Hot cross-species toad sex

Hybrids are sort-of a mystery in evolutionary biology. In the strictest sense, they shouldn't exist because the offspring are often sterile or reproductively impaired. It is to a species evolutionary benefit to limit the number of hybrid offspring in most cases, so biologists often attribute hybrids to mistakes on the animal's part. The animal mistook a similar species for one of its own.

However, Karin Pfennig, publishing in Science, shows that under some circumstances animal's will choose to mate across-species.

Pfennig looked at two species of frogs from the American Southwest: Spea bombifrons and Spea multiplicata.

Here is a video our friend the Spadefoot toad:

(Apparently the toads are actually mined from the Earth and appear by spontaneous generation. Just kidding...)

Anyway, the two species share part of their range, and their offspring are fertile albeit to a lesser extent that purebreed offspring:

Spadefoot toads, Spea bombifrons and Spea multiplicata, risk hybridizing where they cooccur in the southwestern United States across ~20% of S. bombifrons' range. Hybrid offspring are viable and can reproduce, albeit with reduced fertility: Hybrid males can be sterile [although the frequency of sterility among hybrid males is unknown], and female hybrids produce fewer eggs than pure-species females (15). Hybridization between these species has historically been spatially variable, with hybrid frequency ranging from 0 to 40% across populations.

These observed patterns of hybridization may be explained if S. bombifrons females can benefit from hybridization. Spadefoots breed in ephemeral pools, and their tadpoles often fail to metamorphose before ponds dry. S. multiplicata develop more rapidly than S. bombifrons, and hybrid tadpoles metamorphose sooner than pure S. bombifrons tadpoles. Thus, for S. bombifrons females, hybridization may enhance offspring survival. (Citations removed. Emphasis mine.)

Though they are less fertile, the hybrids do develop more quickly -- which is really good if you have to grow up fast in a rapidly drying pond. Pfennig hypothesized that because of this rapid development, female toads would be more likely to choose hybrid mates in shallower ponds -- suggesting drought -- than in deeper ponds.

She performed the following experiment (summarized ably here):

To investigate this further, Pfennig played recordings of songs from the males of each species: the baritone croak of S. multiplicata (video) from one speaker and the tenor quack of S. bombifrons from another (video). She then recorded which speaker the female S. bombifrons approached.

Female S. bombifrons were more likely to approach the call of an S. multiplicata male when in shallow waters, she found.

This suggests that S. bombifrons might benefit in some way from cross-species hybridization when waters are shallow. One possibility is that the toads need their offspring to develop more quickly when water is in short supply, so that the tadpoles turn to toads before the shallow puddles run dry. S. bombifrons are slower to develop; S. multiplicata and hybrids are both faster.

Field tests confirmed that hybrid tadpoles were more likely to survive through metamorphosis in rapidly drying pools.(Links in the original.)

This is really interesting stuff because it shows that there are certain circumstances where it could be favorable to hybridize. The animal's recognize these circumstances and act accordingly. This explains why hybrids might persist in spite of their reproductive impairment. Pfennig concludes:

These findings suggest that facultative switches in female mate-choice behavior contribute to adaptive hybridization and explain localized hybridization in habitats where hybrids may have higher fitness. In addition, these results suggest how hybrids may persist in the face of a general pattern of selection against them. Generally, when hybrids are disfavored, selection should promote the evolution of behaviors in sympatry that preclude hybridization (i.e., reinforcement). As expected, S. bombifrons females in sympatry, but not allopatry, discriminate against heterospecifics when hybridization is costly. If, however, females facultatively hybridize when it is beneficial, hybridization may persist locally in the face of a global pattern of reinforcement. The presence of hybrids in systems that have seemingly undergone reinforcement is often attributed to mistakes in mate choice, constraints on female choice, or forced copulation by males. However, facultative adaptive hybridization potentially explains the persistence of hybrids despite the prediction that reinforcement should eventually eliminate hybridization. (Emphasis mine. Citations removed.)

Hat-tip: Nature