Julia P. Myatt

Group hunting within the Carnivora: physiological, cognitive and environmental influences on strategy and cooperation

Cooperative hunting is believed to have important implications for the evolution of sociality and advanced cognitive abilities. Variation in the level of hunt organisation amongst species and how their cognitive, behavioural and athletic adaptations may contribute to observed patterns of cooperative hunting behaviour, however, are poorly understood. We, therefore, reviewed the literature for evidence of different levels of hunt organisation and cooperation in carnivorans and examined their social and physical adaptations for hunting. Descriptions of group hunting were scarce for many species and often of insufficient detail for us to be able to classify the level of hunt organisation involved. However, despite this, reports of behaviour fitting the description of collaboration, the most advanced level of hunt organisation, were found in over half the carnivorans reported to hunt cooperatively. There was no evidence that this behaviour would require advanced cognitive abilities. However, there was some evidence that both social mechanisms reducing aggression between group members and information transfer amongst individuals may aid cooperative hunting. In general, the cooperative strategies used seemed to depend partly on the species’ locomotor abilities and habitat. There was some evidence that individuals take on consistent roles during cooperative hunts in some species, but it was not clear if this reflects individuals’ physical differences, social factors or life experiences. Better understanding of the social, cognitive and physical mechanisms underlying cooperative hunting, and indeed establishing to what degree it exists in the first instance, will require more data for multiple individuals and species over many hunts.

Is the true ‘wisdom of the crowd’ to copy successful individuals?

Diversity of expertise at an individual level can increase intelligence at a collective level—a type of swarm intelligence (SI) popularly known as the ‘wisdom of the crowd’. However, this requires independent estimates (rare in the real world owing to the availability of public information) and contradicts peoples bias for copying successful individuals. To explain these inconsistencies, 429 people took part in a ‘guess the number of sweets’ exercise. Guesses made with no public information were diverse, resulting in highly accurate SI. Individuals with access to the previous guess, mean guess or a randomly chosen guess, tended to over-estimate the number of sweets and this undermined SI. However, when people were provided with the current best guess, this prevented very large (inaccurate) guesses, resulting in convergence of guesses towards the true value and accurate SI across a range of group sizes. Thus, contrary to previous work, we show that social influence need not undermine SI, especially where individual decisions are made sequentially and then aggregated. Furthermore, we offer an explanation for why people have a bias to recruit and follow experts in team settings: copying successful individuals can enable accuracy at both the individual and group level, even at small group sizes.

Nest-building orangutans demonstrate engineering know-how to produce safe, comfortable beds

Nest-building orangutans must daily build safe and comfortable nest structures in the forest canopy and do this quickly and effectively using the branches that surround them. This study aimed to investigate the mechanical design and architecture of orangutan nests and determine the degree of technical sophistication used in their construction. We measured the whole nest compliance and the thickness of the branches used and recorded the ways in which the branches were fractured. Branch samples were also collected from the nests and subjected to three-point bending tests to determine their mechanical properties. We demonstrated that the center of the nest is more compliant than the edges; this may add extra comfort and safety to the structure. During construction orangutans use the fact that branches only break half-way across in “greenstick” fracture to weave the main nest structure. They choose thicker branches with greater rigidity and strength to build the main structure in this way. They then detach thinner branches by following greenstick fracture with a twisting action to make the lining. These results suggest that orangutans exhibit a degree of technical knowledge and choice in the construction of nests.

