Thesis

Non-invasive measurements of physical traits in wildlife: photogrammetry applied to deer antlers

Authors:
  • Frankfurt Zoological Society / Bavarian Forest NP / University of Freiburg
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Abstract

Photogrammetry has great potential in animal ecology because it can be used to obtain measurements of animal features such as antler, horn, and body size from photographs in a remote, non-invasive fashion. Little use of photogrammetry has been made so far, however, mostly limited by the need to get sufficiently close to the target animal or have an object of known size in frame to scale it. Here, I aim to unleash the further potential of photogrammetry by validating it for measuring more complex traits using three-dimensional deer antlers for the first time. I calibrated and validated two different photogrammetric systems by measuring an object of known size in photographs from distances up to 200 meters. I then demonstrated the high accuracy of my photogrammetry in measuring a suite of antler features from dozens of roe and fallow deer antlers attached to skulls placed at various distances to emulate a free-living animal. I also developed and validated an efficient correction protocol to deal with curved and tilted antler features. My antler measurement estimates were characterised by mean errors lower than 5% when compared to true antler sizes, with my correction protocol for antlers particularly effective and validated with independent antler sizes. Most importantly, the ecological meaning of my antler estimates was not altered by my approach, because the ranking of the deer based on true antler sizes was identical to the ranking estimated by photogrammetry. My procedures can be followed to estimate highly precise and accurate antler sizes. As I was able to obtain highly precise and accurate measurements of deer antler features with photogrammetry, these techniques can be transferred to measure other complex traits (e.g., horn size) and body size in general (e.g., shoulder height). As photogrammetry is non-invasive, remote, and fast, it fits perfectly into modern wildlife conservation and management techniques. The technique is particularly suited to (but not limited to) the establishment of long-term studies that allow repeated measurements of individuals without the need to capture and handle them.

