# Shared Reproductive State Enhances Female Associations in Dolphins

Authors: Luciana M. Möller; Robert G. Harcourt
Journal: Research Letters in Ecology
 (2008)

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## Abstract

Female bottlenose dolphins (genusTursiops) usually associate at moderate level with other females within social clusters called bands or cliques. It has been suggested that reproductive state may play the predominant role in determining associations within female T. truncatus bands. Here, we test the hypothesis that reproductive state correlates with associations of female Indo-Pacific bottlenose dolphins (T. aduncus). We found that females in similar reproductive state, which included females from late pregnancy to the first year of their calves' life or females from early pregnancy to their calves' newborn period, had
higher-association coefficients with each other than they did with females in different reproductive states (females with older calves or without calves). This was observed both within and across social clusters suggesting that reproductive state, at least for pregnant females and those with young calves, plays an important role in determining who to associate with. However, a female's most frequent associate was not always with another in similar reproductive state. We suggest that several factors, including reproductive state, may be of importance in determining associations of female bottlenose dolphins.


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## Body

## 1. Introduction

Group
living among female mammals is widespread. The advantages of group living
include enhanced access to resources and communal rearing, reduced risk of
predation, and protection from male sexual coercion, while costs generally involve
competition for critical resources (e.g., [1, 2]).
Therefore, the size and composition of groups usually reflect a balance between
benefits and costs of group living [3]. In fission-fusion
societies, female social relationships are usually variable (e.g., [4, 5]), probably reflecting particular socioecological
requirements, which may be dependent on the females' developmental stage and
condition (e.g., [6, 7]).Adult female bottlenose dolphins
(genusTursiops) live in complex
societies and usually associate at moderate level with other adult females, apart
from associating closely with their calves [5, 8, 9]. In Shark Bay, Western Australia, female Tursiops sp.  are found within a large
social network, where they usually associate with stable subsets of females [9]. Similarly, adult females T.
truncatus in Sarasota Bay, Florida, form clusters of frequent associates
that share core areas within their home range [5], a pattern
also observed for T. aduncus in Port
Stephens, southeastern Australia [8].Wells et al. [5] suggested that while kinship
may be of primary importance in the formation of female T. truncatus bands (but see [8]), associations within
bands are based on the presence, absence, and imminence of a calf. In a review
of dolphin social relationships, Connor et al. [10] proposed that female
bottlenose dolphins in similar reproductive state may benefit from associating
with each other because they share similar requirements for food and protection.
This “similarity principle” has been suggested to underlie social bonds among other female mammals, such
as in captive Rhesus monkeys [6]. However,
evidence of a correlation between female associations and reproductive state in
the wild is still lacking. Here, we investigate whether associations of female Indo-Pacific
bottlenose dolphins (T. aduncus) correlate
with similar reproductive state.


