4 Galagos, lorises and pottos
Activity 6 is divided into several parts, on successive pages, and asks you to answer a number of questions. Look ahead to this activity now and highlight, or list, the different 'process' words, which tell you what you need to do with the content You should have found 'describe', 'contrast', 'evaluate' and 'explain'. If you look back over earlier activities in this study period, you will also find 'compare', 'discuss' and 'give a brief account of'. You may find it useful to look up such terms in a dictionary, which would help you to work out what is required of the answer. For example, 'describe' means give an account, 'contrast' means compare in order to show differences, and to 'evaluate' requires making an appraisal of something's worth. If you were asked to 'discuss' something, that means you should investigate or examine by argument, sift and debate, give reasons for and against.
Galagos, lorises and pottos were introduced in Section 4; Figure 2 suggests that they derived from a common ancestor, which diverged about 20 million years ago and now forms two families. Pottos and lorises are more closely related to each other than to galagos. Nearly all are tree dwellers, as their hands and feet reveal. Generally, the thumb and often the big toe are enlarged, diverge strongly and are to some degree opposable on the hand or foot. This feature, together with a second digit that is reduced to a small stub, ensures a wide span and an ability to grip branches. The hands and feet of the potto in Figure 4c and h show all these features; those of the loris, which DA describes in LoM p. 228, are much the same. An unusual dentition helps define the group: in particular, lower incisors and modest canines project forward, forming a toothcomb structure, which is involved in both grooming and in some specialised forms of feeding. In fact, nearly all prosimians have a toothcomb of sorts - the main exception being the tarsiers - and in some species (e.g. Figure 7c) it is especially prominent. Its action normally combines with that of a horny plate under the tongue (the sub-lingua), which helps remove debris such as hair that accumulates within the toothcomb during grooming. Another diagnostic feature of the group as a whole is the 'grooming' claw on the second digit of each foot (Figure 4h).
Galagos (bush-babies) are restricted to Africa and the lorises inhabit India and Southeast Asia. (I'll say very little about the pottos; DA refers to them briefly in LoM p. 230. I'll also ignore the angwantibos of West Africa, although they are in Figure 2.) Activity 6 highlights a few key points about these prosimians, via a number of questions. After several of the answers, I'll expand on some of the points made.
To answer the following questions, you may find it useful to study LoM pp. 227-234 again, including the illustrations, and to watch the video sequence below.
(a) In one sentence, describe the diet of lorises and bush-babies.
Both lorises and bush-babies are primarily insect eaters, though both supplement their diet, often with fruit such as berries.
Some bush-babies are specialised for feeding on the gum that flows from trees that have been damaged - by insects, for example. The toothcomb allows most gum-feeding bush-babies simply to scrape off exuded gum, to supplement other food sources. But the needle-clawed bush-baby is more dependent on this food source. It has modified canines and premolars that allow it to gouge holes in the bark; this strengthened toothcomb is shown in Figure 7c. The gum of trees contains complex sugars, which the usual repertoire of digestive enzymes is relatively ineffective at breaking down. So these bush-babies have a form of microbial fermentation similar to that in some herbivores. The microbes that carry out the fermentation are housed within the caecum (an offshoot of the large intestine), which in the needle-clawed bush-baby is five times bigger than expected for a similar sized mammal with a more conventional diet. (Feeding on gum is a widespread habit amongst tree dwellers.)
(b) In three or four sentences, contrast the methods of locomotion of lorises and bush-babies.
Lorises have a creeping form of locomotion - almost a type of unhurried stretching, via limbs of approximately the same size. Their hands have a very wide span and they are able to keep a forceps-like grip on branches. Bush-babies also have grasping hands, but they are specialised leapers and their hindlimbs are considerably longer than their forelimbs [see the photo on pp. 232-233] and their bushy tails are said to act as stabilisers during their leaps.
Given their permanent location on branches (often high up, in the canopy) the forceps-like grip of lorises is a vital adaptation. It requires sustained muscular contraction and the 'special mesh of blood vessels in its wrists and ankles' that LoM p. 228 refers to underpins that ability; it's more formally termed a rete mirabile (pronounced 'rett, or 'retay', mirabilay', and Latin for 'wonderful net'), or simply rete. DA links this rete to an ability to maintain a grip. How might this happen? Highly active muscles require a continuous supply of oxygen and blood-borne nutrients; muscle fatigue can be minimised by the removal of lactic acid. The accumulation of lactic acid limits the action of our own muscles during heavy exertion; it is also associated with deep prolonged dives in aquatic mammals. In all such events, it is a consequence of anaerobic metabolism, brought into play when a reduced oxygen supply is limiting the ability of muscles to contract. The local accumulation of lactic acid would alter the acidity around the contracting muscles and render them ineffective; the rete mirabile perhaps helps transport the lactic acid away, to be dealt with by the liver. Most examples of a rete that you're likely to come across promote the exchange of heat, but the rete here is an example of unusual biological engineering!
Figure 5a (below) shows the rete mirabile in the thigh region of a slow loris. What's evident is a mesh of finely branched, narrow blood vessels - a more conventional arrangement would be very much fewer and larger vessels, where a few major arteries would convey blood towards the feet and a few veins convey blood back towards the core of the body. The intricacies of the rete mirabile are revealed in the cross-section in Figure 5b, which is of the comparable structure in the potto. A major artery (the brachial artery) is surrounded by a lattice-work of smaller arteries and veins. The anatomical arrangement is very reminiscent of structures primarily invlolved in the countercurrent exchange of heat, but a different mechanism seems to be at work here. An array of smaller arterial branches of this type may give the entire bundle of vessels a degree of rigidity. The veins containing the outflowing blood are less likely to collapse during sustained muscular contraction, as they might do in more conventional arrangements. Keeping the blood flowing in such circumstances avoids the accumulation of lactic acid, enabling the grip to be sustained.
(c) Evaluate the following statement: 'Urine washing in lorises and bush-babies is known to increase the grip of the hands and feet.'
The function of urine washing is uncertain and this statement doesn't recognise alternative or additional explanations. Urine washing may be linked with territorial marking, much as may be the case in the kinkajou. Some researchers claim that urine washing has a cooling function; others suggest that urine has a disinfectant role. Further investigation would be needed before the function of urine washing in lorises could be stated with such certainty.
(d) Some experts have argued that stereoscopic vision of the sort possessed by bush-babies is essential for estimating distances in arboreal leaping. But what tree-dwelling mammals encountered earlier in this study period have laterally-pointing eyes and yet are seemingly expert leapers?
Squirrels provide perhaps the best example [p. 219]. Their laterally-positioned eyes ensure a broad field of view, but they appear not to have a significant degree of stereoscopic vision. It may be that bush-baby-style vision is important for hunters - think of owls, hawks and cats - which squirrels are not.
(e) Explain the function of the tapetum lucidum in the eyes of lorises and bush-babies.
For nocturnal species, the tapetum lucidum maximises the sensitivity of the eye to low light conditions, by reflecting back into the retina light that has initially passed through, providing a 'second-chance' for activation of the light-sensitive receptor cells.
There are two major types of light-sensitive cell located within the retina of mammals, different in shape, chemical composition and function. Rods enable species to see in black and white; the presence of cones provides colour vision. By working out the ratio of rods to cones in the retina, an animal's ability to detect colour can be established. As you'd predict of nocturnal animals, pottos, lorises and bush-babies appear not to have colour vision. In prosimians generally, the proportion of cones to rods within the retina is low.