Young fawns, calves, wild lambs, kids, foals etc. probably learn the basics of their diet from their mother during the period while the little one is following the mother ‘on the heel’ and/or foraging within the herd.
Herding ungulates that form mother-offspring unit groups probably also relay such experience-based knowledge on the group level as the young is not restricted to observing and learning from their mother alone but also from other mothers and other younglings.
(Here it might be interesting to note that in species in which males are not included in the mother-offspring unit herd but in which adult males have a different diet, e.g., due to sexual dimorphism in size or other foraging-related traits/behaviours, the young male might have to figure his diet on his own after having left the mother or else there could be a secondary learning period in the species that form separate male groupings.)
Still, there can be times when the young have to make novel decisions regarding their forage.
Such situations might arise, for example, if the individual disperses from the natal area and perhaps, due to high population density, has to travel rather far before being able to settle.
Ungulates usually do not disperse far but dispersal distance could be increased in expanding populations or dense populations.
It might happen thereupon that the young ungulate finds itself in a habitat rather different from the one it grew up in.
Perhaps the young attempt to select habitats as similar to those of their natal home ranges as possible (and if possible).
Such behaviour might result in ecotypes within populations or even a tendency toward speciation.
However, I wonder how the young (but not exclusively this particular age group) make decisions when faced with novel foraging choices, for example, where the plant community is very different or it is represented by the same genera albeit different species.
In species that form herds, smaller groups or temporary associations, social learning might, once again, help to work out what is edible and what is not.
But, for example, roe deer that are not highly social and that have dispersed to expand their population range might find few associates to learn from and these associates might, as well, have arrived to relatively unfamiliar environmental conditions.
Thereby, there might be some other, individual-based methods that the ungulates have evolved to tell apart plant species and to determine their palatability or toxicity.
Firstly, it would be important to know whether ungulates identify plants by sight, smell, taste.
Vision is probably not the dominant tool but it might be applied where the colour of the plants befall their capacity of colour vision or when the plant has a conspicuous and unique morphology (e.g., thistles, hogweed etc.).
Olfactory cues might also be of significance as many ungulates apparently have a very sensitive sense of smell.
Taste should be the last resort with unknown plant species, especially, if toxicity is being determined.
After the plant has been ingested, it might also make sense to follow up with the physiological responses resulting from the ingestion but such mechanism would probably be relevant only in ‘picky’ species (e.g., browsers) and not bulk eaters (grazers).
Generally, bulk eaters are probably rather adapted to consume a vast variety of plants and to deal with those that are ‘unknown’ without paying close attention to the contents of the forage on a very detailed scale (every single plant species).
However, for ‘picky eaters’ whose digestion is not adapted to ‘rework’ the material until all that there is to obtain has been obtained and who therefore must be more selective about what they eat – making decisions in novel situations could be vital as the wrong secondary metabolites or the lack of the right constituents could considerably reduce the uptake of energy and nutrients.
Some secondary metabolites are volatile and these could be used to determine the palatability and toxicity or plants through olfactory evaluation.
Perhaps ungulates categorize the smells and the tastes that they have encountered and when they encounter similar scents and tastes, they know to either avoid them or to try them out.
Accordingly, ungulates might choose closely related plants to those they have already consumed, e.g., in their natal range because these plants smell and taste similarly.
Also, plants that are not closely related to the familiar plants but that have similar chemical composition resulting in similar taste and smell might be either avoided or selected according to the former experience and the associations.
It would be interesting to study whether ungulates in novel foraging situations:
- choose foraging grounds that are environmentally similar to the habitats they are familiar with;
- choose or avoid plants that have similar morphology and chemistry to the plants they are familiar with (e.g., plants of the same genus or plants that produce similar metabolites);
- ingest smaller amount of unknown plants and proceed to continue their inclusion in diet only after a certain period during which the assessment of their palatability and toxicity has been made based on the bodily reactions).
Social behaviour might also be of observational interest under such circumstances because individuals in novel situations might be more prone to approach other individuals in order to learn from them or they might visit frequently used foraging patches in order to see what other individuals have consumed and what they have avoided.
Experimentally, it might also be studied, for example, whether individuals would choose plants that had been treated with metabolites indicating palatability or toxicity (without, of course, making the plant toxic).
For example, would an ungulate avoid a preferred foraging species if it suddenly smelled toxic and unpalatable?
Would the ungulate eat up unknown plants if they strongly smelled of their preferred species?
It is also possible that some choices are made on the basis where the plants grow (e.g., riparian area, forest ground, grassland etc.) and other plant qualities, e.g., how tall they are (shrubs vs. trees), how soft they are and so on.
Perhaps there are categories within the ungulate’s mind whereby some types of plants are considered highly edible and plants that resemble other plants in that category (e.g., they grow in similar habitats and produce similar growth forms) are considered edible.
For example, grazers in novel situations might select patches where they recognize most species or patches that have similar light/shade/moisture etc. conditions to what they have known previously in their lives, and then they might not pay closer to attention to the proportion of plants they do not recognize within this patch.
There could even be some genetic mechanisms at play whereby some scents or tastes stimulate the response in the ungulate to feed on a specific plant even if in its individual lifetime, the ungulate has never encountered this plant but the plant has been a significant part of the diet of the species that the ungulate represents.
Notes by Ieva 