Antlers have a relentless rhythm.
HORNS: Antlers are not horns. Antlers regrow annually; horns are permanent. Antlers grow at the tips; horns grow at the base. Antlers branch; horns do not.
Over the period of a year, antlers bud, grow covered with velvet, harden and lose velvet, are used in sexual combat, repulse predators, and get discarded. This sequence is then repeated to produce a larger and more complex structure the following year.
Antlers: This posting discusses the white-tailed deer for this deer offers the most antler sightings. However, I have photographed antlers on the four less frequently seen local cervids: moose, elk, caribou, mule deer.
Yet, any observation of antlers is a matter of happenstance. Although this feature of the white-tail deer is shown here, they are only found on males, and males are not as often seen at the valley bottom, so being able to track their vagaries is chancy. Consequently, this sequence of images has been assembled using many different deer taken over multiple years.
Two white-tailed deer pause below a willow tree by the lakeshore. One, with antlers, is clearly male. At this distance, the sex of the other is unclear. Is it a female, or a male that has lost its antlers? Thus begins a exporation of antlers. (Picture courtesy of Cynthia Fraser.)
It is July and is this fawn’s first season. As yet there is no indication of the fawn’s sex and certainly no sign of antlers.
The first sign of a fawn’s antlers to come appears in early August when a dark spot turns up above and behind each eye. It is probably dried blood caused by the pedicle breaking the surface (rather like teething). The pedicle is the base from which antlers will subsequently grow.
Now in September, the fawn’s pedicles can form two buttons on the forehead, but the first antlers will not arrive until the following year.
Nothing then happens until the following April when antler buds appear.
The first year’s antler growth produces little more than simple spikes. Succeeding years produce larger and more complex structures. However, we are getting ahead of ourselves.
Extensions of the pedicles on a deer’s skull, antlers are a composite structure of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They grow faster than any other mammalian bone for they must be grown anew each year. While they grow, they are covered with a vascular skin known as velvet, and it supplies oxygen and nutrients to the growing bone. The velvet covering makes the antlers look bulbous.
These are velvet-covered antlers in mid August. At this time the growth rate is as much as six millimetres per day. However, the velvet seems to be rather sensitive, and deer resist allowing contact with it.
It is now the end of summer. The antler has achieved its full size; the velvet is falling off and the antler’s bone hardens and dies. This dead bone structure is the mature antler, which is now ready for both combat and predator repulsion.
This pair of males at the end of August provides an interesting contrast. Although the antlers are small (perhaps second year) the buck on the left is still in velvet, while the one on the right has mature antlers. The transition is occurring.
It is apple-stealing season and a buck with mature antlers scarfs one. Alas, I have no pictures of the rut — the period of sexual combat between males for access to females. So, a picture of stealing apples will have to suffice.
We now come full circle to the initial picture of two deer under a willow tree taken at the end of January. It is antler shedding time and the deer on the right is seen to be a male that did not have a clean antler loss. They fractured as they fell off. This might have come about as the result of a pedicle or skull injury. However, it is clear that the loss of last year’s antlers is now underway. Soon the growth of new, larger, and more complex antlers will start. The beat goes on.
Dipper under ice
A dipper sometimes forages under ice.
I believe I know when and why it does so.
The odd thing about this, is that I have often watched a dipper stand on the border ice along a creek, dive into the open waters in the middle, retrieve something, and bring it back to the ice to eat. But, I had never seen one dive into the water and then turn to forage under the ice it had been standing upon. Why would a dipper do this?
The insight for me started last November when Liv Grant wrote me about dippers in connection with a nature film, “it is quite surprising to see a bird which is not a penguin swimming under ice!” I responded that I had never seen a dipper do this. Then four days ago, I posted about a Common Goldeneye foraging under ice and mentioned the problem with dippers. Two observers, Derek Kite and Bob Stubbs, then wrote me separately to say they had seen a dipper forage under ice. Superb. But, what occasioned the difference between the dipper’s behaviour during their observations and my own earlier ones?
We have now had a succession of days with temperatures of between -15 °C and -10 °C. Local creeks are icy, so yesterday, I went dipper watching. This time, the dipper was seen repeatedly foraging under border ice and I realized when and why they do so.
First, there is a discussion of the dipper’s behaviour, and then an explanation of it.
A dipper stands on border ice and is about to swallow an aquatic arthropod that it had retrieved from the bottom of the creek.
The dipper stands on the edge of the border ice about to dive.
It would then dive in, but promptly turn and swim under the border ice it had been on.
Finding something under the ice, the dipper brings it to the surface and eats it. It looks rather like a caddisfly larva.
The dipper did this over and over (for as long as I was willing to watch it). Here it can be seen swimming under some really thin ice. Once I saw it travel at least two metres under the ice. It almost always found something worth retrieving and eating.
The dipper doesn’t usually forage under ice. The clue to when it does so is that there has been a succession of days with temperatures below about -10 °C, which festoons the creek with ice. Yet, this doesn’t answer the question of why it forages under ice only sometimes when there is ice. The answer to why lies with the two forms of ice found along a turbulent mountain stream. Border ice, which lies atop the water, forms along the gently flowing creek sides. Anchor ice, which lies on the base of the creek, forms in the turbulent central channel. (See a discussion of how these form.) The dipper acquires most of its eatables along the base of the creek, and the central portion of this is now covered by anchor ice. However, under the border ice, the creek’s base is clear so the hunting is good. The dipper now forages where it can find food, that is, under the border ice. In this earlier picture, the border ice is on the lower left and upper right, while the anchor ice is the yellowish material in the centre.
