Over the last 6 or so years, during the month of December, I have observed trace amounts of blood in roe deer tracks.
During the first years I observed this, I did not make the effort to keep scientific notes.
Accordingly, I am not entirely certain that it was December precisely or exclusively but it would have been the period of Dec – Jan (most likely, Dec).
The blood was usually found in very small amounts and only in one tiny spot in the snow.
It had not been dripping along the trail as would be the case if the animal had been injured, nor the amount was sufficient to suggest a wound as its origin.
It rather resembled marks of urine I have sometimes seen in roe deer tracks although the amount was probably lesser.
It is not uncommon I have observed a slight urination mark in the snow along the roe deer trail surrounding one or two hoof prints.
The blood was found in similar setting.
The blood was not mixed with urine in most spots where I saw it although in some spots I noticed urine that was tinged rusty.
However, I do not know if the cue was given to the urine by blood or the colour was the result of some environmental impact (e.g., chemically altered source of drinking water – we have a few streams here that run ‘rusty orange’ perhaps due to iron or copper deposits).
It is possible that there might have been urine mixed with blood, as well.
I observed these trace amounts of blood in at least 4 different years and in at least 3 different roe deer subpopulations (separated from one another by the distance of at least 1 km).
Accordingly, I have come to believe that it is a regular occurrence in December (winter) that concerns roe deer as a species (not on a localized or individual scale).
I have exchanged a few e-mails with professor Sandro Lovari who has studied roe deer thoroughly among other species and I inquired his thoughts on this observation.
I believed at first it might have been associated with the delayed implantation and the unique roe deer conception/gestation strategy.
I wondered if perhaps blastocysts were either implanted or aborted during this time and if abortion might result in trace amounts of blood.
I had thought this was something known to science but apparently similar observations either had not been made or had not been discussed in the scientific literature (as Mr. Lovari wrote to me he had looked into the issue).
Mr. Lovari did not confirm my blastocyst abortion hypothesis, and it has been difficult for me to find out how the blastocysts are implanted/discarded.
There might not be any blood if the non-implanted blastocyst is simply reabsorbed (resorption), especially, during these early stages when embryo has not even started developing.
However, I do not know either when exactly roe deer implant the fertilized blastocysts (I thought December might be a fit time because in many species these processes are determined by photoperiod and December is a month of the shortest days).
Perhaps this occurs prior to December and the blood is somehow related to aborted embryos.
It is startlingly hard to find further scientific notices on these matters.
I began contemplating what other causes there might be for these blood traces (if blastocyst/embryo abortion/resorption), indeed, failed to provide explanation.
I came up with a strange theory.
Mr. Lovari mentioned three main (interrelated) evolutionary reasons behind the delayed implantation in roe deer:
‘(i) the necessity to give birth when the green-up is at its top or nearly, (ii) the necessity of avoiding snow during the physically tiring courtship runs, and especially (iii) the necessity to adapt reproductive physiology to the effects of glaciations and deglaciations over millennia of roe evolution.’ (Quote from personal correspondence)
As I considered these causes, I began wondering about something I have, personally, never heard of, namely, the possibility of facultative delayed implantation.
I was thinking along these lines – all the variables mentioned regarding the reasons behind evolution of delayed implantation in roe deer, are climate / environment related.
Mr. Lovari especially stressed the adaptation to glacials / interglacials.
What if roe deer evolved delayed implantation not in order to adapt to colder climate but in order to adapt to variability in climate?
Many species (e.g., river otters) do not have to stick to a definite mating period. But they do if they live in northern latitudes or other regions where it gets cold during winter (e.g., where there is seasonality in food abundance).
I am not saying that roe deer did not stick to a definite mating period. But perhaps their original mating period (prior to delayed implantation) used to be in winter (which rather makes sense because it is a smaller-sized species that hardly needs a gestation period from autumn to spring).
Roe deer evolved delayed implantation because in some places and during some periods in the geological history, mating in winter (due to the reasons stated) was impractical or even impossible.
Perhaps delayed implantation, initially, was not a constant but it was triggered by some environmental conditions. And, accordingly, it did not occur at all times (in an obligate manner) but only when it was most adaptive (e.g., when some threshold food abundances were reached).
While in other places/periods roe deer continued mating in winter (because mating in August/September is also not that perfect as this time could have been spent foraging while the vegetation is still lush, fruit are available etc.), in climatically more challenging regions (greater latitudes, higher altitudes), roe deer began evolving delayed implantation in order to respond to the variability in their habitats.
Accordingly, maybe the deer that mated in August/September could no longer mate in winter (e.g. December) because they were impregnated or (those who were not) had exhausted their hormonal or energy capacity.
Perhaps what we are observing is some kind of remnant oestrus that occurs, for example, in females that did not conceive during August/September.
Maybe these barren female are ovulating once more during the period when roe deer used to ovulate before they evolved delayed implantation.
But perhaps they are not fertile despite the fact that they are ovulating (because food has been too scarce between September and December) or they cannot produce hormones at the same level because they already participated in rut and the rut was followed by a period of gradual decrease in forage availability.
Thus, second rut does not functionally occur (but it might in places where vegetation does not stop growing in autumn) but second oestrus occurs.
The second mating might also be constrained due to the social behaviour of roe deer because not as many males are available for the local females (compared to non-territorial species). The male has already mated and spent much energy. But the young males might not be viable yet.
Additionally, young females might experience their first oestrus in the winter of their first year (in accordance with the original mating behaviour of the species prior to delayed implantation). Maybe they bleed slightly but they are not mature, either, so reproduction does not happen.
Accordingly, the bleeding might have come from immature females and mature females that did not conceive during August/September but the oestrus is not ‘realized’ due to some constraints of hormonal / energy / social nature.
I do not know nearly enough about roe deer anatomy and reproductive phenology (not to mention species evolution history) to determine if this makes any sense but I thought I might share this theory.
It might be adaptive for a species to evolve facultative delayed implantation (and facultative earlier reproduction) if it allows to mate more flexibly with respect to resource availability and weather conditions in specific regions or specific periods in climate history.
On the other hand, I have never heard about such phenomenon. And I am not sure if a species can advance its mating date. Because this would mean that the roe deer learned to mate earlier than it was natural for them (before the forage ran out, i.e., following some environmental cue in forage decrease, plant senescence etc.).
It is difficult imagine how that could have come about.
I believe, however, that roe deer might be the species that could have pulled it off as income breeders whose reproductive cycles are adapted to changing environmental conditions and the more immediate food availability (as compared to capital breeders that always ‘plan ahead’ and that might stick to a stricter reproductive routine).
Additionally, as a browsing species, roe deer might not suffer the same forage availability restrictions as grazers or intermediate feeders who probably are forced to mate while the more palatable and nutritionally valuable food resources are not gone entirely.
So far I know very little of the evolutionary history of the roe deer (or, more specifically, evolution of delayed implantation in the species) but it would appear that the species or it close ancestors evolved in Late Miocene as the climate was cooling and drying.
If I understand correctly, it was a period of a series of glaciations and deglaciations preceeded by somewhat tropical environment.
Perhaps the species that gave rise to roe deer were not limited by the same climatic seasonality that affected roe deer during Late Miocene, and they could have had a mating period also as late as December.
Notes by Ieva 