Functional adaptations in the forelimb muscles of non-human great apes

The maximum capability of a muscle can be estimated from simple measurements of muscle architecture such as muscle belly mass, fascicle length and physiological cross‐sectional area. While the hindlimb anatomy of the non‐human apes has been studied in some detail, a comparative study of the forelimb architecture across a number of species has never been undertaken. Here we present data from chimpanzees, bonobos, gorillas and an orangutan to ascertain if, and where, there are functional differences relating to their different locomotor repertoires and habitat usage. We employed a combination of analyses including allometric scaling and ancovas to explore the data, as the sample size was relatively small and heterogeneous (specimens of different sizes, ages and sex). Overall, subject to possible unidentified, confounding factors such as age effects, it appears that the non‐human great apes in this sample (the largest assembled to date) do not vary greatly across different muscle architecture parameters, even though they perform different locomotor behaviours at different frequencies. Therefore, it currently appears that the time spent performing a particular behaviour does not necessarily impose a dominating selective influence on the soft‐tissue portion of the musculoskeletal system; rather, the overall consistency of muscle architectural properties both between and within the Asian and African apes strengthens the case for the hypothesis of a possible ancient shared evolutionary origin for orthogrady under compressive and/or suspensory loading in the great apes.

Hindlimb muscle architecture in non-human great apes and a comparison of methods for analysing inter-species variation

By relating an animal’s morphology to its functional role and the behaviours performed, we can further develop our understanding of the selective factors and constraints acting on the adaptations of great apes. Comparison of muscle architecture between different ape species, however, is difficult because only small sample sizes are ever available. Further, such samples are often comprised of different age–sex classes, so studies have to rely on scaling techniques to remove body mass differences. However, the reliability of such scaling techniques has been questioned. As datasets increase in size, more reliable statistical analysis may eventually become possible. Here we employ geometric and allometric scaling techniques, and ancovas (a form of general linear model, GLM) to highlight and explore the different methods available for comparing functional morphology in the non‐human great apes. Our results underline the importance of regressing data against a suitable body size variable to ascertain the relationship (geometric or allometric) and of choosing appropriate exponents by which to scale data. ancova models, while likely to be more robust than scaling for species comparisons when sample sizes are high, suffer from reduced power when sample sizes are low. Therefore, until sample sizes are radically increased it is preferable to include scaling analyses along with ancovas in data exploration. Overall, the results obtained from the different methods show little significant variation, whether in muscle belly mass, fascicle length or physiological cross‐sectional area between the different species. This may reflect relatively close evolutionary relationships of the non‐human great apes; a universal influence on morphology of generalised orthograde locomotor behaviours or, quite likely, both.

Testing the use/disuse hypothesis: pectoral and leg muscle changes in captive barnacle geese Branta leucopsis during wing moult.

SUMMARY Previous studies on wild moulting waterfowl have demonstrated that flight and leg muscles experience periods of hypertrophy and atrophy. This is thought to be in response to the change in use of the locomotor muscles as described in the use/disuse hypothesis. We tested this hypothesis using captive barnacle geese. Forty geese were dissected before, during and after wing moult, to determine the changes in mass and functional capacity of the flight and leg muscles. Physiological cross sectional areas (PCSA) and mean fascicle lengths of functional muscle groups were calculated to ascertain the force-producing capabilities of the flight and leg muscles. At the onset of moult, flight muscle mass was at a minimum, having atrophied by 35% compared with pre-moult levels, but it returned to pre-moult levels by the end of wing moult. By contrast, the leg muscles hypertrophied during wing moult by 29%, and the PCSA of individual muscle groups increased substantially. Increases in mass, PCSA and fascicle length of individual leg muscle groups during moult suggest that, when flightless, the leg muscles are functionally adapted to provide greater force and/or manoeuvrability to the birds, to aid ground-based escape from predators. Through studying captive animals that are unable to fly, it has been possible to conclude that the major changes in leg and flight muscle in moulting captive geese cannot be explained through use or disuse. Instead, changes seem to be compensatory or to occur in anticipation of changes in locomotor patterns.

Postural strategies employed by orangutans (Pongo abelii) during feeding in the terminal branch niche.

