Despite large changes in demography, the basic pattern of social structure characteristics of this community remained consistent with previous long-term analyses, including definitive social clusters (Elliser and Herzing 2012), sex preferences, and overall association patterns (Elliser and Herzing, in press). This is contrary to what has been described for other species, where demographic changes resulted in altered behavior and social structure/grouping (bottlenose dolphins: Lusseau and Newman 2004; marmosets: Lazaro-Perea et al. 2000; chimpanzees: Lehmann and Boesch 2004; killer whales: Matkin 2008; bottlenose dolphins: Elliser and Herzing 2011). However, some changes in spotted dolphin social structure were observed after the hurricanes. There was lower social differentiation, younger age of alliance formation and increased overall cohesion within clusters and across age class. This suggests that responses to demographic upheaval differ between populations and/or species, with varying degrees of change in social structure as they adapt to new conditions.
One of the most striking results was that despite losing many individuals and an overall decrease in community size, the Northern, Central, and Southern clusters remained discrete (although the Central cluster appeared more closely connected with the Southern cluster prehurricane and then with the Northern cluster posthurricane) and group size remained the same, even though social differentiation within the clusters decreased. Small communities of both dolphins and primates, whether resulting from demographic changes or isolated populations, have been documented to have less fission/fusion dynamics, increased group sizes, strong associations across sex class, and increased cohesiveness, with individuals spending more time with all other members of the population (Lusseau et al. 2003, Lehmann and Boesch 2004). Smaller social groups may be more accepting of outside individuals (such as immigrants or individuals from another cluster) to maintain normal social and behavioral functions (such as foraging, mating, play, and calf care). This would facilitate recruitment of individuals (Schaffner and French 1997). Therefore, it might be expected that the clusters would increase their cohesiveness and combine together; however, the lack of increased number of associations between clusters indicated that the clusters remained an integral part of the community structure.
There was increased cohesiveness of the associations within clusters and across age class overall. The dolphins began associating with all individuals within their cluster (some of which they were not observed to have associated previously), and a few outside their cluster, more than they had during the prehurricane years. Past research on associations from 1991 to 2002 showed observed association percentages similar to the prehurricane years of this study (Elliser and Herzing, in press), indicating a significant change posthurricane. Similarly, the mean CoA for the community posthurricane was almost twice that of prehurricane years, and more than double previous long-term work since 1991 (Elliser and Herzing, in press). Social differentiation posthurricane was much lower, by almost half, which may denote a less stratified society. Similar cohesion within units occurred in the sympatric bottlenose dolphin community. They lost 30% of their community, but an almost equal number of immigrants moved in. Their previously stable community split into two units, with increased number of associations and cohesion within units than had been seen in the previous community (Elliser and Herzing 2011). It seems that for these sympatric species the loss of individuals, regardless of the presence or absence of immigrants, will influence the surviving individuals to associate more with the other surviving members of their unit or cluster.
Similar evidence of increased cohesion has been reported in a community of bottlenose dolphins in Sarasota, Florida. During severe harmful algal blooms the connectivity, ties and density of social network measures increased significantly for both adult and juvenile bottlenose dolphins; they were more gregarious and interacting with more associates than they had previously (McHugh et al. 2010). Changes in human activities have also been shown to alter associations in a similar way, though this led to a large change in bottlenose dolphin community structure (Ansmann et al. 2012). After fisheries regulations greatly reduced trawling activities in Moreton Bay, Australia, the presence of two distinct communities of bottlenose dolphins (trawler and nontrawler dolphins) effectively disappeared. The social network was less differentiated and more compact with increased and stronger associations between individuals (Ansmann et al. 2012). Although there were similar association changes within the clusters of the spotted dolphin community, the clusters and the overall community structure remained intact. Together these results indicate that changes in demography, environment and human behavior can influence dolphin associations. The effects on social and community structure may vary, depending on many factors, including the nature of the disturbance/change, the species, the previously established social structure of the population or community and the social needs and flexibility of the individuals.
