Toad's last supper helps solve Victorian mystery

5 September 2016

A Mexican pine toad

The Mexican pine toad, Incilius occidentalis. Museum scientists have shown that Bufo intermedius, previously thought to be a separate species, is one and the same.

A toad's final meal has helped Museum scientists demonstrate the origins of the species Bufo intermedius, solving a taxonomic mystery dating back over 150 years.

By comparing insects found within a specimen's stomach to beetles held in the Museum collections, researchers have shown that the toad must have been native to Mesoamerica - not South America, as its label originally recorded.

As a result of these findings, and following comparisons with other toads in the collection, the team has shown that Bufo intermedius is one and the same species as Incilius occidentalis - commonly known as the Mexican pine toad.

A Victorian mystery

The toad species Bufo intermedius, known only from a handful of specimens in the Museum collection, has long puzzled scientists.

Jeff Streicher, curator of amphibians at the Museum, said: 'It's a strange toad species. It was originally described in the 1850s, but we don't know much about it.'

The specimens were collected by the Victorian zoologist Louis Fraser during his travels through the New World in the first half of the nineteenth century.

Fraser's field notes indicated that they were collected in South America, and the specimen label recorded the locality as 'Guayaquil and the Andes of Ecuador'. Yet attempts to place the specimens within any species native to South America failed.

Four toads shown from above, below and the side

The type specimens for Bufo intermedius, held in the Natural History Museum collection


'According to the specimen label, it's supposed to be from Ecuador', adds Streicher, 'but nothing resembling it has ever been seen in that country. In fact, it looks like a toad from Mexico.'

As early as 1902 scientists were noting the physical similarities between Bufo intermedius and various species native to Mexico, casting doubt on the locality of the specimens.

Toads revisited

Until recently, research into the specimens' origins had been restricted to morphological analysis: studies comparing the physical characteristics of the specimens with those of other species native to the Americas.

Joseph Mendelson, director of research at Zoo Atlanta, worked with a team from the Museum to perform further morphological research into Bufo intermedius.

Their research showed that key physical attributes of the specimens - including the size of the crests on their heads and the coloration of their bodies - were indistinguishable from Incilius occidentalis, a species native to Mexico and more commonly known as the pine toad.

Building on their conclusions about the specimens' physical attributes, the team discovered that one of the specimens had a full stomach. Following careful dissection, the stomach contents were found to include a few small rocks, as well as the remains of several flies, ants and - most significantly - beetles.

Introducing the beetles

Two beetle specialists from the Museum - Michael Geiser, curator of beetles, and Max Barclay, collections manager for beetles and true bugs - helped establish the species that the remnants belonged to.

Fragments of beetles, labeled alphabetically

Beetle remnants found in the stomach of one of the Bufo intermedius specimens


By comparing the beetle remains with specimens in the Museum's entomology collection, the team were able to identify two species of leaf beetle from the toad's stomach: Zygogramma signatipennis and Megalostomis dimidiata.

Crucially, both beetle species are predominantly found in Mexico, and neither has been found to occur further south than Costa Rica.

These findings confirmed that the South American locality recorded in earlier publications and on the specimen jar labels was in error, and that Bufo intermedius and Incilius occidentalis are one and the same species.

Max Barclay said: 'A 160-year-old mystery has been solved by looking at the last meal of a toad that died before Darwin wrote the Origin of Species. These beetles had been safely preserved within the specimen's stomach all this time, just waiting to reveal their information.

'That wouldn't have been possible without the Natural History Museum's unique collections and taxonomic expertise. It makes you wonder what else is waiting to be discovered in these huge collections.'

  • By Steve Hopkinson

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