Choosy Cannibals Preferentially Consume Siblings with Relatively Low Fitness Prospects

Abstract

When an individual can selfishly cannibalize a relative or altruistically set it free, the benefits of altruism will be positively associated with the relative's fitness prospects (the benefits it receives from altruism). We tested the prediction that altruism should be preferentially directed toward high-quality relatives using larvae of the New Mexican spadefoot toad (Spea multiplicata), a species in which tadpoles plastically express omnivore and carnivore ecomorphs. In a no-choice design, we presented carnivores with sibling or nonsibling omnivores varying in developmental stage, which is positively associated with survival in this toad's ephemeral larval environment. There was a significant interaction between relatedness and developmental stage on the probability of cannibalism: carnivores were overall more likely to cannibalize less developed omnivores, but this effect was exaggerated when the potential victim was a sibling. This evidence that altruists favor relatives with high fitness prospects highlights the numerous factors shaping altruism's payoffs.

Cannibalism and Infectious Disease: Friends or Foes?

“…By consuming weak or recently infected individuals, parents could protect their healthier offspring from later infection through eliminating current parasites and a reduction in clutch/litter density (Polis 1981;Hart 1990;Elgar and Crespi 1992;Richardson et al 2010;Lehtonen and Kvarnemo 2015). This possibility ties well to recent work suggesting that the strength of kin selection varies with estimated future fitness (Dugas et al 2016) and suggests that filial cannibalism could be another estimation of risk (including the risk of disease transmission) balanced against offspring potential (Lehtonen and Kvarnemo 2015). While additional explanations for filial cannibalism exist, such as maintaining balance between future costs of offspring rearing and resource levels (Fox 1975;Rohwer 1978;Polis 1981;Manica 2002;Klug and Bonsall 2007), the casting of filial cannibalism as a mechanism that reduces the prevalence of parasites is similar to a smallerscale version of the culling through cannibalism concept outlined above.…”

Size-dependent aggression towards kin in a cannibalistic species

“…In animals where aggression can escalate to cannibalism, the majority of studies focus on the causes that underlie the killing and consumption of conspecifics. This previous work has been primarily done in insects (but see Dugas et al 2016b for a similar study done with spadefoot toads) and has yielded a variety of results (interaction between relatedness and size: Dobler and Kölliker 2011 , relatedness effect only: Bilde and Lubin 2001 ; Roberts et al 2003 ), providing no consistent pattern to extrapolate to cannibalistic vertebrates. In D. tinctorius , where there is high offspring investment (i.e., male parental care and low fecundity), we found that large tadpoles (where size is relative between pairs) from sibling dyads were the least aggressive, expressing almost half the amount of aggression compared with large tadpoles from non-sibling dyads ( Figure 2 ).…”

“…In systems with sibling aggression, fighting represents an important potential advantage in early development for securing resources ( Drummond et al 2003 ; Naidenko and Antonevich 2009 ); in cannibalistic species, the factors that shape opponent assessment are vital, as there is the threat of interactions escalating to death. Thus, cannibalism is often conditional on the assessment of either the environment (food availability: Mayntz and Toft 2006 ; Dugas et al 2016a , conspecific density: Maret and Collins 1994 , Frankino and Pfennig 2001 ; or a combination of the two: Wildy et al 2001 ) or the opponent (size and relatedness: Dugas et al 2016b , condition: Ibáñez and Keyl 2010 ).…”

“…This has been shown to be an important factor in several cannibalistic species that demonstrate kin discrimination and avoid eating kin ( Pfennig et al 1994 ; Pfennig and Frankino 1997 ; van den Beuken et al 2019 ), although there are also examples of cannibals consuming their kin without avoidance ( Boots 2000 ; Gray et al 2009 ). Although differences in opponent size and relatedness have individually been identified as variables that shape cannibalistic decisions, the interaction between these two variables has yielded diverse results across taxa where, for example, studies have reported a strong interactive effect in earwigs ( Dobler and Kölliker 2011 ), the absence of size effect in spiders ( Bilde and Lubin 2001 ; Roberts et al 2003 ), and both a stage and phenotype dependent adversity where spadefoot toads are less likely to cannibalize other cannibals ( Pfennig 1999 ) as well as more developed siblings ( Dugas et al 2016b ). Ultimately, more work is needed to tease apart the factors influencing decision-making in juvenile cannibals in a broader range of taxa.…”

Habitat ephemerality affects the evolution of contrasting growth strategies and cannibalism in anuran larvae

“…Differences in growth strategies generally follow predictions of adaptation to ephemeral conditions ( Richter-Boix, Llorente & Montori, 2006 ; Dittrich et al, 2016 ), but we found that this effect was contingent on growing density and food availability. We also found lower cannibalism rates among tadpoles from ephemeral streams, which may be accounted for by the indirect fitness costs involved ( Pfennig, 1997 ; Dugas et al, 2016 ). By comparing two geographically isolated populations using a common garden experiment, we have confirmed that the observed differences at least partly have a genetic basis.…”