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The acquisition of accurate information on the size of traits in animals is fundamental for the study of animal ecology and evolution and their management. We demonstrate how morphological traits of free-ranging animals can reliably be estimated on very large observation distances of several hundred meters by the use of ordinary digital photographic equipment and simple photogrammetric software. In our study, we estimated the length of horn annuli in free-ranging male Alpine ibex (Capra ibex) by taking already measured horn annuli of conspecifics on the same photographs as scaling units. Comparisons with hand-measured horn annuli lengths and repeatability analyses revealed a high accuracy of the photogrammetric estimates. If length estimations of specific horn annuli are based on multiple photographs measurement errors of <5.5 mm can be expected. In the current study the application of the described photogrammetric procedure increased the sample size of animals with known horn annuli length by an additional 104%. The presented photogrammetric procedure is of broad applicability and represents an easy, robust and cost-efficient method for the measuring of individuals in populations where animals are hard to capture or to approach.
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Size and symmetry of secondary sexual traits are supposed to be honest signals of male phenotypic quality in vertebrates. Antler size and symmetry, male quality and mating success have not been fully demonstrated to be correlated in cervids. Such correlations can be particularly intriguing in the case of species adopting costly mating strategies, which imply territorial defence without feeding. In these cases, body condition appears to be crucial at the onset of the rut, and large and symmetrical antlers may be borne by successful males. For these reasons, during four consecutive years, we analysed growth rate, size and symmetry of 26 fallow bucks’ antlers in relation to individual mating strategy and success in a lekking population. Territorial (T) males, which gained higher mating success in the lek, showed a faster antler growth (about 10 g/d per antler) than non-territorial (NT) males (3.6–5.2 g/d per antler) during the velvet period, and this was likely because of optimized foraging strategies. At the onset of the rut, when antler growth was completed, T males had larger antlers than NT males. Possibly because of worsened body conditions, NT males showed a pronounced antler directional asymmetry, while T males did not. However, no direct link between antler symmetry and mating success was found, thus confirming the ambiguous role of antler asymmetry as an indicator of fitness. The faster the antler growth, the larger its final size and the higher its beholder’s mating success. Our results confirmed that, like groaning and scent marking, antler size reflects social status and dominance in male fallow deer, and therefore represents an honest advertisement of phenotypic quality.
Article
New insights into anti-predator adaptations and ways of quantifying antler mass in relation to neonatal investment allow a test of the hypothesis that antler size in male deer tracks the inherent reproductive capacity in females. Cursorialism (speedy and enduring running) as an anti-predator strategy maximizes perinatal investment by the female. In deer it leads to maximum antler size in males. In saltatorial hiders (species that avoid detection by hiding and escape by rapid leaping over obstacles and by taking cover), neonates secret themselves away in cover. This reduces the need for perinatal investment, or for large antler size in males. The capacity to spare nutrients from body growth is reflected in females in reproduction and in males in antler growth which permits females to choose males with the best capacity to spare nutrients from growth, or with the best capacity to forage. Female choice is expected to be proportional to the need of neonates to run from predators soon after birth. Courtship, but not dominance displays by male deer, varies interspecifically in proportion to antler size.
Article
A central aim of the study of animal communication is to identify the mode and content of information transferred between individuals. The lateral presentation of the antler palm between male fallow deer has been described as either a signal of individual quality or an attempt to avoid fighting. In the first case two phenotypic features have been proposed by which transmission of individual quality may be facilitated. These are antler size and antler symmetry. The alternative hypothesis proposes that the lateral presentation of antlers occurs as a consequence of averting a threatening posture and may signify a reluctance to fight. We examined whether mature fallow deer use lateral palm presentation as a display during fights to indicate antler size and symmetry. We found no relationship between presentation rate of the antler and antler size and symmetry. Furthermore, males did not preferentially present their larger antler to their opponent. We also investigated whether the rate at which males presented antlers laterally during a fight was related to their ability to win the fight. Our results show that the male who performed more presentations during a fight was more likely to lose it. There were behavioural differences in the way in which a bout of presentation ended; subsequent losers tended to turn their body away from their opponent and subsequent winners tended to lower their antlers to an opponent which we interpret as an invitation to continue fighting. We conclude that the lateral palm presentation serves to de-escalate fighting between mature fallow deer. It is not a mechanism by which to communicate individual quality but rather an indication that a male is less committed to continuing investment in the current contest.
Article
The quantification of nutritional status (e.g., total body fat) of animals is difficult, because the linear dimension (body length) required for the calculation of proxy parameters, such as the physique or body mass indices, cannot be measured without capture. One solution is photogrammetry of body length, provided the following two criteria are met: (1) the camera axes and subject are oriented vertically, and (2) anatomical landmarks are easily identified with low measurement error. By modifying Mori's (Primates 20:371-397, 1979) approach, we devised an accurate photogrammetric method that uses a horizontal bar with an attached ruler for the monkey to traverse, and the anatomical landmarks of the eye and upper border of the ischial callosity to measure body length. We tested the applicability of this method on 11 adult female, habituated, free-ranging Japanese macaques (Macaca fuscata). Somatometric body length (crown-rump length and anterior trunk length) was statistically compared with the body length obtained using photogrammetry. The significant correlation of the photogrammetric body length with each somatometric measurement verified that the former could be employed to calculate various indices that are used to characterize fat mass (nutritional status) in Japanese macaques. The advantages and disadvantages of photogrammetry are also discussed.
Article
Representation of generalized additive models (GAM's) using penalized regression splines allows GAM's to be employed in a straightforward manner using penalized regression methods. Not only is inference facilitated by this approach, but it is also possible to integrate model selection in the form of smoothing parameter selection into model fitting in a computationally efficient manner using well founded criteria such as generalized cross-validation. The current fitting and smoothing parameter selection methods for such models are usually effective, but do not provide the level of numerical stability to which users of linear regression packages, for example, are accustomed. In particular the existing methods cannot deal adequately with numerical rank deficiency of the GAM fitting problem, and it is not straightforward to produce methods that can do so, given that the degree of rank deficiency can be smoothing parameter dependent. In addition, models with the potential flexibility of GAM's can also present practical fitting difficulties as a result of indeterminacy in the model likelihood: Data with many zeros fitted by a model with a log link are a good example. In this article it is proposed that GAM's with a ridge penalty provide a practical solution in such circumstances, and a multiple smoothing parameter selection method suitable for use in the presence of such a penalty is developed. The method is based on the pivoted QR decomposition and the singular value decomposition, so that with or without a ridge penalty it has good error propagation properties and is capable of detecting and coping elegantly with numerical rank deficiency. The method also allows mixtures of user specified and estimated smoothing parameters and the setting of lower bounds on smoothing parameters. In terms of computational efficiency, the method compares well with existing methods. A simulation study compares the method to existing methods, including treating GAM's as mixed models.
The intensity of sexual selection predicts weapon size in male bovids
  • J Bro-Jorgensen
  • D Chapman
  • N Chapman
Bro-Jorgensen, J. (2007). The intensity of sexual selection predicts weapon size in male bovids. Evolution, 61(6), 1316-1326. doi:10.1111/j.1558-5646.2007.00111.x Chapman, D., & Chapman, N. (1997). Fallow Deer: Their History, Distribution and Biology (2 ed.). Machynlleth, Wales: Coch-y-Bonddu Books.
Mountain Goats: Ecology, Behavior, and Conservation of an Alpine Ungulate
  • J W Durban
  • K M Parsons
Durban, J. W., & Parsons, K. M. (2006). Laser-metrics of free-ranging killer whales. Marine Mammal Science, 22(3), 735-743. doi:10.1111/j.1748-7692.2006.00068.x Festa-Bianchet, M., & Côté, S. D. (2008). Mountain Goats: Ecology, Behavior, and Conservation of an Alpine Ungulate. In. Washington DC: Island Press.
Postcapture movement rates can inform data-censoring protocols for GPS-collared animals
  • A C D Quinn
  • D M Williams
  • W F Porter
Quinn, A. C. D., Williams, D. M., & Porter, W. F. (2012). Postcapture movement rates can inform data-censoring protocols for GPS-collared animals. Journal of Mammalogy, 93(2), 456-463. doi:10.1644/10-mamm-a-422.1