1. The ultimate causes of communal breeding and joint parental care in various species of Nicrophorus burying beetles have not been resolved satisfactorily. One hypothesis suggests that females remain on the carcass for extended periods of time because joint defence affords them improved probabilities of retaining the carcass successfully in the face of intense competition from intra-generic competitors.
2. In a field experiment designed to test this hypothesis in N. defodiens (Mannerheim), breeding associations of two females and a male were no more successful at retaining their carcass than were monogamous pairs, lending no support to the hypothesis.
3. Intra-generic intruders that usurped already-buried carcasses were typically much larger than the original residents.
4. The body size of original residents affected both the burial depth and the probability of a takeover. Larger beetles buried the carcass deeper and were more likely to retain possession of the carcass. Group composition also did not affect the depth at which carcasses were buried.
5. Severe and even fatal injuries incurred by some residents indicated the occurrence of violent and damaging fights between competitors over carcasses in the field.