke (t = −7·068,
P < 0·001, n = 178, 156) compared to periods of solitary swimming
(black bars).
(T = 19949·00, P < 0·001, n = 178, 156) and reduced distance
per stroke (Fig. 4). For a given speed, mothers swimming in
echelon increased tailbeat frequency by 17%, which resulted
in a 13% decrease in distance covered per stroke compared to
solitary swimming.
Fig. 3. Swimming kinematics of the mother in relation to the
swimming speed of the mother. Peak-to-peak fluke stroke amplitude
(a) and tailbeat frequency (b) were both correlated with swim speed
for mother dolphins swimming in echelon with their calf (white
symbols; r = 0·400, P < 0·001, n = 178 and r = 0·799, P < 0·001, n =
178, respectively) and swimming solitarily (black symbols; r = 0·277,
P < 0·001, n = 156 and r = 0·885, P < 0·001, n = 156, respectively).
The relationship between swim speed and peak-to-peak fluke stroke
amplitude appears to be nonlinear. Given the strong linear correlation between swim speed and tailbeat frequency (SF) linear regressions
are provided for echelon (speed = 1·61 SF + 0·27; r2 = 0·638, F =
310·794, P < 0·001) and solitary (speed = 2·09 SF + 0·13; r2 = 0·783,
F = 555·610, P < 0·001) swimming. A different symbol is used for
each of the three individual dolphins.
was the same; average swim speed during echelon swimming
was significantly slower than during solitary swimming
(t = −11·644, P < 0·001, n = 94, 145). The absolute maximum
swim speed for each mother swimming with their calf in
echelon was 4·23, 4·39 and 4·32 m s–1 for animals 1, 2 and 3,
respectively. This compares to 5·65, 6·32 and 5·11 m s–1 for
animals 1, 2 and 3 swimming solitarily, respectively. As a
result, mean maximum swim performance was significantly
slower when mothers were swimming in echelon (4·31 ± 0·05 m s–1)
compared to solitary swim periods (5·69 ± 0·35 m s–1; t = −4·816,
n = 3, P = 0·053).
Peak-to-peak fluke stroke amplitudes (Fig. 3a) and tailbeat
frequencies (Fig. 3b) were correlated with swim speed for
mother dolphins swimming in echelon and swimming
solitarily. Because average swim speed was significantly different
between the two swim categories, swim effort was standardized
for swim speed. Mothers in echelon demonstrated significant
increases in effort compared to periods of solitary swimming as evident by greater normalized tailbeat frequency
Discussion
Infant carrying provides a solution for mothers who must
locomote in their environment while accompanied by their
young offspring. Echelon swimming in cetacean mother–
infant dyads is a type of ‘infant carrying’ that improves calf
swim performance (Noren et al. 2008), but it appears to come
with a maternal cost. Average and maximum swim speeds
were significantly slower for mothers swimming in echelon
with their calves compared to periods of solitary swimming.
For example, the mean maximum swim speed of the mothers
swimming in echelon only represented 76% of the mean
maximum performance of these mothers swimming solitarily.
The maximum speed was assumed to represent the animal’s
extreme performance, which is the method used to qualify
physiological capacity (Weibel et al. 1987), thus this result
implies that the presence of a calf is a detriment to maternal
swim performance.
An alternate hypothesis for the decreased performance of
dolphin mothers swimming in echelon is that the mother can
only travel as fast as the infant can actively swim because the
infant must occasionally stroke during the echelon swim
behaviour. However, 0–1 month-old calves are capable of
independent swim speeds of 0·58–4·20 m s–1 (Noren et al.
2006) and this performance is increased when the calf is in
echelon (Noren et al. 2008) because it receives up to 60% of
the thrust from its mother (Weihs 2004). Thus, it is unlikely
that the swimming ability of an entrained calf constrains
maternal echelon swim performance. Studies of chimpanzees
(Pan troglodytes) also suggest that infant carrying decreases
maternal travel speed (Wrangham 2000; Williams, Hsien-Yang
& Pusey 2002).
© 2007 The Author. Journal compilation © 2007 British Ecological Society, Functional Ecology
4
S. R. Noren
In addition to decreased performance, dolphin mothers
swimming in echelon were required to increase effort compared to periods of solitary swimming. For a given speed,
mothers swimming in echelon significantly increased tailbeat
frequency with the result that distance covered per stroke
significantly decreased by 13% compared to solitary
swimming (Fig. 4). Interestingly, the proportion of decreased
distance per stroke in ‘infant carrying’ dolphin mothers is
strikingly similar to the 17% decrease in distance per leap
measured empirically for marmosets (Callithrix jacchus)
carrying weights equivalent to their newborn twin offspring
(Schradin & Anzenberger 2001). Furthermore, although
female wallabies carrying a load (approximating the mass of
a fully developed offspring) did not alter stride frequency,
they increased the time the foot applied force on the ground,
which likely increased the stored elastic strain energy to levels
necessary to transport the additional load (Baudinette &
Biewener 1998). Regardless of habitat (aquatic vs. arboreal)
or forces to overcome (hydrodynamic drag vs. gravity) in these
systems maternal effort must change to support an increased load.
