Hello All. I received an email last night asking this question, and instead of responding to just one person, thought that everybody might appreciate seeing what the data shows for the research project over the last five years.
Here you go....and, by the way, thanks for the email, it's a great question.
We have been checking pregnancy rates through the course of my research. On the 13D project, we’ve been simultaneously tracking pregnancy rates and conducting summer surveys since 2009. We did not survey in summer 2011 but other than that it’s been flown consistently.
The question you are asking is one that we asked right as we started the project; that is “Do low ram numbers result in depressed pregnancy rates?”
At this point in the project I’m pretty confident that the problem is nutritional restriction rather than lack of rams. I’ll lay out the case for what I’ve seen and then add a few caveats at the end of the discussion.
First, keep in mind that the test itself will not tell us if the ewes were or were not being bred (potentially as a result of low ram numbers). It just measures the level of progesterone in the blood which is elevated in pregnant animals. The test is extremely precise, and I am told that it is sensitive enough that it will show a negative within about a week to ten days after the ewe has lost the pregnancy.
The 13D study area (between the Matanuska and Tazlina glaciers in western 13D) has had extremely low hunter harvest rates as a result of the initially conservative strategy employed in the DS 160/260 any ram drawing permit hunt which was instituted beginning with the 2008 season. It was harvested heavily up until 2008 however.
Here are the numbers and ages of the rams harvested in the study area since the drawing permits were instituted: 3 in 2008 (ages 6, 6, and 4) 3 in 2009 (ages 6, 6, and 9); 3 in 2010 (ages 7, 7, and 10); 3 in 2011 (ages 10, 8, and 9); 3 in 2012 (ages 5, 8, and ); and 6 in 2013 ages (5,9,9, 5, 3, and 9). As an aside, there were additional animals taken on the 160/260 permit in 2012 that were not harvested in the study area (in the permit unit but outside the study area boundaries)
Over the last five years, surveys show steady increases in the ratios of both legal and sublegal rams per 100 ewes. In 2009 we recorded 3.8 legals and 27.5 sublegal rams/100 ewes; in 2010, 5.1 legals and 32 sublegals/100; in 2012, 7.5 legals and 34.0/100 sublegals; and a slight drop in legals to 6.5/100 but a continued increase to 39.9/100 sublegals in 2013.
Overall population numbers increased slightly from 2009-2011 but then decreased somewhat in 2012-13, probably as a result of the extremely heavy snowfall in 2011-2012. The late spring in 2013 didn’t help. 2009 – 574 sheep counted; 2011 – 708 sheep counted, 2012 – 491 sheep counted, and 2013 –486 sheep counted.
Pregnancy rates have remained low in the collared ewes in the 13D study area with the exception of two of five years. 13D ewes had a 62% pregnancy rate in 2009, increased to 88% in 2010, dropped again to 69% and then 21% in 2011 and 2012 respectively, and then rebounded to a more normal 91% in 2013.
For comparison, Steve Arthur reported typical pregnancy rates of 85-100% during his 1999-2003 project in the AK range, with only one low year at 74%. And, Canadian biologist Mari Wood, working on Stone’s sheep in BC, reported rates of 95-100%, testing 45-50 ewes annually over a four year project.
I suspect if ram numbers were the limiting factor we would have seen a steady increase in pregnancy rates as the ram/100 ewe ratios increased between 2008-2013 rather than the back and forth shift between low and higher rates. I suspect that the back-and-forth shift is indicative of animals lambing, taking a year off, lambing again, taking a year off, and so forth which is supported when we look at the individual reproductive history of collared ewes. This is also a typical strategy we have observed in adjacent moose populations when they are nutritionally stressed. I also suspect that the 91% rate we recorded last year is a rebound effect as most ewes did not have lambs in summer 2012 and thus had a bit of a nutritional surplus going into 2013 and were able to maintain a pregnancy.
Finally, and most importantly, these animals are very skinny when we capture them. A good description would be “skin and bones”. We know from animal physiology that animals below a certain threshold of body fat reserves will not maintain pregnancy, or may not enter estrus in the first place.
However, as I mentioned there are a couple caveats that need to be acknowledged.
First, keep in mind that these sheep surveys cover most of the core sheep habitat, though some area is not surveyed. Also, observers, pilots, and counting conditions may vary from year to year, meaning that survey data should be referred to as minimum counts, and taken as an indication of population trend rather than an exact population census. Even under the best conditions, we will never see all the sheep in the unit. Composition data (rams / 100 ewes) from these surveys are generally considered fairly accurate representations of the population.
Second, rams produce viable sperm at 18 months. So, a shortage of mature rams may not ever actually result in a lowered pregnancy rate. Along the same lines, there’s no way to definitively state that that population did or did not experience a low enough ram numbers that could have resulted in decreased pregnancy rates. I’ve heard—though have not ever seen scientific documentation for—that male:female ratios below 20/100 are suspected to be approximately the threshold for decreased pregnancy rates in some other ungulate populations.
Third, there are several hypotheses that are largely untested regarding the value of maintaining older rams in a population. These range from Geist’s dominance related mortality, to others supported anecdotally by historical reports of movement patterns and range use that suggest that these older rams are critical to teach younger rams travel routes, and winter and summer ranges, to older animals serving as a sort of ‘buffer’ in case of a hard winter or increased predator load. So, keep in mind that harvesting all –or even a majority of –the mature rams in a population could have effects that we cannot quantify at this point in time.
And, finally, keep in mind that I’m not saying that this sheep population doesn’t have problems. I’m just saying that I don’t think that depressed ram numbers are the cause of low pregnancy rates we’ve been observing.
I’m totally swamped right now trying to get geared up for captures, scientific meetings, Board of Game meetings, etc. So, I’ll do my best to answer questions but can’t promise that I’ll be able to answer all of them in a reasonable time frame.
I am planning on attending the forum gathering on the 25th in Eagle River for an informal Q&A, and Brian M. is currently locking in a venue and a time for me to present the complete research talk, at which point I’ll make sure we have a long time window available for Q&A after the talk. Keep an eye out for details.