The distribution of parasites among mammalian hosts is complex and represents a differential ability or opportunity to explore host phylogeny. Using data on over 1400 parasite species that have been documented to infect between 1-81 mammal host species, we characterize the generalism of parasites using standard effect sizes for three metrics describing host relatedness: mean phylogenetic distance (PD), span or maximum PD, and degree of aggregation of hosts (the tendency for the hosts of a parasite to cluster within the phylogenetic tree). We explore variation in these metrics in terms of parasite taxonomy and transmission mode. Collectively, this macroecological perspective helps to generate a broad picture of how certain types of parasite and transmission mode are differentially linked with the tendency to associate with multiple host species, captured by a set of complementary metrics defining the dimensions of parasite generalism.
Of the multi-host parasites, the majority are generalists, with the average relatedness of their mammalian host species being roughly equivalent to that expected from a random sample of mammals. Only a minority of parasites are associated with host sets that are more related than expected by chance. The degree of host relatedness is affected by both parasite taxonomy and transmission mode; bacteria and arthropod parasites are typically the most generalist, helminths are intermediate, and viruses and protozoa are on average the most specialist. While viruses are specialist as a group, the group contains some of the most generalist parasites. Transmission mode also influences the degree of parasite generalism, with closer relatedness of hosts exhibited for parasites that rely on close contact or intermediate hosts, compared to those with environmental or vector-borne transmission. Both the span of a parasite across the host tree and the tendency to infect clusters of related hosts vary among parasites. The taxonomy and transmission modes of parasites with only one known host species are broadly reflective of multi-host parasites, with rarely single-host viruses being a notable exception. Lastly, a host species' evolutionary distinctiveness is a weak predictor of the number of parasite species associated with it.