Hybrid hoax: A fake cross between Black Vulture and Turkey Vulture

But who forged this hybrid vulture?

Some readers might be familiar with the hoax of the Piltdown Man. In 1912, Charles Dawson announced that he had found the “missing link” between early apes and man near Piltdown, East Sussex. Bones and artifacts at the site were attributed to one individual that was given the scientific name Eoanthropus dawsoni (“Dawson’s dawn-man”). In 1953, scientists published evidence to show that the Piltdown Man was a forgery. It was composed of a human skull of medieval age, a 500-year-old lower jaw of an orangutan and chimpanzee fossil teeth. The identity of the forger remains unknown, but numerous suspects have been put forward (I can recommend the essays of Stephen Jay Gould about this peculiar case). Creationists regularly mention the hoax to highlight the alleged dishonesty of paleontologists that study human evolution, although scientists themselves exposed the Piltdown hoax. As Richard Dawkins once said: “Science. It works, b*tches!”

While looking into vulture hybrids, I came across another – lesser known – hoax. A forged hybrid between Black Vulture (Coragyps atratus) and Turkey Vulture (Cathartes aura).

A Sea of Black Heads

Our story begins in the winter of 1937 when E.A. McIlhenny is trapping vultures at Avery Island, Louisiana. Late in the afternoon of the February 6, he checked the trap with four assistants and found the pen packed with Black Vultures. Then he suddenly discovered a peculiar individual.

In the sea of black heads, I noticed one red head, and remarked that there seemed to be a young Turkey Vulture in the catch. When, on handling the birds, this red-headed ‘buzzard’ was brought to me, I saw at once that it was a hybrid between the Black Vulture (Coragyps aratus atratus) and the Turkey Vulture (Cathartes aura septentrionalis). The predominating color and shape were those of the Black Vulture, but the beak, eyes, eyelids and fore part of the head were like those of the Turkey Vulture, as were also rear part of the head and neck, and all of the body plumage. The shape of the tail was as in the Black Vulture. The legs and feet were more Turkey Vulture than Black Vulture, both in coloring and in shape. The primaries were definitely Turkey Vulture. The remainder of the wing plumage was Black Vulture.

The putative hybrid was shipped by express Saturday afternoon to the US Biological Survey. Close inspection of the specimen revealed that it was not a hybrid. While describing the Turkey Vulture in the Handbook of North American birds, Palmer (1986) noted that “an individual, alleged at the time (McIlhenny 1937) to be a hybrid with the Black Vulture, proved to be the latter when red paint that has been applied to it wore off its head.”

The quote about the alleged hybrid and some drawings of Turkey Vultures from the Handbook of North American Birds by Palmer (1988).

Whodunit

Similar to the Piltdown Man, it is unknown who forged the Black Vulture x Turkey Vulture hybrid. Did McIlhenny himself apply the red paint, or is one of his assistants to blame? Or perhaps an unknown passerby decided to perform a practical joke. We will never know.

To this date, no reliable wild hybrids between Black Vulture and Turkey Vulture have been reported. But that does not mean that this particular cross is impossible. In an overview of raptors bred in zoos, Cade (1986) mentioned a captive hybrid between these two species. However, I could not verify the reliability of this particular case. Another mystery to explore.

References

Cade, T. J. (1986). Propagating diurnal raptors in captivity: a review. International Zoo Yearbook, 24(1), 1-20.

McIlhenny, E. A. (1937). A hybrid between turkey vulture and black vulture. The Auk, 54(3), 384-384.

Palmer, R. S. (1988). Handbook of North American birds (Volume 4). Yale University Press, New Haven and London.

Featured image: Black vulture (Coragyps atratus) © Charles J. Sharp | Wikimedia Commons

Resolving the phylogenetic tree of tinamous

A recent study combined molecular and morphological data.

In 2021, I published a short paper on hybridization patterns in tinamous, a group of Neotropical birds that together with several flightless ratites (e.g., ostriches and emus) form the Palaeognathae. My paper introduced a scoring scheme to assess the reliability of hybrid records. Applying this idea to the tinamous revealed one well-documented case and three doubtful records that require further investigation. In addition to the literature supporting certain hybrid records, I also took into account the distribution of the parental species and their divergence time. Surprisingly, I could not find reliable estimates for divergence times due to a lack of genetic studies. I had to rely on a maximum date that corresponds to the split between the subfamilies Nothurinae and Tinaminae (17 million years ago).

