Food caching allows foraging decisions to include both present and future food value. We developed and tested a conceptual framework for measuring animals’ perceptions of future vs. present food value. Fox squirrels (Sciurus niger) are well-known caching animals in North America. We measured giving-up densities (GUDs) of fox squirrels to test the following hypotheses: (1) all else equal, animals should prefer cacheable to non-cacheable foods, and (2) the option value of a cacheable food should change seasonally and be highest preceding lean periods. We presented squirrels with experimental food patches containing either shelled hazelnuts, hazelnuts with their shells intact (to affect cacheability) or both kinds. Our data support both hypotheses. Squirrels exploited food patches with cacheable nuts more thoroughly and left them at lower GUDs. The squirrels’ perception of the future value of hazelnuts with their shells intact was found to be 32% higher than their present value. GUDs also varied by season, fall > spring >winter > summer, likely corresponding to food availability. Season and food cacheability interacted to further determine GUDs. The difference in GUDs between cacheable and non-cacheable nuts varied similarly: fall > spring > winter > summer, likely corresponding to changing food availability, reproductive opportunities and pending energetic shortfalls.
Marius van der Merwe, Joel S. Brown and Burt P. Kotler
Oldřich Kopecký and Jiří Šichtař
Sexual selection based on visual stimuli was recently studied in several amphibian species with permanent or temporary dichromatism. The Alpine newt (Ichthyosaura alpestris) is a Caudate species with prominent sexual dichromatism during the breeding period. We focused on the intersexual differences in the orange, carotenoid-base ventral colouration of this widespread European species. We used an image analysis approach to compare the content of the red colour and saturation of the belly in 80 male and 62 female aquatic adult newts captured during spring migration in two localities in the Czech Republic. Both studied colour parameters of the ventral side of the Alpine newt are connected with sex. Males have significantly higher values of saturation on both localities, but the relationship with red content was not so clear. Individual characteristics (body condition, body length) affect colour variables of males and females similarly. Effect of body condition on colour variables was not demonstrated. Therefore we presume that the pronounced colouration of males is a cue facilitating detection, localisation or interspecies identification, rather than a direct component of male quality as assessed by females.
Bayesian posterior probabilities are wrongly considered by many systematists as indicative of character support, and equivalent to non-parametric bootstrap frequencies. Here I argue against this view. Non-parametric bootstrap is indicative of the amount of evidence in a data matrix supporting each clade in the tree, while Bayesian posterior probabilities are not intended to represent that property. Clades with high posterior probability may not have a large amount of characters favouring them, and their frequencies are the result of the particular sampling procedure of the Bayesian Markov chain Monte Carlo method, which tends to sample very similar topologies according to their posterior probabilities. Both metrics may relate to the notion of confidence, but depict different properties.