Increased reproduction success, enhanced foraging and reduced predation risk are usually regarded as major factors favouring the evolution of social behaviour. Here we formulate a series of hypotheses relating sexual, ecological and behavioural factors to evaluate their explanatory value for 13 extant otter species, estimating the extent to which each factor contributes to the sociality of each species. We also compare individual behaviours within some of the species. Four otter species are obligatory social; four are obligatory solitary; five present both types of social organization. Social organizations of otter species are not related to their phylogenetic relationships. However, many otter species exhibit intra-species patterns of flexible social lifestyles. Both solitary and social otters adjust their social patterns in response chiefly to food availability, but also to habitat features and competition.

Group living is more common when intraspecific competition is reduced or trophic resources replenish rapidly. Under these circumstances, group members often forage individually. When otters forage individually, they often switch prey type when they compete with other conspecifics. Social structures of otters fall into seven types: (1) family groups; (2) extended family groups, often with an alpha dominant pair; (3) highly social groups with helpers; (4) collective hunting groups; (5) solitary lifestyle; (6) unstable mixed-sex groups; and (7) single-sex bachelor groups. When an individual of a species with variable sociality adopts one type of sociality, this may be only temporary. Variations in social life are actually based on a series of events that induce individuals to make decisions taking ecological factors into account.

Although ontogenetic factors can influence delayed dispersal of otters, social factors rather than ecological factors could play an important role in the formation of groups, and cohesiveness and kinship appear to be secondary effects of reduced dispersal more than primary causes for living in a group. Appropriate adjustment of group behaviour reduces the cost of sociality because individuals avoid social interactions when benefits are low but gather together when group living provides real advantages. Although any one model is unlikely to explicate all facets of sociality, evolution towards a social group results mainly from interactions within a family.

繁殖成功的增加，觅食的增加和捕食风险的降低通常被认为是促进社会行为发展的主要因素。在这里，我们制定了一系列有关性，生态和行为因素的假设，以评估其对13种现存水獭物种的解释价值，估计每种因素在多大程度上促进了每个物种的社会性。我们还比较了某些物种内的个体行为。水獭有4种是必须的。四是强制性的单身；五个代表了两种类型的社会组织。水獭物种的社会组织与它们的系统发生关系无关。然而，许多水獭物种表现出灵活的社会生活方式的种内模式。

当种内竞争减少或营养资源迅速补充时，群体生活更为普遍。在这种情况下，小组成员经常单独觅食。当水獭单独觅食时，当它们与其他物种竞争时，它们通常会切换猎物类型。水獭的社会结构分为七种类型：（1）家庭群体；（2）大家庭群体，通常以阿尔法占主导地位。（3）有帮手的高度社交团体；（四）集体狩猎团；（5）孤独的生活方式；（6）不稳定的混血人群；（7）单性单身汉群体。当具有可变社会性的物种的个体采用一种社会性时，这可能只是暂时的。社会生活的变化实际上是基于一系列事件，这些事件促使个人在考虑生态因素的情况下做出决定。

尽管个体发育因素可以影响水獭的延迟传播，但社会因素而不是生态因素可以在群体形成中发挥重要作用，凝聚力和亲缘关系似乎是分散传播的次要影响，而不仅仅是群体生活的主要原因。对群体行为的适当调整降低了社会成本，因为当收益低时个人会避免社交互动，而当群体生活提供真正的利益时会聚在一起。尽管任何一种模式都不太可能说明社会的所有方面，但向社会群体的演变主要来自家庭内部的互动。