Pathogens as a factor limiting the spread of cannibalism in tiger salamanders

Abstract

Intraspecific predation is taxonomically widespread, but few species routinely prey on conspecifics. This is surprising as conspecifics could be a valuable resource for animals limited by food. A potential cost of cannibalism that has been largely unexplored is that it may enhance the risk of acquiring debilitating pathogens or toxins from conspecifics. We examined how pathogens affect variation in the incidence of cannibalism in tiger salamander larvae (Ambystoma tigrinum nebulosum), which occur as two environmentally-induced morphs, typicals and cannibals. Salamanders from one population were more likely than those in another to develop into cannibals, even when reared under identical conditions. Variation in the propensity to become a cannibal may be caused by variation in pathogen density. In the population with cannibals at low frequency, bacterial blooms in late summer correlated with massive die-offs of salamanders. The frequency of cannibals correlated significantly negatively with bacterial density in ten different natural lakes. In the laboratory, cannibals exposed to a diseased conspecific always preyed on the sick animal. As a result, cannibals wre more likely to acquire and die from disease than were typicals that were similarly exposed, or cannibals that were exposed to healthy conspecifics. Since conspecifics often share lethal pathogens, enhanced risk of disease may explain why cannibalism is generally infrequent. Pathogens may constrain not only the tendency to be behaviorally cannibalistic, but also the propensity to develop specialized cannibal morphologies.

Ranavirus in Wood Frogs (Rana Sylvatica): Potential Sources of Transmission Within and Between Ponds

“…Notably, it remains unknown to what extent hosts may evolve responses that minimize infection (Pfennig et al, 1991) or how ranavirus persists between outbreaks. Brunner et al (2004) suggested that A. tigrinum virus may be maintained by being passed from parent to offspring, but to our knowledge, evidence for vertical transmission is lacking.…”

Ecology and pathology of amphibian ranaviruses

“…Several studies have documented that ingestion of ranavirus-infected animal tissue can cause infection (Jancovich et al 1997, Pearman et al 2004, Harp & Petranka 2006, Cunningham et al 2007a). Thus, ranaviruses may limit the occurrence of cannibalistic phenotypes in amphibian populations through differential survival (Pfennig et al 1991, Parris et al 2005. Larval salamanders that were infected with ranavirus were depredated 48% less by dragonfly Anax junius larvae than controls (Parris et al 2004), although the mechanism could not be identified.…”

Ranavirus Ecology and Evolution: From Epidemiology to Extinction

“…First, there is a negative correlation between disease frequency and cannibal frequency among salamander populations throughout Arizona (Pfennig et al 1991 ). Although cannibals enjoy a performance advantage by preying on conspecifi cs (Reilly et al 1992 ), cannibalism comes at the cost of increased risk of acquiring ATV (and other pathogens) from conspecifi cs, such that reduced cannibalism might prevent disease spread (Pfennig et al 1991 ;Bolker et al 2008 ). Common garden experiments suggest these patterns are genetically based and thus likely result from past selection (Parris et al 2005 ).…”