One of the most interesting differences between the spotted dolphin community and a similarly demographically altered chimpanzee community was that strong and/or long-lasting mixed sex associations were predominant in the latter (Lehmann and Boesch 2004), but not in the spotted dolphins. Generally strongest and/or long-term associations were between members of the same sex (Wells et al. 1987; Connor et al. 2000; Rogers et al. 2004; Elliser and Herzing 2011; Elliser and Herzing, in press). These sex preferences also remained evident posthurricane in the sympatric bottlenose dolphins, and may have been the driving force for the changes in social structure that emerged because acceptance of immigrants differed between the sexes (Elliser and Herzing 2011). Despite the loss of individuals and decreasing community size, sex preferences still strongly influenced association patterns in this spotted dolphin community, further supporting that sex preferences have a primary role in cetacean social organization.
The loss of individuals had little effect on the association patterns of the female spotted dolphins in this community. Their associations varied little from that of prehurricane years (Elliser and Herzing, in press). Associations with other females continued to be constrained within the clusters, strong associations were often between reproductively active females, strong associations were not limited to same age class pairs and strong mother/offspring relationships continued past weaning, sometimes into adulthood.
The high female mean CoA seen posthurricane indicated increased cohesiveness and may be related to female reproduction and sociality. The stress of losing so many individuals, and the lower birth rate observed in these years (DLH and CRE, unpublished data), may have initiated a social tightening between females within clusters. Females generally associate with others in the same stage of life (Wells et al. 1987, Herzing and Brunnick 1997). Lowered reproduction indicates that more females were in similar reproductive states during this time, allowing a greater number of females to more easily associate with one another. These females would have some level of social familiarity with each other as most females interact to some degree with all other females (and often with their offspring as well) within the cluster. Social familiarity has been shown to be important since closely associated females may have been close associates as calves or juveniles (Möller and Harcourt 2008; Elliser and Herzing, in press). In addition, allomaternal care is an important aspect of female sociality and has been documented in other bottlenose dolphin populations (Wells et al. 1987, Shane 1990, Mann and Smuts 1998, Rogers et al. 2004), this spotted dolphin community (Elliser and Herzing, in press), and primates (nursery groups in chimpanzees: Pepper et al. 1999; nonreproductive helpers in marmosets: Stevenson and Rylands 1988). Sociality has been documented to influence fitness traits, revealing the adaptive value of female sociality (Frère et al. 2010). The results of this study showed that even through demographic upheaval, normal female association patterns remain evident and further support that female sociality depends greatly on reproductive status and social familiarity.
Spotted dolphins have male alliance social patterns like those of their closely related cousins, bottlenose dolphins (T. aduncus) in Shark Bay, Australia, including first and second order alliances (Elliser and Herzing, in press). Of the first order alliances, some pairs survived the hurricanes and continued their long-term associations, some since 1985, lasting up to 22 yr. Long-term alliances of this magnitude have been documented in Sarasota and Shark Bay (Connor et al. 2000). Other alliances changed after the loss of a member, where the surviving member began an alliance with a new individual, which has also been documented in Sarasota (Wells et al. 1987), Shark Bay (Smolker et al. 1992), sympatric bottlenose dolphins in this study area (Rogers et al. 2004) and previous long-term work on this spotted dolphin community (Elliser and Herzing, in press).
The greatest difference in male association patterns after the hurricanes was the decrease in complexity between male alliances. Following the storms, only one second order alliance was observed and only first order alliances remained (both old and new). In many species, alliances are usually attributed to increased access (directly or indirectly) to females (primates: Watts 1998; lions: Wilson et al. 2001; bottlenose dolphins: Wells 1991, Connor et al. 1992) and successful mating (Krützen et al. 2004, Wiszniewski et al. 2012). In this community of spotted dolphins, male coalitions often monopolize females (Herzing and Johnson 1997; Elliser and Herzing, in press). A genetic study has revealed that first order alliance membership may increase reproductive success (Green et al. 2011), indicating that access to mates is also a key role of alliances in spotted dolphins. These studies, along with the changes posthurricane in this study, indicate that first order alliance formation may be more critical for access to mates than second order alliances, especially when changes in demography and sex ratio occur.
