• individual specialization;
  • intraspecific variation;
  • kin;
  • kin selection;
  • polyphenism;
  • spadefoot toads;
  • Spea multiplicata

When resource competition occurs between close relatives, the negative effects of competition are potentially amplified. However, kin selection theory predicts that natural selection should promote the evolution of mechanisms that minimize the intensity of competition between kin. Experimental tests of these hypotheses are mixed, however. Moreover, there is little consensus regarding the generality of either outcome, suggesting that the conditions important in determining the effects of competition between kin are likely complex and not fully understood. We performed two experiments using spadefoot toad tadpoles (Spea multiplicata) to evaluate the hypothesis that individuals can minimize the negative effects of exploitative competition by using alternative resources when competing with close relatives. Supporting our hypothesis, we found that only when individuals had access to alternative resources were the negative effects of competition between siblings less than between unrelated competitors. We suggest that mechanisms to lessen kin competition may be more likely to evolve in environments where alternative resources are available, and that selection to minimize exploitative competition between kin may promote the evolution of resource polyphenism. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 689–695.