The Chillingham cattle are said to be the only survivors of the wild herds which once roamed freely through the forests of Great Britain. Credit: Sarah Burthe
Climate change has caused more of the British wild cattle of Chillingham to be born in winter, when the chance they will survive to reach a year old is lowest. The cattle, which have lived on a 365 acre park since the 13th century, show that climate’s influence on biological event timing could be greater than realised. “The proportion of births in winter was correlated with the timing of the start of the plant growing season in the previous spring, when winter born calves would have been conceived,” explained Sarah Burthe from the UK’s Centre for Ecology and Hydrology. “More calves were born in winter when the plant growing season started early in the previous spring. Although more births are happening now in the winter than they were 60 years ago, winter-born calves do not do very well relative to calves that are born during summer and are more likely to die before they reach the age of one.”
Though Burthe is currently getting up a 3.30am to study birds on the Isle of May in Scotland, the Chillingham cattle’s unique story made studying the recent climate’s impact on them less gruelling for her. As well as reputedly being the only survivors of wild herds that once roamed the land, thanks partly to the encouragement of Charles Darwin information has been collected about them since 1860. The data Burthe and her colleagues used runs back from the modern herd, which numbered 93 cattle in December, to the unusually cold winter of 1947, which only 13 cattle survived. “This study would not have been possible without the amazing data collected by the Chillingham wild cattle association since 1947,” she said.
Studies of how climate affects key dates in species’ lives – known as phenology – have already shown that recent warming has affected plants and animals. The Chillingham cattle help add to the picture, as they’re not limited to particular breeding seasons like more familiar farmed cattle herds. “There are remarkably few studies in mammals and, to our knowledge, no studies of species that are able to breed year-round,” Burthe said. The ability to have calves all year apparently comes from farmers selectively mating the Chillingham cattle before they went wild, to make their population as large as possible. While this could possibly help them when facing difficult conditions, Burthe noted other wild animals previously bred to reproduce as much as possible have suffered when environmental conditions worsen.
Warmer climate, colder birthdays
A young Chillingham heifer has come into season and is skittishly flicking her tail, showing she will soon be ready to mate. She is escorted by a mature bull while three others look on with envy. Such scenes have been happening earlier in the year as climate has warmed. Credit: Sarah Burthe
In the few other mammal phenology studies, births had been seen occurring earlier in the year with warming, and that was also found in the Chillingham cattle. In a research paper published in the Journal of Animal Ecology last week, Burthe’s team found that on average birth dates had moved forward by one day per year over the past 60 years. “Warming is thought to have significantly increased after the late 1970s and overall we see an increase in the proportion of winter births from this time,” Burthe explained “However, there is still a relationship between winter births and spring temperature before 1979.” The CEH, University of Lincoln and Biomathematics and Statistics Scotland researchers found that this was closely linked to changes in when plants began growing. “This may be due to females coming into condition suitable for reproduction earlier in the year when the plant growth is also earlier,” Burthe said.
That has made a big difference to when the cattle are born, given the typical 280 day duration of pregnancy. Due to their year-round breeding pattern the biggest impact of earlier cattle births was the increased number happening in winter and the reduced survival chance that brings. Burthe and her team now also want to see whether this has any impact on the mothers’ survival, and if it affects the balance of sexes in the herd. Yet they say that their findings already underline that the influence of climate on when biological events happen may be larger than realised and that impacts may be negative, even for species with relatively flexible breeding strategies. “Understanding the consequences of phenological change and how widespread responses are helps us to predict and identify the potential magnitude of changes caused by a warming climate,” Burthe said. “Very little is known about how mammals or feral animals would respond to climate change, despite the fact that they are often important components of ecosystems and used as tools for managing habitat.”