## 2. Materials and Methods

The
study was conducted in Port Stephens, southeastern Australia
(32042'S, 152006'E). Bottlenose dolphins
can be observed year-round in this area, with about 90 individuals sighted on a
regular basis [11]. Based on association patterns and spatial
analysis, four main social clusters of females were identified in the area [8].Between December 1998 and April 2000, 43 boat surveys were conducted in Port Stephens
for determining membership of dolphin groups through photo-identification. During
this period, 138 individual dolphins were identified. Dolphins were defined as part of the same group if within a 100 m
radius and if the animals were traveling, they were heading in the same
direction [8].
Details on surveys and photo-identification
procedures can be found in [11]. For analysing association
patterns, 158 groups were used out of 218 observed. Groups were selected if at
least four times the visually estimated group size was taken in identification
photographs, and there was no apparent fusion with animals from other groups
while the photographs were being taken [8]. Females were then selected
to calculate pairwise associations using the half-weight index (HWI)   [12] , if they were sighted in at least 6 groups (the median number
of sightings or higher; [8]), using programme SOCPROG 1.2 [13].
The HWI was chosen because it has been used in other dolphin studies based on the premise that individual dolphins are more likely to be identified when apart than when together in the same group (e.g., [9]). Individuals
were considered females if they were observed repeatedly with a calf or if they
were genetically sexed as such [8]. Only noncalf females were
included for analyses. Of all
individuals identified during the study period, 65 were of unknown sex, 33 were
males, and 40 were females. Since only known females sighted more than 6 times were
used for the association analysis, some females (see results) and individuals
of unknown sex were excluded.Pairwise associations between females,
which  we were able to ascertain that were insimilar reproductive state during the study period, were compared
with pairwise associations of these females with others that were in different reproductive state. Since the
study spans from the beginning of one breeding season to the end of the next
breeding season, associations between females in similar reproductive state refer to associations between those females
that were in late pregnancy at the beginning of the study and that by the end
of the study had a calf of approximately one year of age, as well as associations
between those females that were receptive at the beginning of the study and
that by end of the study had a newborn calf. Associations between females in different reproductive state refer to
associations between those females above with females that had different-aged
calves or no calves. Associations between females with possible same-aged
calves but of older age classes (among those females that already had calves
but not newborns when the study commenced) were not included as similar state because the calves' age
could not be determined with certainty. Females without calves were also not
included as similar state females because
we could not discern with certainty whether these were nulliparous females or
females without dependent calves.Two comparisons were made: one
considering associations of females overall (both within and across social
clusters) and the other only associations within social clusters (see [8] for information on social clusters). Differences in the mean of female
associations were compared using a two-sample randomisation test implemented in
the programme RT 2.1 [14]. Randomisation
tests make no assumption about the distribution or interdependence of the data
and therefore are most appropriate for this data set.


## 3. Results

Thirty-five
known females were sighted 6 or more times, 28 of which had a closely
associated calf (Table1). Of these
females, 10 gave birth during the study period, with five calves born in the
first summer and five others in the second summer (Table 1). Overall (i.e.,
within and across social clusters), females in similar reproductive state were more likely to associate with each
other than they were with those in different reproductive states (Figure 1(a)). Within social clusters, there was also a
significantly higher mean association between similar-state females compared to different-state females (Figure 1(b)). The most frequent female associate
that was in similar reproductive condition was always within its social cluster.
However, its closest associate within the cluster was not always an individual
in similar reproductive state (Table 1).Table 1
Social cluster membership (from [8]). Reproductive state and number of sightings of female Indo-Pacific bottlenose dolphins, including an individual's highest association
and its closest associate.
Female
IDSocial
clusterReproductive
stateNumber of
sightingsHighest
associationClosest
associate ID1W≥ 1 yr110.43363N≥ 1 yr170.33834N≥ 1 yr, new 2110.55426WNew 1150.385210WNew 1220.421111W≥ 1 yr160.551817N≥ 1 yr90.44418W≥ 1 yr200.744520S≥ 1 yr190.815421S≥ 1 yr170.622022≥ 1 yr130.362029SNo calf60.402036WNo calf130.485239≥ 1 yr60.244242NNo calf, new 2150.55445WNo calf120.741852W≥ 1 yr160.483654S≥ 1 yr180.812061KNew 170.776662K≥ 1 yr60.676663KNo calf, new 2140.746864K≥ 1 yr, new 260.676165KNew 190.606366KNew 160.776167K≥ 1 yr80.676168KNo calf, new 2100.746371K≥ 1 yr90.626473S≥ 1 yr110.3011075N≥ 1 yr110.40479SNo calf80.292183NNo calf110.33387SNo calf60.4754110S≥ 1 yr130.5554124S≥ 1 yr60.25110132KNo calf70.6262 and 66Reproductive state refers to estimated age
or absence of calf at the beginning of the study, and summer of parturition thereafter.
New 1 and new 2 refer to newborn in summers 1 and 2, respectively. Females with
no cluster membership could
not be unambiguously classified as member of any particular main cluster.Mean association of female Indo-Pacific
bottlenose dolphins in similar and different reproductive states: (a) overall
and (b) within social clusters.
(a)(b)