Alliance formation and size of the alliance are strongly affected by the mean number of males competing for a female and the factors that impact this, such as the density of females, operational sex ratio, and encounter rate with females (Whitehead and Connor 2005). Möller (2012) hypothesized that the development of male alliances in delphinids is related to both small male-biased sexual size dimorphism and male-biased operational sex ratio (due to differences in parental investment). Alliances and/or coalitions will form when the female encounter rate increases such that the cost of sharing copulations is outweighed by the benefits of cooperative female defense (Connor and Whitehead 2005). Coalitional mate guarding, previously unknown in chimpanzees, was found to develop in large mating parties when the groups had too many males for single males to maintain exclusive access to estrous females (Watts 1998). Prior to the hurricanes, the sex ratio of spotted dolphins was skewed towards females (32 males, 42 females), possibly supporting the formation of both first and second order alliances as more females were available. After the hurricanes, the sex ratio was reduced to roughly 1:1 (23 males, 24 females). In this scenario the cost of sharing mating opportunities with other alliances may be too great as the encounter rate with different females is much lower, especially within clusters. The benefits of having one or two other males to aid in gaining access to females may still outweigh the costs of sharing mating opportunities; however, the cost may be too high to share with another entire alliance while female numbers are reduced.
This fitness cost could also be related to the kinship level of alliances, which varies between and even within populations (e.g., Möller et al. 2001, Krützen et al. 2003). Genetic relatedness of the alliances in this study is currently unknown. However, the lack of second order alliances after the hurricanes could be explained if the first order alliances were more highly related than the second order alliances, increasing the individual fitness cost of second order alliances during posthurricane years. Further genetic analysis will help determine whether kinship played a role in these changes in alliance membership.
Spotted dolphin alliances are also important for interspecific interactions with sympatric bottlenose dolphins on LBB (Herzing and Johnson 1997; Elliser and Herzing, in press). Behavioral research on regularly occurring interactions has shown bottlenose dolphins, which are larger and more dominant, are usually the aggressors (Herzing and Elliser, in press) and that it takes six spotted dolphins to chase away one bottlenose dolphin (Herzing and Johnson 1997). After the hurricanes, however, these types of interactions decreased significantly becoming almost nonexistent (Elliser 2010). During this time of restructuring for both communities, these interspecific interactions may not have been a priority for either species. The lack of second order alliances after the hurricanes may have occurred if a primary reason for second order alliance membership is to aid each other (similar to aiding in consortships seen in Shark Bay) during these interspecific encounters.
Alliance-strength relationships are rare in young juveniles and have not been previously documented in this spotted dolphin community (Elliser and Herzing, in press). The bonds between males apparently grow from relationships developed in subadult groups or earlier and crystallize during sexual maturity (Wells 1991; Elliser and Herzing, in press). More affiliative associations between juveniles, which are less constrained by the mating system and social organization of adults, may indicate the early stages of alliance formation (Gero et al. 2005). The dynamics controlling alliance formation may have been altered after the hurricanes so that the loss of individuals effectively sped up the development of alliance formation. Juvenile social behavior has been shown to change due to other environmental disturbances. Increased sociality (and increase in number of associates) was documented in juvenile bottlenose dolphins after severe harmful algal blooms in Sarasota, FL (McHugh et al. 2010). Factors that alter social behavior may affect young animals to a greater degree, due to the fact that juveniles typically socialize at higher rates than adults (McHugh et al. 2010). Alternatively, the strong alliance level associations between juveniles in this study may be due to lack of individuals. With fewer choices, individuals that associated previously may increase their associations to a stronger level. Future work detailing the stability and processes of juvenile and alliance development will shed light on the origin of alliance formation.