Mammary Glands and Solenodon

One of the more interesting aspects of Solenodons is the location of their teats, which are located near the buttocks.
Generally, mammary glands are located in several regions. They can be located anteriorly (as in primates, elephants, sea cows and bats), posteriorly (as in horses, cows, sheep, and whales) or serially (as in litter bearing species such as dogs and cats). Anterior mammary glands are located in the thoracic region, whereas posterior mammary glands are located in the inguinal region.
In addition to supplying young Solenodon with nourishment, the teats provide a secondary function. Namely, they aid in the transport of solenodon young. From an interesting article on Solenodons:

We observed a unique mode of maternal-young contact which we have referred to as ‘teat transport’. This phenomenon is well known among some rodents (2), but unreported in insectivores. At seven weeks of age the youngster will accompany its mother on her foraging activities by clinging to one of the two inguinal teats. At this time the teats are very elongated, up to 2 cm in length, enabling the youngster to cling to a teat as it is dragged along close behind. As the infant grows it is able to assert its own locomotion and simply seize the teat and follow, moving when the mother moves, stopping when she stops. The 21/2 month old infant may still show this response and even scratch itself while standing behind the mother holding on to a teat. It would seem that if the solenodon has to change burrows from time to time, then such a teat-transport mechanism enables the female to move with still very dependent young, pulling them along behind her on her teats, rather than attempting to carry them in her mouth. Mouth transport, of course, is a wide-spread phenomenon in small insectivores; however, the solenodon’s teat-transport mechanism is probably quite efficient since the young remain dependent for a long time. During this dependency period, the female can forage and be accompanied by the young.

This is similar to the function of inguinal teats in marsupials – the one difference being that in marsupials the teats used for transport do not produce milk. I could find no information on the actual anatomy of the Solenodon mammary gland so I can’t compare it to, say, marsupials, cows, sheep, or humans. Although, since Solenodon are pretty ancient (as a species – dating back to the time of the dinosaurs) I would expect there to be some similarities with marsupials.
I also wonder if the teat transport might not explain why there are so few venomous mammals – most mammals transport their young by carrying them with their teeth. I have no data on the subject so this is pure speculation. One of the reasons Solenodon was free to evolve venom is that they used teat transport to move their young and wouldn’t run the risk of accidentally poisoning their young (incidentally, nursing is one way mammals build up immunity so a possible test of this might be to examine young Solenodon’s immunity to their own poison). Other mammals, then would have been precluded from developing venom because an alternative transport mechanism was not available for co-option. If I understand Gould correctly, this would be a case of exaption.

An interesting link Comparative Mammary Gland Anatomy

Friday Solenodon Blogging

This falls under the category of “So ugly it’s cute”.

Solenodon 

Or maybe just “Soooo ugly”!

Solenodon 2 

Solenodon are related to the insectivores and can be found in Hispaniola and Cuba. The Solenodons are small, nocturnal omnivores. They eat insects, grubs, small reptiles, fruit and other plant matter. They may produce two litters a year. How are the young fed? The female’s two teats are placed on the edge of the buttocks near the tail.

Warning: Gratuitous Spider Sex Link
Now if I were PZ Myers that would be reason enough to talk about them. Maybe even try to find a picture of it and talk about the developmental genetics of it all. But I’m not.

So instead I’m going to talk about venomous mammals. Solendons are one of a few venomous mammals. The others being the duckbilled platypus and several species of shrew (including the North American Short Tailed Shrew) The saliva of a solenodon is venomous and is injected via narrow grooves on it’s second lower incisor.

solenodon jaw 

Recent genetic research indicates that the solenodon lineage split off from that leading to moles, shrews and hedgehogs approximately 76 million years ago – somehow surving the event that caused the extinction of the dinosaurs.

Curiously enough, a fossil has recently been found dating to about 60 million years ago.

Bisonalveus browni 

The fossils consisted of several mandibular fragments and a cranial fragment. Previous finds of this particular creature contained molars only but the new find also had canines – which contained grooves similar to the solenodon incisor (an interesting difference). The fossils have been named Bisonalveus browni and belong to a small shrew like mammal. Researchers have several hypothesis to explain the teeth:

“The likelihood that the saliva was toxic and was required to subdue active prey is high,” he said. “But one must also consider that if the animal was a highly active forager … introduction of saliva for digestive reasons could also be important.”

Did other mammals of the time have venom:

“The discovery that B. browni and, in all likelihood, a few other extinct mammals used venom to secure prey suggests that venomous mammals were more widespread in the past.
As the fossil record of mammals from B. browni’s era improves … even more venomous mammals will be discovered.”

Why don’t modern mammals have venom:

“… venom may be scarce among mammals today because predatory mammals use surprise, speed, and strength so efficiently in their attacks, and can inflict lethal damage with teeth and claws”.
“The kill can be immediate … whereas a venom, however sophisticated, takes time.”

Added later: PZ Myers has a great post on Bisonalveus browni, including better pictures of the teeth.

It will be interesting to see how these issues are answered.