Recently, a study in the Zoological Journal of the Linnean Society attempted to fill this knowledge gap on divergence times. Francisca Almeida and her colleagues combined molecular and morphological data to determine the phylogenetic relationships within the tinamou family.

Two Major Groups

The researchers produced a molecular dataset of three mitochondrial and five nuclear genes, representing 32 of the 45 species. The morphological dataset included all species and comprised information on 249 characters. Analyses of both datasets – separately and combined – converged upon the same phylogenetic backbone, supporting the idea of two major groups within tinamous: the open-area Nothurinae (genera Tinamotis, Eudromia, Taoniscus, Nothura, Nothoprocta and Rhynchotus) and the forest-dwelling Tinaminae (genera Nothocercus, Tinamus, Crypturellus).

All genera were monophyletic with the exception of Nothoprocta and Nothura. The Dwarf Tinamou (Taoniscus nanus) was embedded with the genus Nothura, and the phylogenetic position of the Rhynchotus species rendered the genus Nothoprocta paraphyletic. A taxonomic revision might be warranted here.

Phylogenetic relationships of the tinamous using a combined matrix of molecular and morphological data. From: Almeida et al. (2022).

Divergence Times

The phylogenetic relationships are quite clear, but what about the divergence times? The answer is not that straightforward. The authors “obtained four different estimates based on different calibration schemes and matrices.” Despite this uncertainty, it seems that the Tinamidae originated between 31 and 40 million years ago. The split between the two subfamilies appears to be older than the previous estimate of 17 million years. Their diversification started in the late Eocene (38–33 Mya) or the Oligocene (33-23 Mya) when the woodland savannahs spread in South America. Many questions remain, but we are slowly learning more about this mysterious group of birds.

References

Almeida, F. C., Porzecanski, A. L., Cracraft, J. L., & Bertelli, S. (2022). The evolution of tinamous (Palaeognathae: Tinamidae) in light of molecular and combined analyses. Zoological Journal of the Linnean Society, 195(1), 106-124.

Featured image: Pale-browed Tinamou (Crypturellus transfasciatus) © Nick Athanas | Flickr

Ghost introgression of a supergene in the Ruff?

Unexpectedly low mutational load in an inversion.

Inversions are sections of DNA that have been flipped around. These chromosomal rearrangements can be very large, housing numerous genes. The resulting genomic sections are often inherited in one big chunk, giving rise to so-called supergenes. The genetic content of these inversions can lead to dramatic phenotypic differences. A classic example concerns the Ruff (Calidris pugnax) where an inversion of ca .4.5 million DNA-letters (containing 125 genes) explains the occurrence of several morphs that differ in morphology and behavior. In a previous blog post, I briefly described the three morphs.

This chromosomal rearrangement resulted in two distinct morphs: the large and territorial independent morph (with the “normal” section of DNA) and the smaller faeder morph (with the inversion). The latter morph developed a new mating strategy in which males sneakily try to copulate in the territories of independents by pretending to be a female. Later on, a third morph arose through the recombination between the ancestral DNA-section and the inversion. The resulting satellite morphs are semi-cooperative, they display on the territories of independent males to attract more females, even though they don’t always manage to mate.

A recent study in the journal Molecular Biology and Evolution took a detailed look at these inversions. Genomic analyses revealed some intriguing patterns that suggest a complex evolutionary history.

Mutational Load

Inversions can be useful when they bring together an adaptive collection of genes. However, they also experience reduced recombination rates (as the “normal” and inverted DNA-sequence cannot align properly). This lack of recombination can have serious consequences, because it reduces the effectiveness of purifying selection to remove deleterious alleles. Hence, inversions are expected to accumulate mutational load. Moreover, suppressed recombination and inefficient selection also provide an opportunity for transposable elements to gather and expand within the inversion.

The inversion in the Ruff is about 4 million years old and might thus experience a high mutational load and potentially harbor many transposable elements. When the researchers analyzed the inversion in several individuals they found “no substantial expansion of repeats and only a modest mutation load on the satellite and faeder haplotypes despite high sequence divergence to the non-inverted haplotype.” An unexpected finding.

An overview of the three Ruff morphs and the accompanying genomic rearrangements. From: Hill et al. (2023).