Obtaining food in an arboreal habitat is complex due to the irregular and flexible nature of the supports available. As the largest predominantly arboreal primate, orangutans are expected to have developed particular postural strategies to enable them to feed successfully. In particular, they need to be able to cope within the terminal branch niche (TBN) as this is where the smallest, most compliant supports are, and also where the majority of the fruit and leaves are situated. We recorded feeding posture, along with a number of ecological and behavioral variables from different age-sex classes to enable analysis of the interactions between these and the compliance of the supports (as estimated from stiffness score). Suspensory postures with a pronograde orientation were used on the most compliant supports for all age-sex classes and appeared to play a particular role in facilitating safe use of the TBN by distributing body weight and using limbs for balance across multiple supports. This contradicts the idea that orthograde suspension evolved in response to the demands of feeding in the TBN. Adult males appear to use the same postures and feeding zones as the other age-sex classes, but appear to use stiffer supports where possible due to their larger body mass. Feeding method differed between the age-sex classes in relation to support stiffness, with larger adult males taking fewer risks due to their larger size, compared to infants and juveniles.

Distribution patterns of fibre types in the triceps surae muscle group of chimpanzees and orangutans

Different locomotor and postural demands are met partly due to the varying properties and proportions of the muscle fibre types within the skeletal muscles. Such data are therefore important in understanding the subtle relationships between morphology, function and behaviour. The triceps surae muscle group is of particular interest when studying our closest living relatives, the non‐human great apes, as they lack a significant external Achilles tendon, crucial to running locomotion in humans and other cursorial species. The aim of this study, therefore, was to determine the proportions of type I (slow) and type II (fast) fibres throughout these muscles in chimpanzees and orangutans using immunohistochemistry. The orangutan had a higher proportion of type I fibres in all muscles compared with the chimpanzees, related to their slower, more controlled movements in their arboreal habitat. The higher proportion of type II fibres in the chimpanzees likely reflects a compromise between their need for controlled mobility when arboreal, and greater speed and power when terrestrial. Overall, the proportion of slow fibres was greater in the soleus muscle compared with the gastrocnemius muscles, and there was some evidence of proximal to distal and medial to lateral variations within some muscles. This study has shown that not only do orangutans and chimpanzees have very different muscle fibre populations that reflect their locomotor repertoires, but it also shows how the proportion of fibre types provides an additional mechanism by which the performance of a muscle can be modulated to suit the needs of a species.

Energy cost and return for hunting in African wild dogs and cheetahs

African wild dogs (Lycaon pictus) are reported to hunt with energetically costly long chase distances. We used high-resolution GPS and inertial technology to record 1,119 high-speed chases of all members of a pack of six adult African wild dogs in northern Botswana. Dogs performed multiple short, high-speed, mostly unsuccessful chases to capture prey, while cheetahs (Acinonyx jubatus) undertook even shorter, higher-speed hunts. We used an energy balance model to show that the energy return from group hunting and feeding substantially outweighs the cost of multiple short chases, which indicates that African wild dogs are more energetically robust than previously believed. Comparison with cheetah illustrates the trade-off between sheer athleticism and high individual kill rate characteristic of cheetahs, and the energetic robustness of frequent opportunistic group hunting and feeding by African wild dogs.

Additive opportunistic capture explains group hunting benefits in African wild dogs.

African wild dogs (Lycaon pictus) are described as highly collaborative endurance pursuit hunters based on observations derived primarily from the grass plains of East Africa. However, the remaining population of this endangered species mainly occupies mixed woodland savannah where hunting strategies appear to differ from those previously described. We used high-resolution GPS and inertial technology to record fine-scale movement of all members of a single pack of six adult African wild dogs in northern Botswana. The dogs used multiple short-distance hunting attempts with a low individual kill rate (15.5%), but high group feeding rate due to the sharing of prey. Use of high-level cooperative chase strategies (coordination and collaboration) was not recorded. In the mixed woodland habitats typical of their current range, simultaneous, opportunistic, short-distance chasing by dogs pursuing multiple prey (rather than long collaborative pursuits of single prey by multiple individuals) could be the key to their relative success in these habitats.