Spatial and seasonal dietary patterns of the desert horned lizard (Phrynosoma platyrhinos): harvester ant specialist or generalist ant feeder?

Newbold, T. A. Scott; MacMahon, James A.
February 2009
Canadian Journal of Zoology;Feb2009, Vol. 87 Issue 2, p112
Academic Journal
A suite of factors including prey availability and prey defenses influence prey selection by consumers and ultimately define an animal’s feeding strategy (e.g., generalist or specialist). Here we examined the relationship between availability and selection of ants by the desert horned lizard (Phrynosoma platyrhinos Girard, 1852) to test the hypothesis that P. platyrhinos are specialist predators on harvester ants, and to investigate which factors influence the prey preference of lizards. Variation in ant availability and lizard diets was assessed using 83 plots established along a Great Basin shrub-steppe bajada in northwestern Utah, USA. Across the study site, 14 of 20 ant species were represented in the lizard diet. However, 70% of that diet consisted of just two species. In contrast to previous studies, P. platyrhinos did not show a disproportionate preference for harvester ants. Instead, larger ants of any species (including harvester ants) were preferentially selected over medium-sized and smaller ants. These findings suggest that desert horned lizards demonstrate an opportunistic feeding strategy in which lizards select ants based on their body size and abundance. More generally, these results emphasize the importance of assessing predator response across naturally varying gradients in prey availability to evaluate feeding specialization. Une série de facteurs, dont la disponibilité des proies et leurs moyens de défense, influence la sélection des proies par les consommateurs et, en fin de compte, définit la stratégie alimentaire (par ex. généraliste ou spécialisée) d’un animal. Nous examinons ici la relation entre la disponibilité et la sélection des fourmis par le phrynosome cornu (Phrynosoma platyrhinos Girard, 1852) afin de tester l’hypothèse selon laquelle les P. platyrhinos sont des prédateurs spécialisés des fourmis moissonneuses et de déterminer les facteurs qui influencent les préférences de proies chez ce lézard. Nous avons évalué la variation de la disponibilité des fourmis et du régime alimentaire des lézards dans 83 parcelles établies le long d’une bajada de buissons et de steppes dans le Grand Bassin dans le nord-ouest de l’Utah, É.-U. Dans l’ensemble du site d’étude, 14 des 20 espèces de fourmis se retrouvent dans le régime des lézards. Cependant, deux espèces à elles seules constituent 70 % de ce régime. Contrairement à ce qui a été observé dans des études antérieures, P. platyrhinos n’affiche pas de préférence démesurée pour les fourmis moissonneuses. Au contraire, les fourmis de plus grande taille de toutes espèces (y compris les fourmis moissonneuses) sont sélectionnées de préférence aux fourmis de petite ou moyenne tailles. Ces résultats laissent croire que les phrynosomes cornus possèdent une stratégie alimentaire opportuniste dans laquelle ces lézards choisissent les fourmis en fonction de leur taille et de leur abondance. Plus généralement, nos résultats soulignent l’importance d’évaluer la réaction des prédateurs dans des gradients de variation naturelle de disponibilité des proies lorsqu’on veut évaluer la spécialisation de l’alimentation.


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