Friday mystery object #50 answer

On Friday I gave you a multipart mystery object from the wonderful Grant Museum of Zoology:

This was easy in part and horribly difficult in part. The bones themselves were quickly identified as being bacula (plural of baculum) or os penes (penis bones) by Shane and SmallCasserole, but then there was confusion about which species they might  belong to. Matthew Partridge got one right, but after that the guesses went a bit wide. Here’s an unedited image that has the labels attached:

Now the front label is a bit confusuing, since it refers to A, B, C and D, which are not marked on the specimens and moreover the order of the label is not the same as that of the bacula. The labels with each specimen are more helpful, except that they are not all legible and the scientific names are very old and in one cases misspelled.

Problems aside, these bones are as follows (from left to right): Dog Canis lupus familiaris Linnaeus, 1758; White-nosed Coati Nasua narica (Linnaeus, 1766); Kinkajou Potos flavus (Schreber, 1774); Red Fox Vulpes vulpes (Linnaeus, 1758).

You probably noticed that the Dog and Fox bacula, despite being different sizes, both have a similar shape and a groove (called the urethral sulcus), which helps identify these as belonging to related species.

The Kinkajou apparently has a forked baculum, but according to a species description by Ford and Hoffman (1988) they should have a “… unique baculum that ends in four short, radiating, round-tipped branches“.  Perhaps this simply doesn’t show up on the photograph, perhaps the specimen is damaged, perhaps the individual had a deformed baculum, or perhaps it has been misidentified.

Similarly the White-nosed Coati baculum in the photo doesn’t clearly match the description by Gompper (1995), “The distal tip of the baculum is bilobed rather than spatulate” which distinguishes the White-nosed Coati from the Brown-nosed Coati. I think the photo is probably at fault. Another relative of the Coatis are the two species of Ringtails (Bassariscus sp. Coues, 1887), which again can be differentiated on the basis of their baculum – here’s a detail of a Ringtail from the Grant Museum with its baculum in place:

According to Poglayen-Neuwall & Toweill (1988) the “Bacula of B. astutus have a shaft that bends upward, then downward, and the distal end is flattened dorso-ventrally with a lateral expansion; in contrast, the shaft is nearly straight with a condyle-like tip that has two ventral tubercles proximal to it in B. sumichrasti“. So I think we can confidently identify this Ringtail specimen as Bassariscus sumichrasti on the basis of that description.

Although bacula are distinctive, helping closely related species to be distinguished from one another, not every species has a baculum – marsupials and monotremes don’t have them at all and within the placental mammals there are quite a few examples of the baculum being lost (horses, rabbits, hyaenas, etc.). Humans fall into this category too, although all other primates have bacula. This has led some people to speculate that the ‘rib’ from Adam that was used to create Eve in the Biblical creation myth should have been translated as baculum. Nice story, but the lack of a baculum in humans is more likely to relate to reproductive strategies and indicators of male fitness, since the lack of a supporting bone makes stressed male humans vulnerable to erectile dysfunction.