This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science.
When it comes to sex, it makes sense to stick to your own species. Even putting aside our own innate revulsion, inter-species liaisons are a bad idea because they mostly fail to produce any young. In the few instances they do, the hybrid progeny aren't exactly racing ahead in the survival stakes and are often sterile (think mules).
But having poor unfit young is still better than having no young at all and if an animal's options are limited, siring a generation of hybrids may be a last resort. Karin Pfennig from the University of North Carolina found that the plains spadefoot toad uses just this strategy in times of need.
Female toads breed just once a year, so it pays for them to make the right choice. According to Pfennig's work, they take their health and their environment into account when choosing mates. If their bodies are weak and their surroundings are precarious, the benefits that another species' genes can provide to their young are enough to outweigh the risks.
The south-western United States is home to two species of spadefoot toads with overlapping ranges - the Mexican spadefoot, Spea multiplicata and the Plains spadefoot, Spea bombifrons (more Kermit-like, according to Pfennig). Where both species mingle, they can breed and, as usual, the hybrid young are worse at spawning the next generation than their pure-blooded peers. Hybrid males are often sterile, and hybrid females lay fewer eggs.
Nonetheless, up to 40% of toads in certain areas can be hybrids and this intrigued Pfennig. She wanted to work out whether this was just incidental, or if some circumstances nudged the toads towards mating with individuals from a different species.
Their breeding grounds provided the answer; spadefoots lay eggs in temporary ponds and it's often a race for tadpoles to turn into frogs before the water dries out. Pfennig noticed that hybrids were more common in shallower ponds that dry out quicker, and that's because the two toad species develop at different rates.
On average, Mexican spadefoot tadpoles take less time to make the transition into frog-hood than Plains spadefoot ones, and hybrid tadpoles lie somewhere in the middle. This means that a Plains spadefoot female that's faced with a short-lived pond might do better if she mates with a Mexican spadefoot male, for her young will be more likely to grow up in time.
Pfennig tested this idea by placing Plains spadefoot females in tanks simulating shallow and deep ponds and letting them choose between recorded calls from males of both species. In deep water, they favoured their own kind about 65% of the time, but in the shallower pools, they had no such preferences.
In contrast, Mexican spadefoot females also showed no willingness for breed with other species. Since their tadpoles develop quickly anyway, they gain nothing by courting Plains spadefoot males. Pfennig also found that only Plains spadefoot females that lived in the same areas as Mexican spadefoots had the ability to switch their mate preferences. In parts of the States where the two species are geographically segregated, females never made this choice.
A Plains spadefoot female's health also affects which species she fancies. If she is fitter, she could provision her eggs with more nutrients and her tadpoles would grow faster. That would obviate her reliance on Mexican spadefoot males, even in shallower ponds.
Pfennig's experiments confirmed her idea; the unhealthiest females were the most likely to switch their preferences, from mating with their own kind in deep ones to preferring the other species in shallow ones.
Biologists are used to viewing a female's choice of partners solely in terms of the physical traits of males. But Pfennig's results show that it isn't just about which male has the flashiest colours, the most melodious song or the most impressive antlers. For females, mate choice is a much subtler affair, influenced by environment, personal health and probably many other factors that we have only begun to consider.
Reference: Pfennig. 2007. Facultative mate choice drives adaptive hybridization. Science 318: 965-7.
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This is very interesting, I remember when this study came out. There are also a lot of counter examples in other anurans in which character differences between two closely related species are more different at the contact zone then in allopatry (where the species do not co-occur). Emily Moriarty Lemmon demonstrated it in chorus frogs. It has also been shown in narrow mouthed frogs in the southern US (see here.
Yet another fine reminder of the Physicians instruction to not become dehydrated.
I wondered what that toad was doing humping my leg the other day (not that day, the other one). Now I have a serious case of spadefoot and have to get around in a wheelchair....
Neat to see this on your blog (I began reading your blog only a few months ago, so did not see the original post). Ironically, I wrote a research profile on Pfenning for her college's research magazine just last fall after she received a $1.5M NIH grant to continue this line of inquiry.My story is on pg. 12 of the PDF magazine (pg. 14 of the PDF): http://www.delene.us/PDFs/TDB-UNC-CAS-ToadTracker.pdf. I thought about her work when Kays' new coywolf paper came out in September. Can't help but wonder if it's a similar scenario that led the eastern Canadian wolves to interbreed with the eastern-expanding coyotes. (Kays paper: http://rsbl.royalsocietypublishing.org/content/early/2009/09/23/rsbl.20….)
The prevalence of genes telling Plains spadefoot females when to switch preferences means some of those hybrid descendants are reintegrating into the Plains spadefoot gene pool. If they're bringing Mexican spadefoot genes with them, then that's a one-way gene flow between species.
It would be interesting to look at the behavior of the hybrid females - presumably some of them breed with Mexican spadefoots, at least under shallow water/poor health conditions. That seems like an avenue for gene flow to eventually go into the "pure" Mexican spadefoot gene pool.
Ed Yong, you da man! I am fascinated with intelligent people because they concern themselves with things that probably don't concern those of us on the low side of the Bell curve. It's like, who cares about frogs most days? I enjoyed the article because I found a gramatical error in the boy's piece. "females also showed no willingness for breed with other species" That's allright, I live in the deep south and I don't talk right most of the time and I was born here. Randomly it has dawned me that Asians are obviously the superior race of people. 'course you would expect a judgement like that form a southerner. Anyway, I'm glad the world has people smarter than me so that we have someone to blame when things go wrong. See, some people would have said "when things go south" but I resent that remark. Just kidding. I'm just sayin'.