Ghost Introgression

What can explain the modest mutational load in the Ruff inversion? One possibility is that the inversion has been residing in the Ruff more recently than the genetic divergence (ca. 4 million years) suggests. The inversion might have introgressed from another species. However, additional analyses did not provide clear evidence for this idea.

However, amplicon data from 15 Calidris species, whole-genome resequencing data from 4 other Calidris species, and a search for other divergent haplotypes across the genome failed to identify candidate donor taxa or indications of introgression elsewhere in the genome.

We should not discard the introgression scenario though. Perhaps the inversion came from a now extinct species, so-called ghost introgression (such as in Asian Leaf Warblers). An exciting hypothesis that remains to be investigated.

References

Hill, J., Enbody, E. D., Bi, H., Lamichhaney, S., Lei, W., Chen, J., … & Andersson, L. (2023). Low mutation load in a supergene underpinning alternative male mating strategies in ruff (Calidris pugnax). Molecular Biology and Evolution, 40(12), msad224.

Featured image: Ruff (Calidris pugnax) © Åsa Berndtsson | Wikimedia Commons

Nest design and migration influence conformity to Bergmann’s Rule

Behavior can impact biogeographical patterns.

I have always been fascinated by biogeographical patterns, such as Gloger’s Rule (i.e. darker animals in more humid environments) or Bergmann’s Rule (i.e. bigger animals in colder climates). Numerous studies have reported taxa that follow these rules, but the underlying mechanisms remain unclear. Bergmann’s Rule, for example, is mostly explained by the thermoregulatory benefits of having a lower surface-to-volume ratio. In other words, bigger animals are better at conserving heat. An alternative explanation suggests that bigger animals might be more resilient to starvation in harsher climates.

One aspect that has been largely ignored in the study of Bergmann’s Rule is behavior. Particular behaviors might help species cope with cold conditions, and consequently affect their adherence (or not) to Bergmann’s Rule. In a recent Ecology and Evolution study, Mark Mainwaring and Sally Street took a closer look at the impact of nest design and migration in Western Palearctic birds. Do these traits impact the conformity to Bergmann’s Rule?

Migration and Nest Design

Overall, the researchers found support for Bergmann’s Rule in Western Palearctic birds. The species’ body mass increased with breeding range latitude. When they added nest design and migration to the statistical models, some interesting patterns emerged. Whether a group of species followed Bergmann’s Rule (i.e. a correlation between body size and climate) depended on their migratory behavior and their nest design. Migration showed the strongest impact.

We find that body mass increases in colder temperatures only among sedentary species, while short- and long-distance migrants do not conform to Bergmann’s rule at all. These findings support the idea that migrating Western Palearctic species are completely buffered against selection for large body size in colder climates as they avoid exposure to the coldest winter temperatures at high latitudes by spending the nonbreeding season in warmer environments in southern Europe or Africa.

Within the sedentary species, nest design also played an important role. Species that build an open nest showed the strongest relationship between body mass and climate, whereas this pattern was weaker in species with semi-open or enclosed nests.

Statistical analyses show the impact of migration and nest design on the relationship between body mass and breeding latitude. The steeper the slope, the bigger the effect. These patterns suggest that sedentary species (left figure) and species with open nests (right figure) adhere most strongly to Bergmann’s Rule. From: Mainwaring & Street (2021).

Niche Construction

Taken together, these patterns indicate that birds can not only adapt to colder climates by a large body size (and thus following Bergmann’s Rule), but also by building enclosed nests or migrating to other regions. Species that don’t migrate or that build open nests are exposed to colder conditions, resulting in natural selection for larger body size. Behavior matters in conformity to Bergmann’s Rule.

The idea that species can impact their own evolutionary trajectory through their behavior fits within “niche construction theory“. A classic example concerns the beaver’s dam which can create lakes that drastically change riparian ecosystems. The importance of niche construction in evolutionary theory is currently a matter of debate, specifically between advocates of an extended evolutionary theory and proponents of the “standard” evolutionary theory. An interesting discussion that I will leave for another blog post.

References

Mainwaring, M. C., & Street, S. E. (2021). Conformity to Bergmann’s rule in birds depends on nest design and migration. Ecology and Evolution, 11(19), 13118-13127.

Featured image: A bird nest © Bugaga | Wikimedia Commons