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Parental effects are often considered an evolved response, in which parents transmit information about the environment to enhance offspring fitness. However, these effects need not be adaptive. Here, we provide a striking example by presenting evidence that overfeeding of adult Mexican spadefoot toads, Spea multiplicata, is associated with decreased offspring survival. After a temporary change to their standard feeding regimen, S. multiplicata in our captive colony developed a much higher body condition (i.e. body mass for a given body length) than those in the wild. We analysed data from three subsequent experiments and found that although the body condition of a father was positively correlated with tadpole survival, mothers with a higher condition had lower tadpole survival. Our study highlights how obesity can negatively impact future generations via maladaptive maternal effects. Such effects could be especially likely for animals living in variable environments (such as spadefoots) that have evolved ‘thrifty phenotypes’ that make them prone to obesity. Our study also illustrates how husbandry conditions typically regarded as beneficial might be harmful. Given that captive breeding programmes are increasingly used to combat worldwide amphibian declines, these programmes must consider the ecology and evolutionary history of the focal species to minimize any maladaptive parental effects.

 

Mating with another species is often maladaptive because it generally results in no or low-fitness offspring. When hybridization is sufficiently costly, individuals should avoid mating with heterospecifics even if it reduces their ability to mate with high-quality conspecifics that resemble heterospecifics. Here, we used spadefoot toads, Spea multiplicata, to evaluate whether females alter their preferences for conspecific male sexual signals (call rate) depending on heterospecific presence. When presented with conspecific signals against a background including both conspecific and heterospecific signals, females preferred male traits that were most dissimilar to heterospecifics—even though these signals are potentially associated with lower-quality mates. However, when these same females were presented with a background that included only conspecific signals, some females switched their preferences, choosing conspecific signals that were exaggerated and indicative of high-quality conspecific mates. Because only some females switched their preferences between these two chorus treatments, there was no population-level preference for exaggerated conspecific male signals in the absence of heterospecifics. These results show that hybridization risk can alter patterns of mate choice and, consequently, sexual selection on male signals. Moreover, they emphasize that the strength and expression of reproductive barriers between species (such as mate choice) can be context-dependent. 

Adaptive radiations are characterized by the rapid proliferation of species. Explaining how adaptive radiations occur therefore depends, in part, on identifying how populations become reproductively isolated––and ultimately become different species. Such reproductive isolation could arise when populations adapting to novel niches experience selection to avoid interbreeding and, consequently, evolve mating traits that minimize such hybridization via the process of reinforcement. Here, we highlight that a downstream consequence of reinforcement is divergence of conspecific populations, and this further divergence can instigate species proliferation. Moreover, we evaluate when reinforcement will––and will not––promote species proliferation. Finally, we discuss empirical approaches to test what role, if any, reinforcement plays in species proliferation and, consequently, in adaptive radiation. To date, reinforcement’s downstream effects on species proliferation remain largely unknown and speculative. Because the ecological and evolutionary contexts in which adaptive radiations occur are conducive to reinforcement and its downstream consequences, adaptive radiations provide an ideal framework in which to evaluate reinforcement’s role in diversification.

 

Hybridization—interbreeding between species—is generally thought to occur randomly between members of two species. Contrary to expectation, female plains spadefoot toads (Spea bombifrons) can increase their evolutionary fitness by preferentially mating with high-quality males of another species, the Mexican spadefoot toad (Spea multiplicata). Aspects of Mexican spadefoot males’ mating calls predict their hybrid offspring’s fitness, and plains spadefoot females prefer Mexican spadefoot males on the basis of these attributes, but only in populations and ecological conditions where hybridization is adaptive. By selecting fitness-enhancing mates of another species, females increase hybridization’s benefits and exert sexual selection across species. Nonrandom mating between species can thereby increase the potential for adaptive gene flow between species so that adaptive introgression is not simply happenstance.

 

Males of many species aggregate in large groups where they signal to attract females. These large aggregations create intense competition for mates, and the simultaneous signaling by many individuals can impair any given male’s ability to attract females. In response to this situation, male signals can be modified, either evolutionarily or facultatively, such that the detectability of the signal is enhanced. The way in which signals are modified varies among even closely related species, yet few studies have evaluated what causes such variation. Here, we address this issue using male spadefoot toads (Spea multiplicata and Spea bombifrons), which call to attract females. Using data from natural populations, we examined if, and how, male calls of 3 different call types (S. multiplicata with a slow call, S. bombifrons with a slow call, and S. bombifrons with a fast call) varied depending on competition with other males. We found that in both call types consisting of slow calls, call pulse rate decreased with increasing competition. By contrast, in the call type consisting of fast calls, call rate decreased with increasing competition. Moreover, we found that the relationship between competition and male call effort—a measure of the energy that males expend in calling––differed between the call types. Such variation in male signals in response to competition can have important implications for explaining diversity in male signals and patterns of sexual selection.