The increase in maternal effort for dolphin mothers
swimming in echelon compared to periods of solitary swimming
may be associated with changes in water flow patterns and
drag. The presence of the calf may disrupt the boundary flow
around the mother causing it to separate, which would
increase turbulent flow. In addition, the entrained calf could
increase the surface area of the mother, which effectively
increases the drag of the swimmer (Webb 1975). More power
is required to overcome increased turbulent flow and drag
(Webb 1975). As a greater proportion of maternal power
output is utilized to accommodate increased turbulent flow
and drag, there is less energy available to propel the animal
forward. As a result, locomotor performance decreases
because power output per stroke is limited by mechanical
constraints (Fish & Hui 1991) and total work is limited by the
animal’s metabolic scope (Weibel et al. 1987). These relationships may explain the observed decrease in swim speed and
distance covered per stroke for mothers swimming in echelon
position. Although Weihs (2006) suggested that the gain in
forward forces by the following body (calf ) is larger than the
added cost to the leading body (mother), a more detailed
theoretical examination of the drag and flow patterns for the
leading body in echelon position is warranted to validate the
hypotheses for the proposed decreased maternal performance
observed in the present study.
Given that infant carrying mothers must forage and evade
predators, one of many constraints on newborn offspring
mass may be its impact on maternal locomotor performance.
As a result, newborn dolphins and marmosets represent a
similar proportion of maternal mass, 15% (mass data from a
mother–calf pair in this study) and 17% (Tardif, Harrison &
Simek 1993), respectively. Ultimately as an infant ages and
increases in size, maternal costs associated with infant
carrying theoretically increase in aquatic (Weihs 2004, 2006)
and arboreal (Schradin & Anzenberger 2001) environments.
Optimal theory predicts that maternal carrying costs should
not outweigh the sum of the costs of independent locomotion
by the mother and her offspring (Kramer 1998). Therefore,
the transition to offspring locomotor independence may
represent a parent–offspring conflict (Trivers 1974) as energy
expenditure to carry an infant may limit the mother’s available
energy to invest in future offspring (Kramer 1998). As
cetacean and primate offspring increase in size, there is an
increase in the active prevention and/or avoidance of infant
carrying by mothers in both groups (Altmann 1980; Taber &
Thomas 1982; Mann & Smuts 1999), such that with age there
is a decrease in the time cetacean infants swim in echelon
(Taber & Thomas 1982; Mann & Smuts 1999; Noren &
Edwards 2007) and primate infants are carried (Altmann &
Samuels 1992; Salvage et al. 1996; Pontzer & Wrangham
2006). Examination of human infant carrying behaviour also
suggests that by a certain mass and age, mothers encourage
their infants to walk independently (Kramer 1998).
In summary, this study provides the first empirical evidence
of the maternal consequences of ‘infant carrying’ in an aquatic
environment. Although infant carrying provides a solution
for mothers that must manoeuvre within their environment
while accompanied by their underdeveloped offspring, this
behaviour is associated with maternal costs regardless of environment, as evident in arboreal (Schradin & Anzenberger 2001)
and aquatic (present study) regimes. The decreased locomotor
performance and increased locomotor effort associated with
infant carrying undoubtedly impacts maternal energy budgets,
foraging efficiency and predator evasion. Given the prevalence
of infant carrying behaviour across diverse taxa and habitats
and the consequences of this behaviour, it is surprising that
the energetics of infant carrying have largely been ignored.
Future investigations are warranted, particularly in a volant
environment, which remains unexplored.
Acknowledgments
I thank Dolphin Quest, particularly J. Sweeney and R. Stone, for providing the
experimental facilities and animals and for funding portions of data collection
and analyses. I also thank Southwest Fisheries Science Center (SWFSC),
particularly S. Reilly and E. Edwards, for funding portions of data collection.
In addition, I thank the staff at Dolphin Quest Hawaii (particularly G. Biedenbach
and C. Buczyna), T. Williams of the University of California Santa Cruz for the
use of her Peak Motus system, and J. Redfern of SWFSC for assistance with
data management.
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Received 12 June 2007; accepted 27 September 2007
Handling Editor: Francisco Bozinovic
© 2007 The Author. Journal compilation © 2007 British Ecological Society, Functional Ecology