Tuesday, 29 July 2014

A downside to long gestation periods

Giraffe with calves - John Storr (Wikimedia Commons)
Giraffes (Giraffa camelopardalis) and the forest-dwelling okapi (Okapia johnstoni) are the survivors of a much broader radiation. Recently Clauss and Rössner (here) used the New and Old Worlds (NOW) data base of fossil mammals to examine the prevalence of various types of ruminant over time.

Rise and fall of tragulids 

The most basal group is Tragulidae, represented by present day chevrotains or mouse deer. In the Early Miocene they were the most prevalent ruminants but were subsequently displaced by the pecorans. The principal difference was the evolution in pecorans of an additional forestomach, the omasum, and this seems to have given them the edge over tragulids. 

Both tragulids and pecorans have synepitheliochorial placentation where binucleate trophoblast cells fuse with uterine epithelial cells at the fetal-maternal interface. However, tragulids lack the cotyledons typical of the placenta in pecorans.

Rise and fall of giraffoids

Among pecorans, Giraffidae is basal to both Bovidae (cattle, sheep, antelopes) and Cervidae (deer). Looking at the equivalent fossil clades, Clauss and Rössner found that Giraffoidea were much more  abundant in the later Early to Middle Miocene but then steadily declined to be supplanted by Bovoidea in Africa and by Bovoidea and Cervoidea in Eurasia. Why?

Based on extant species, the most striking difference between giraffids and other pecoran ruminants is their extremely long gestation - in excess of a year. Assuming this also was the case in fossil species, could that explain their decline? Clauss and Rössner argue that they were unable to develop a seasonal breeding pattern. This put them at a disadvantage compared to other pecorans with shorter gestation times such as bovids and cervids.  

Perhaps in support of this narrative, fossil giraffoids occupied a much greater range of ecological niches and many were grazers. The giraffes grew a long neck and survived as browsers in a special niche. The okapi is highly adapted to its restricted forest habitat.

Placentation in the giraffe has recently been described (see previous post).   

Thursday, 3 July 2014

Menstruation in elephant shrews

Etendeka Round-eared Elephant Shrew or Sengi Macroscelides
micus. Image credit John P. Dumbacher
This post is prompted by the discovery in Namibia of a new species of elephant shrew or sengi, the smallest yet of the 19 species in the Order Macroscelidea. Elephant shrews are so named because of their mobile proboscis.

Midgestation conceptus of the Four-toed Elephant Shrew Petrodomus
tetradactylus. The embryo and fetal membranes are enclosed in the embryo
chamber by the decidua pseudocapsularis. Reproduced from
Oduor-Okelo et al. (here) (c) 2004 with permission from Elsevier
The report (here) mentions two pregnant females carrying one fetus in each horn. This suggests they may resemble other elephant shrews in that implantation of the blastocyst occurs in a preformed embryo chamber as shown above for Petrodumus tetradactylus. There is one such chamber in each horn. 
Should a female fail to become pregnant, the chamber is discarded in a process akin to menstruation. In most mammals transformation of the endometrial stroma to decidua occurs only following implantation. Exceptions are the catarrhine primates, including humans; if pregnancy does not ensue the decidua is shed together with blood and fluids (see previous post). In the 1940's, when human menstruation was poorly understood, Professor C. J. van der Horst of the University of Witwatersrand proposed using elephant shrews as a model. His suggestion was not followed as the establishment of a breeding colony of macaques at the Carnegie Institution of Washington provided a better alternative.
Placentation in elephant shrews was studied by van der Horst and others and more recently has been described by Dominic Oduor-Okelo (here and here). The placental disc has a labyrinth with a haemochorial structure and a spongy zone. In addition there is a paraplacenta. The allantoic sac is large and divided into four lobes. This last feature is a synapomorphy for the superordinal clade Afrotheria (discussed here).