Exploring the formation, maintenance, and adaptive significance of multi-male groups in feral horse societies

概要

Animals adapt their behaviour to maximize their fitness in response to differences in their social and
ecological environment, as well as competitive capabilities, so that we may expect variation in mating
behaviour. In polygynous mammals, male’s mating success is highly variably and therefore competition
for mates is intense with only a few individuals achieving reproduction. Consequently, some males may
employ alternative mating strategies. In feral horses (Equus caballus), females (mares) can associate in
year-round social groups with one male (single-stallion (SS) groups) or multiple adult males (multi-stallion
(MS) groups). However, as stallions compete to monopolize mares, MS groups raise questions regarding
their formation, maintenance and adaptive value. This thesis aimed to provide new insights on stallion
mating strategies by reassessing previously proposed hypotheses and exploring intra-specific variation in
stallions’ traits, using a multidisciplinary approach and non-invasive technologies. In Chapter 2, I explored
if morphological features are a differentiating factor between stallions of SS and MS groups. In Chapter 3,
I tested the costs and benefits predicted for MS groups by previous studies. In Chapter 4, I looked into
patterns of stallion-mare and stallion-stallion socio-spatial behaviour in feral horses and their contributing
social factors.
Methods
Data was collected from a feral Garrano horse population in Northern Portugal. In Chapter 2, I used a noninvasive, laser distance meter to measure distance to target and determined individual body length and
height by overlapping photos of individual horses and a measuring tape. I then tested the accuracy of remote
measurements by manually and remotely measuring domestic horses. For Chapters’ 3 and 4, I used the
population’s demography data over a six-year period (2016-2021) to analyse group size, stability and
foaling rate of MS and SS groups. For behaviour analysis, I focused on a total of 14 SS groups and four
MS groups, and the data collection period varied between the months of May and August (comprising the
breeding and birthing season) from 2018 to 2020. I conducted daily 1-h focal group observations,
representing a total of 314 h of observation (ranging from 6–40 h per group). Simultaneously, every five
minutes I scanned the activity of focal group’s members to compare feeding and resting time of SS and MS
groups. Aerial photo scans were taken every 10 min to record the spatial behaviour of all individuals. ...