Harrison's rule scales up to entire parasite assemblages but is determined by environmental factors

Abstract

1. Harrison's rule states that parasite body size and the body size of their hosts tend to be positively correlated. After it was proposed a century ago, a number of studies have investigated this trend, but the support level has varied greatly between parasite/host associations. Moreover, while the rule has been tested at the individual species level, we still lack knowledge on whether Harrison's rule holds at the scale of parasite and host communities. 2. Here, we mapped flea (parasites) and rodent (hosts) body sizes across Mongolia and asked whether Harrison's rule holds for parasite/host assemblages (i.e. whether a parasite's average body size in a locality is positively correlated with its host's average body size). In addition, we attempted to disentangle complex relationships between flea size, host size and environmental factors by testing alternative hypotheses for the determinants of fleas' body size variation. 3. We gathered occurrence data for fleas and rodents from 2,370 sites across Mongolia, constructed incidence matrices for both taxa and calculated the average body sizes of fleas and their hosts over half-degree cells. Then, we applied a path analysis, accounting for spatial autocorrelation, trying to disentangle the drivers of the correlation between parasite and host body sizes. 4. We found a strong positive correlation between average flea and host size across assemblages. Surprisingly though, we found that environmental factors simultaneously affected the body sizes of both fleas and hosts in the same direction, leading to a most likely deceptive correlation between parasite and host size across assemblages. 5. We suggest that environmental factors may, to a great extent, reflect the environmental conditions inside the hosts' burrows where fleas develop and attain their adult body size, thus influencing their larval growth. Similarly, rodent body size is strongly influenced by air temperature, in the direction predicted by Bergmann's rule. If our findings are valid in other host-parasite associations, this may explain the dissenting results of both support and lack thereof for Harrison's rule.

Latitudinal gradients in body size and sexual size dimorphism in fleas: males drive Bergmann's pattern

“…Nevertheless, when average flea and host sizes across assemblages, rather than sizes of individual species, were considered, it was found that environmental factors simultaneously affected the body sizes of both fleas and hosts in the same direction, so that both average flea and host sizes increased toward higher latitudes, and the correlation between parasite and host size across assemblages was considered to be misleading (Maestri et al. 2020).…”

Functional biogeography of parasite traits: hypotheses and evidence

“…Second, a comparative study on ticks found no correlation between body size and local annual mean temperature, which itself varies with latitude [ 66 ]. Finally, a comparison across communities of fleas infecting small mammals in Mongolia reported that the average parasite body size, computed across all locally occurring species, increased toward higher latitudes and lower temperatures [ 67 ].…”

Harrison’s rule corroborated for the body size of cleptoparasitic cuckoo bees (Hymenoptera: Apidae: Nomadinae) and their hosts

“…Also, HR has been documented in many parasitic organisms: parasitic nematodes and their hosts 12 , parasitic barnacles and decapods 17 , avian lice and birds 18 , and fleas and rodents 19 . HR has been tested for several insect groups previously, in terms of both traditional parasitism and herbivory, but most insect groups remain unexplored 11 .…”