We develop a game-theoretic model to explore the question of whether two animals should cooperate in the dangerous activity of obtaining a rich and essential resource. We consider variation in the risks incurred to individuals and in how the activities of the two animals interact to influence the probability of success. We also consider that the animals may be relatives and thus share evolutionary interests. The model is general and can, for instance, be applied to mammalian predators attempting to capture and subdue large and dangerous prey or to female parasitoid wasps that attack and, if successful, paralyse aggressive hosts that then provide the only feeding resource for their offspring. This minimal model of cooperation contains three dimensionless parameters: vulnerability (the ratio between the average time for a lone attacker to subdue the defending resource and the average time for the defender to fatally strike the attacker), the dilution ratio (the extent to which attack by animals acting in tandem reduces a defender's ability to kill its attackers) and the relatedness between the potential attackers. The model predicts that higher values of all three parameters favour cooperation and that for small values cooperation is not evolutionarily stable. Cooperation can arise from an ancestral state of non-cooperation if values of all parameters are sufficiently high but cannot arise among non-relatives, irrespective of other parameter values. Once cooperation has emerged in a population, it can be maintained among nonrelatives at modest values of dilution ratio and vulnerability. We discuss these general predictions in particular relation to the parasitoid genus Sclerodermus, in which multiple females may attack unusually large and aggressive hosts and in which host attack behaviour is mediated by kinship. (c) 2021 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Conifer trees, including Norway spruce, are threatened by fungi of the Heterobasidion annosum species complex, which severely affect timber quality and cause economic losses to forest owners. The timely detection of infected trees is complicated, as the pathogen resides within the heartwood and sapwood of infected trees. The presence of the disease and the extent of the wood decay often becomes evident only after tree felling. Fourier-transform infrared (FT-IR) spectroscopy is a potential method for non-destructive sample analysis that may be useful for identifying infected trees in this pathosystem. We performed FT-IR analysis of 18 phloem, 18 xylem, and 18 needle samples from asymptomatic and symptomatic Norway spruce trees. FT-IR spectra from 1066 – 912 cm−1 could be used to distinguish phloem, xylem, and needle tissue extracts. FT-IR spectra collected from xylem and needle extracts could also be used to discriminate between asymptomatic and symptomatic trees using spectral bands from 1657 – 994 cm−1 and 1104 – 994 cm−1, respectively. A partial least squares regression model predicted the concentration of condensed tannins, a defense-related compound, in phloem of asymptomatic and symptomatic trees. This work is the first to show that FT-IR spectroscopy can be used for the identification of Norway spruce trees naturally infected with Heterobasidion spp.