Abstract
Ranging behaviour and temporal patterns of individuals are known to be fundamental sources of variation in social networks. Spatiotemporal dynamics can both provide and inhibit opportunities for individuals to associate, and should therefore be considered in social analysis. This study investigated the social structure of a Lahille's bottlenose dolphin (Tursiops truncatus gephyreus) population, which shows different spatiotemporal patterns of use and gregariousness between individuals. For this, we constructed an initial social network using association indices corrected for gregariousness and then uncovered affiliations from this social network using generalized affiliation indices. The association‐based social network strongly supported that this dolphin population consists of four social units highly correlated to spatiotemporal use patterns. Excluding the effects of gregariousness and spatiotemporal patterns, the affiliation‐based social network suggested an additional two social units. Although the affiliation‐based social units shared a large part of their core areas, space and/or time use by individuals of the different units were generally distinct. Four of the units were strongly associated with both estuarine and shallow coastal areas, while the other two units were restricted to shallow coastal waters to the south (SC) and north of the estuary (NC), respectively. Interactions between individuals of different social units also occurred, but dolphins from the NC were relatively more isolated and mainly connected to SC dolphins. From a conservation management perspective, it is recommended that information about the dolphin social units should be incorporated in modeling intrapopulation dynamics and viability, as well as for investigating patterns of gene flow among them.
Highlights
Social structure is a synthesis of the nature, quality, and pattern‐ ing of the relationships among members of a population (Hinde, 1976)
Using ten years of photo‐ID data and social network analyses, this study showed that the Lahille’s bottlenose dolphins inhabiting the Patos Lagoon estuary and adjacent coastal waters in southern Brazil show preferred and/or avoided associations and form social units likely driven by their gregariousness, spatiotemporal use pat‐ terns and social preferences
This pattern of social relationships and space/time use led to the identification of three major dolphin units or subpopulations based on spatial use patterns: a large unit composed by four affiliation‐based social units (SUs1–4) composed by resident individuals which use the entire study area but are mostly found in the estuary mouth and its adjacencies; and two coastal affiliation‐based social units (SU5 and SU6) composed by some residents, but with seasonal inputs from transient individuals, which, in general, do not use the inner estuary; one preferentially using the southern area, and the other the northern area
Summary
Social structure is a synthesis of the nature, quality, and pattern‐ ing of the relationships among members of a population (Hinde, 1976). Among dolphins that were never observed inside the estuary, some appear to travel during the colder months from Uruguay to PLE’s adjacent southern coast (ca 250 km southward; Laporta et al, 2016), while others, tend to use the area immediately to the north of the PLE during warmer months (R.C.G., personal observation) The objectives of this long‐term study on this Lahille’s bot‐ tlenose dolphin population were to (a) categorize and group in‐ dividuals according to their patterns of spatial use and temporal fidelity to the area; (b) identify the most adequate analytical method to describe its social structure; and (c) verify the pres‐ ence of social units and elucidate their role within the popula‐ tion’s social network
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