## 4. Discussion

Findings
from this study corroborate the hypothesis that similar reproductive state may be
an important factor determining female bottlenose dolphin associations [5], at least for pregnant females and those with young calves. Given
that bottlenose dolphin females late in gestation and with young calves may be sexually
coerced by males [15], these similar reproductive state females
may be at considerable risk of injury or separation from their calves. Circumstantial
evidence also suggests that males may kill infants (e.g.,
[16]), and so avoidance of males may be a trait shared by females in similar
condition. Sharks also pose a significantly higher risk of predation for
mothers and calves, compared to other age-sex classes (e.g.,
[16]). Hence, association with other females in similar reproductive state may
provide protection through shared level of risk. In addition, there is evidence
from other delphinids that feeding habits may vary with reproductive state.
Lactating spotted dolphins (Stenella
attenuata) feed preferentially on flying fish, and less so on squid
compared to pregnant females  [17] . Altered nutritional
requirements and/or restrictions on foraging ability might, therefore, also
drive enhanced associations for pregnant female dolphins and those with young
calves. In plain zebras (Equus burchellii),
harems containing lactating females are more likely to lead herd movements,
possibly to gain preferential access to scarce water [18].Port
Stephens' female dolphins in similar reproductive state had their highest
association with females of their own social cluster, but a female's most
frequent associate within its cluster was not always with another in similar
reproductive condition. This indicates that other factors may also play a role
in association preference. These factors may include kinship, age, maternal
experience, and social familiarity.Kinship has been shown to shape
associations of many female mammals (e.g., several primates [19]) and has
been suggested to be of importance in the formation of female T. truncatus bands [5].
In Port Stephens, both genetic relatedness and maternal kinship have been found
to correlate with female associations [8]. Protection from
predators and males, and assistance for infant rearing are potential advantages
of group living for female bottlenose dolphins [8, 10]. If this is the case, then related individuals may be preferred for
association because they may be more reliable cooperators [2] and because
individuals may gain through inclusive fitness effects  [20] .In highly social mammals, close social
bonds may also form between similar-aged individuals (e.g., baboons, [21]). This,
however, may represent a preference of females for associating with paternal half-sisters,
especially in species where paternal kin recognition may be present, and
reproductive skew is high [21]. The possibility that paternal
relatedness and age play a role in female T.
aduncus associations remains to be tested.Females may also associate with others
depending on their maternal experience. In one population ofT. truncatus, the most experienced
mothers associated most frequently with other mothers with dependent calves,
and primiparous mothers primarily with either nulliparous females, males, or
females without dependent calves [22].Another possibility is that females
associate closely with other females with whom they are socially familiar (e.g.,
[21]). This is
particularly likely to occur in bottlenose dolphins, given their long-term
associations [5, 10] and capability for
individual recognition (e.g., [23]). Bottlenose dolphin calves usually stay in close
association with their mothers until they are between 3 and 6 years of age, and
associate with other calves during this period (e.g., [5]), probably
those of their mothers' closest associates. Later, as juveniles, they associate
with other juveniles in mixed-sex groups [5]. Therefore, as
adults, close female associates may perhaps include the daughters of their
mothers' closest associates, with whom they spent part of their infancy, or
females with whom they spend their juvenile period. Moreover, even though adult
females may not share similar reproductive state at present, they may have done
so in the past. Since female inter-birth interval in bottlenose dolphins is
variable, and calf mortality is relatively high in the first 3 years of a
bottlenose dolphin's life (e.g., [24]), the probability that females will continue to share similar
reproductive state in subsequent pregnancies is likely to be low. Thus, while
females on average associate more frequently with females in similar
reproductive state, probably due to shared requirements, previously close
associations may not completely dissipate. Therefore, other close associates
may possibly include females with whom they have previously shared similar
reproductive state. Further study should consider a multivariate analysis of these
potential factors, in addition to reproductive state, in order to evaluate
their relative strength on determining associations in these animals.



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*Source: 498390-2008-02-17.xml*