Individuals often associate socially with those who behave the same way. This principle, homophily, could structure populations into distinct social groups. We tested this hypothesis in a bottlenose dolphin population that appeared to be clustered around a specialized foraging tactic involving cooperation with net-casting fishermen, but in which other potential drivers of such social structure have never been assessed. We measured and controlled for the contribution of sex, age, genetic relatedness, home range and foraging tactics on social associations to test for homophily effects. Dolphins tended to group with others having similar home ranges and frequency of using the specialized foraging tactic, but not other traits. Such social preferences were particularly clear when dolphins were not foraging, showing that homophily extends beyond simply participating in a specific tactic. Combined, these findings highlight the need to account for multiple drivers of group formation across behavioural contexts to determine true social affiliations. We suggest that homophily around behavioural specialization can be a major driver of social patterns, with implications for other social processes. If homophily based on specialized tactics underlies animal social structures more widely, then it may be important in modulating opportunities for social learning, and therefore influence patterns of cultural transmission.

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SUMMARY A patient is described in whom diagnosis of isolated pulmonary valve endocarditis was made by M-mode and two dimensional echocardiography. Angiography confirmed the presence of vegetations on the pulmonary valve. At cardiac surgery a quadricuspid, non-stenotic valve with ruptured medial leaflet covered by necrotic vegetations and a small ventricular septal defect were found.Echocardiography is known to be useful in diagnosing valvular endocarditisl 2 of the mitral and aortic valves but only two cases of pulmonary valve endocarditis have been previously diagnosed in this way. We report another. Case reportA 46-year-old man, known to have an undiagnosed asymptomatic congenital heart defect, had a high temperature in November 1979. He was treated with antibiotics which were at first effective but pyrexia reappeared two weeks later. Cephalosporin was given intravenously for 10 days, but the patient became very weak, and haemorrhagic purpura appeared on the legs; evening pyrexia persisted. Finally, in May 1980, he was admitted to this hospital; bacterial endocarditis was suspected and penicillin started.At physical examination the patient appeared very ill and febrile, with gross hepatosplenomegaly. A grade 4/6 harsh pansystolic murmur was heard in the second and the third left intercostal space followed by a grade 1/6 diastolic murmur. The second heart sound was almost inaudible on the pulmonary area.The electrocardiogram and chest x-ray film appeared to be within normal limits. The M-mode echocardiogram ( Fig. 1) Treatment with cefuroxime led to a brief remission of fever with apparent clinical improvement but high fever reappeared after 10 days despite continuous treatment and finally the patient developed a left lower lobe infarction with pleural effusion. As he was so ill, cardiac catheterisation and angiocardiography, previously planned, were abandoned and cardiac surgery was performed on the evidence of the echocardiographic diagnosis on 5 June. The pulmonary artery was incised transversely above the level of the valve. A quadricuspid non-stenotic valve was seen; the medial leaflet was ruptured and covered by an overgrowth of necrotic tissue (Fig. 3); a small ventricular septal defect was found just below the pulmonary valve which was excised. A biological prosthesis was inserted, and the ventricular septal defect sutured with four stitches on Teflon pledgets.Culture of the valve grew Staphylococcus cutis.