Forging a New Link: Wren Mothers Teach Offspring PasswordsPublished 19 November, 2012
Returning to the question of deception, last week there was a substantial amount of media coverage of a really interesting recent paper in Current Biology looking at deception in superb fairy wrens (pictured), a species subject to brood parasitism by cuckoos. Using the language of my prior post, cuckoos break the Link between, on the one hand, an egg being in a mother bird’s nest and, on the other, the bird inside the egg being the offspring of that mother bird. This Link usually holds because under normal circumstances only eggs laid by the nesting mother bird are in the nest; cuckoos break this Link by laying their own eggs in another bird’s nest, “deceiving” the mother bird into treating them as her own.
The costs of nurturing individuals who are not one’s own offspring are obviously very high, which selects for counter-strategies to defend against parasitism. I wrote about one such strategy previously, which had to do with the coloration and pattern of eggs. In the new , “Embryonic Learning of Vocal Passwords in Superb Fairy-Wrens Reveals Intruder Cuckoo Nestlings,” by Colombelli-Ne´grel et al., the authors investigate, as they say in their title, the idea that parents supply their nestlings with “passwords” during incubation.
The researchers continuously recorded 15 nests during the period that the wrens were nesting, finding what they labeled an “incubation call,” vocalizations that female birds produced while incubating their eggs, but stopped producing after the eggs hatched. Looking at the calls made by the chicks after they hatched, the research team found that while different chicks made different calls across nests, chicks imitated a key element of their particular mother’s incubation call, singing back to her the “password” she taught them while in the egg.
By itself, this analysis leaves open the possibility that the chick-mother call similarity and between-mother differences are due to genetic variation. However, a cross-fostering study adds additional evidence. They moved eggs from one wren mother to another, and found that fostered young’s calls were more similar to their foster mother than to their biological mother, as one would expect if the identifying element was being acquired during incubation. Finally, using playback experiments, the researchers found that when adults were presented with begging calls from individuals who were not their own, the adults fed nestlings less than they did when presented with calls from their own offspring.
These findings raise the question (note: avoided the obvious pun because that’s not what “begs the question” means. Or what it used to mean, anyway. Similarly, it turns out that “literally” now literally can mean “figuratively,” according to a note at Dictionary.com. Sigh.), why don’t the cuckoos simply learn the same vocalization? The answer is illustrated in a nice figure in the paper. Recall that brood parasites tend to hatch earlier than the chicks from the host species, giving them, it has been argued, an advantage over the host species, allowing them to get a head start in their competition with host chicks. In this case, the cuckoos hatch 12 days after incubation while wrens hatch 15 days after incubation. By starting the incubation calls at day 10, mothers give the embryos five days to learn the calls, while the cuckoos have only two days which, the authors argue, might be insufficient time.
So, in essence, this system establishes a new Link, one between, on the one hand, a chick making a call with the crucial auditory element and, on the other, that chick being the mother’s actual offspring. One might expect that there might be selection on cuckoos to break this Link, whether through depositing eggs in the nest earlier, a better leaning mechanism, or some other counter-strategy.
Colombelli-Négrel D., Hauber M., Robertson J., Sulloway F., Hoi H., Griggio M. & Kleindorfer S. (2012). Embryonic Learning of Vocal Passwords in Superb Fairy-Wrens Reveals Intruder Cuckoo Nestlings. Current Biology, 22 (22) DOI: 10.1016/j.cub.2012.09.025