Tuesday, December 06, 2016

Report on the Unhatched Eagle Egg from N2B

Unhatched egg, N2B
We received the results on the unhatched egg from N2B. As watchers might recall, one of the eggs failed to hatch. We thought the embryo might have died early in formation, but according to tests run by Iowa State University, the egg was never fertile. How could that be?

A quick primer on egg fertilization in birds. Sperm needs to encounter an ovum at the infundibulum, or site of fertilization.  If sperm are too early, they will die prior to the arrival of an ovum. If sperm are too late, they can't penetrate the eggshell layers that form around the ovum in the female's oviduct. So how do birds assure fertilized eggs? They:
  • Copulate regularly. Regular copulation helps assure a good supply of sperm - especially important in an animal that regularly clears its cloaca when eliminating waste!
  • Store sperm. Sperm storage tubules maintain sperm viability, prevent stored sperm from being ejected, and continuously release sperm to the infundibulum.
  • Concentrate sperm at the infundibulum. Released sperm are passively carried to the infundibulum. Their continuous release and relatively slow drift help ensure that sperm are present when an ovum arrives. 
After removal from N2B
It is good to know that egg number one didn't contain an embryo, but why wasn't it fertilized? Three suggestions based on a simple idea: sperm wasn't present at the infundibulum when the first ovum arrived. 
  • While eagles have been observed copulating ten months out of the year, males don't produce sperm year round and they don't store it very long once production is underway. Perhaps Dad wasn't producing sperm in time for Mom's first egg. Sperm production is not required for pair bonding.
  • Mom may not have had enough sperm stored to concentrate sperm at the infundibulum in time for ovum #1.  
  • Age might be impacting reproductive success in either Mom or Dad. While free-living animals don't tend to have 'menopause' - a long stretch of time in which they do not bear young - age does impact fertility.
Some watchers have expressed concern that age might be a factor in the failure of egg number one. It isn't especially likely in Mom based on what we know. In general, a female bird that produces a healthy, intact egg is most likely fertile. Reduced fertility in our 'elderly' female peregrine falcons tends to be accompanied by changes in the amount of eggs laid, egg color, shape, and condition. Eggs might be unusually colored, pitted, or shaped - all things we saw at Xcel Energy's Sherco facility in 2014 and 2015. While we don't have a lot of data about senescence and egg production in bald eagles, wild eagles are generally assumed to live for 20-30 years. Given that Mom is just fourteen years old, age-related fertility impairment seems unlikely. 

So how about Dad? We believe he is at least 19 years old, although we don't know exactly how old he is. Senescence and sperm production in birds is a little complicated. Research indicates that aging impacts sperm quantity, quality, and motility in birds, and eggs fertilized by older males hatch at reduced rates when compared to those fertilized by younger males. But even with reduced motility, the sperm of older males tends to perform better than the sperm of younger males in a female bird's body. In short, older birds have less sperm than younger birds, and the sperm they have is less motile and of lower quality. But studies have found that older birds are more likely to fertilize eggs than their younger counterparts, even if those eggs are less likely to hatch.

Why are older male birds more successful at fertilizing eggs? The study Senescent sperm performance in old male birds found that obstacles to sperm movement in a female bird's reproductive tract affected older males less than younger males. I would love to see research on the role that skill and pair bonds play in sperm retention (I am defining 'skill' as actions taken by the male to assure a high degree of receptivity in his partner). We know that at least some female birds are able to preferentially reject the sperm of less desirable males, that female birds who mate with familiar males often produce more fertilized eggs with more egg mass than those who mate with novel males, and that the success of novel mating is highly dependent on male behavior. Reproduction is clearly much more complicated than we used to think, and it is obviously past time to drop the pejorative term bird-brained!

Having said that, we are back to the question of Dad's fertility. One infertile egg doesn't really give us enough data to come to any conclusions, but it is very helpful to know why the egg didn't hatch. We will be documenting whether or not nest production continues to decline at N2B. Thanks to John Howe, Kike Arnal, Pat Schlarbaum, Dr. Ensley, and Iowa State University for giving us more insight into the lives of the birds we follow. We are hoping for the best for our beloved Mom and Dad!


Did you know?
  • Sperm competition can result in speedier sperm among animals that have multiple mates while ovulating. Female mice, for example, take multiple mates and can't reject or store sperm long, so speed is important! http://www.sciencemag.org/news/2014/04/scienceshot-how-make-speedy-sperm
  • Why didn't the egg explode? A decomposing embryo rapidly produces gasses that can explode an egg, but this egg didn't contain anything except (presumably) Mom's blastodisc. It was also kept relatively cool and protected from direct sunlight by grass and nest detritus. John really had to dig for it! He told us that he dug through roughly a 5-gallon peregrine gravel pail worth of stuff to find the egg, including a squirrel's skull!
  • Would a receptive female in a long-term pair be likely to produce more eggs? I had very little time to follow up on the question, but here is one study that touches on it:
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817173/
Things that helped me learn about this topic
So does this mean it is clearly the magic in Dad's many, many sticks? ;) All jokes aside, Dad spends a great deal of time working on the nest and providing food, although Mom is more than capable of building and hunting on her own. We tend to define the reproductive season as starting at the beginning of active copulation or egg-laying. But today's daily activities help cement the bond between Mom and Dad and may result in increased productivity months from now: more fertilized eggs with higher egg mass, more hatched eggs, and heavier weights in nestling eaglets during critical stages of growth. We are learning that eagle reproductive success depends on far more than the brief period of time they spend engaging in productive copulation.
Posted by at
Labels: , , , , ,